bluuebunny/crossref_metadata_2025 huggingface dataset:
Early afterdepolarizations: mechanism of induction and block. A role for L-type Ca2+ current.
Authors: January C T , Riddle J M
Published: June 2012

Early afterdepolarizations (EADs) are a type of triggered activity found in heart muscle. We used voltage-clamped sheep cardiac Purkinje fibers to examine the mechanism underlying EADs induced near action potential plateau voltages with the Ca2+ current agonist Bay K 8644 and the effect of several interventions known to suppress or enhance these EADs. Bay K 8644 produced an inward shift of the steady-state current-voltage relation near plateau voltages. Tetrodotoxin, lidocaine, verapamil, nitrendipine, and raising [K]o abolish EADs and shift the steady-state current-voltage relations outwardly. Using a two-pulse voltage-clamp protocol, an inward current transient was present at voltages where EADs were induced. The voltage-dependence of availability of the inward current transient and of EAD induction were similar. The time-dependence of recovery from inactivation of the inward current transient and of EAD amplitude were nearly identical. Without recovery of the inward current transient, EADs could not be elicited. The inward current transient was enhanced with Bay K 8644 and blocked by nitrendipine, but was not abolished by tetrodotoxin or replacement of [Na]o with an impermeant cation. These results support a hypothesis that the induction of EADs near action potential plateau voltages requires 1) a conditioning phase controlled by the sum of membrane currents present near the action potential plateau and characterized by lengthening and flattening of the plateau within a voltage range where, 2) recovery from inactivation and reactivation of L-type Ca2+ channels to carry the depolarizing charge can occur. Our results suggest an essential role for the L-type Ca2+ "window" current and provide a framework for understanding the role of several membrane currents in the induction and block of EADs.
Inhibition of Na-K pump current in guinea pig ventricular myocytes by dihydroouabain occurs at high- and low-affinity sites.
Authors: Mogul D J , Rasmussen H H , Singer D H , Ten Eick R E
Published: June 2012

Binding of cardiac glycosides to the Na+,K+-dependent ATPase has been shown to occur at both high- and low-affinity sites. However, recent reports suggest that glycoside-induced inhibition of electrogenic Na-K pump current occurs with simple first-order binding kinetics at relatively low-affinity sites. This implies that high-affinity binding sites have little to do with Na-K pump inhibition during exposure to cardiac glycosides. To better understand the role of the high-affinity site, we investigated the concentration dependence of Ipump inhibition by dihydroouabain (DHO) in guinea pig ventricular myocytes through use of wide-pore patch pipettes to "fix" internal Na+ activity at approximately 30 mM and to voltage clamp at -40 mV (T = 34 degrees C). DHO was found to have no effect on membrane conductance at a holding potential of -40 mV. Holding current was monitored and the difference between steady-state holding current before and during external exposure to nine concentrations (range, 0.01-1,000 microM) of DHO was measured and normalized to cellular membrane capacitance. The concentration dependence of the inhibition of Na-K pump current was biphasic and well fitted to a two-binding site model with inhibitory KD values of 0.05 microM and 64.5 microM. This is consistent with previously reported 3H-ouabain binding studies in guinea pig myocardium. These findings indicate that the electrogenic properties of the Na-K pump can be inhibited by glycoside binding to both high- and low-affinity sites.
Release of different relaxing factors by cultured porcine endothelial cells.
Authors: Boulanger C , Hendrickson H , Lorenz R R , Vanhoutte P M
Published: June 2012

Experiments were performed to determine the effect of ouabain on the release of relaxing factor(s) from cultured endothelial cells, and its action on the effect of the relaxing factor(s) on arterial smooth muscle. A column of porcine aortic endothelial cells grown on microcarrier beads in suspension culture was perfused with modified Krebs-Ringer bicarbonate solution. The release of relaxing factor(s) by the endothelial cells was detected under bioassay conditions by measuring the relaxing activity of the perfusate overflowing a ring of canine coronary artery (without endothelium) contracted with prostaglandin F2 alpha. Incubation of the endothelial cells with ouabain did not affect the relaxation of the bioassay ring under basal conditions or upon stimulation of the endothelial cells with ADP but impaired the relaxation induced by bradykinin or the calcium ionophore A23187. Incubation of the bioassay ring with ouabain reduced the relaxation under basal conditions as well as the relaxation induced by ADP but did not affect the relaxation observed upon stimulation with bradykinin and A23187 and the endothelium-independent relaxations induced by nitric oxide. These experiments suggest that cultured porcine aortic endothelial cells release two endothelium-derived relaxing factors; one is released under basal conditions and upon stimulation with adenosine diphosphate and the other (which presumably is nitric oxide) upon stimulation with bradykinin and the calcium ionophore A23187.
ATPase activity of the cross-linked complex between cardiac myosin subfragment 1 and actin in several models of chronic overloading. A new approach to the biochemistry of contractility.
Authors: Lauer B , Van Thiem N , Swynghedauw B
Published: June 2012

Myosin ATPase activity is usually considered to reflect the contractile capacity of a given muscle since it correlates with the maximum initial speed of shortening of the unloaded muscle (Vmax). There are several exceptions to this scheme, and it was the goal of this study to determine if the Mg2+-ATPase activity of the covalently bound actomyosin S1 is a more physiological index of contractility. On polyacrylamide gels, the complex obtained after condensation of fast skeletal myosin S1 to skeletal actin is identical to that obtained with myosin S1 from the ventricles of different species, including rat, guinea pig, and human, cross-linked to cardiac or skeletal actin. In every condition, the ATPase activity of the complex is 700-fold higher than that of myosin S1. It correlates linearly with the Vmax both in phylogeny and in conditions in which an isomyosin shift has been reported, such as hypothyroidism and chronic cardiac overload. Such a relation indicates that, in species that already have a low Vmax, a small change in myosin ATPase may induce dramatic consequences in the shortening velocity. Cardiac hypertrophy in humans, where the drop in Vmax is not associated with a myosin change, does not fit into this scheme. The enzymatic activity of the complex is also unmodified in this condition, which shows that, in humans, the myosin ATPase is not a determinant of Vmax and suggests that other mechanisms may be involved. Measurement of this type of ATPase activity provides a new tool to explore contractility biochemically, which is more reproducible and, from a technical point of view, easier to perform than a kinetic assay. It also correlates better with mechanical data obtained with skinned fibers than with those measured on fresh papillary muscles.
Macromolecular transport within heart valves.
Authors: Tompkins R G , Schnitzer J J , Yarmush M L
Published: June 2012

The present study documents the permeability characteristics of heart valvular endothelium to low-density lipoprotein (LDL), albumin, and horseradish peroxidase (HRP). Using quantitative autoradiography, LDL and albumin concentrations were measured within aortic valves of squirrel monkeys and rabbits after 30 minutes of in vivo circulation. The valvular concentration profiles were analyzed using theoretical mathematical models based on fundamental transport principles. In vivo transvalvular concentration profiles of LDL and albumin displayed the highest tissue concentrations immediately beneath the endothelium and displayed the lowest concentrations near the midline of the valve. Tissue concentrations of LDL and albumin displayed large differences in magnitudes between different regions of individual valve leaflets suggesting marked spatial variation in the permeability properties of the valvular endothelium to LDL and albumin; this was also seen visually with HRP. The results of the theoretical analysis showed that 1) the aortic valvular endothelium limits the uptake of LDL and albumin into the valvular tissue, 2) the permeability of the valvular endothelium differs widely from one region of a valve to another and even from one side of the valve to the other within a single valvular region, and 3) intramural diffusion is the predominant mode of transport for LDL and albumin within the aortic valve, even in valvular regions exposed to large pressure differences across the valve.
Angiotensin II causes formation of platelet activating factor in cultured rat mesangial cells.
Authors: Neuwirth R , Satriano J A , DeCandido S , Clay K , Schlondorff D
Published: June 2012

Angiotensin II may contribute to the progression of renal glomerular diseases. Beneficial effects of converting enzyme inhibition in models of renal disease are, however, not always explicable by hemodynamic consequences of angiotensin II inhibition. Angiotensin increases intracellular calcium in glomerular mesangial cells and activates phospholipase A2, factors required for the formation of the lipid mediator of inflammation platelet activating factor (PAF). We therefore examined whether angiotensin II could stimulate PAF production in cultured rat mesangial cells. During a 15-minute incubation angiotensin II caused formation of PAF in a dose-dependent manner with a threshold around 10(-9) M. In four experiments PAF formation in response to angiotensin II (10(-8) M) occurred within 5 minutes and was 29 +/- 8 pmol PAF/mg protein. The amount of PAF detected then declined to 9 +/- 2 and 13 +/- 3 pmol after 15 and 30 minutes of incubation with angiotensin II. More than 90% of the PAF remained cell-associated. The PAF formation was confirmed by negative ion chemical ionization mode of mass spectrometry. A single species of PAF was detected and identified as hexadecyl PAF. We speculate that part of the detrimental effects of angiotensin II in progressive renal disease may relate to PAF formation. The PAF generated may in turn influence glomerular function, platelets, and eicosanoid synthesis, all factors implicated in renal disease. Furthermore, we speculate that angiotensin II-induced PAF formation may contribute to microvasculature pathology in general.
Characterization and pharmacological relevance of high affinity binding sites for [3H]LY186126, a cardiotonic phosphodiesterase inhibitor, in canine cardiac membranes.
Authors: Kauffman R F , Utterback B G , Robertson D W
Published: June 2012

[3H]LY186126, an analogue of the cardiotonic agent indolidan, was shown to bind reversibly and with high affinity (Kd = 4 nM) to a single class of binding sites within canine myocardial vesicles. Binding site density measured in various cardiac membrane fractions correlated well with Ca2+-ATPase activity (r = 0.94; p less than 0.01), but not with Na+,K+-ATPase or azide sensitive ATPase, indicating a localization of these sites within sarcoplasmic reticulum membranes. Divalent cations were required for binding and displayed the following order of activation: Zn2+ greater than Mn2+ greater than Mg2+ greater than Ca2+. Differential activation of [3H]LY186126 binding by various divalent cations was due to alterations in binding site density, rather than affinity. cGMP and selective inhibitors of type IV membrane-bound phosphodiesterase (SR-PDE), for example, indolidan, milrinone, imazodan, and enoximone, selectively displaced bound [3H]LY186126 caffeine, theophylline, and rolipram were relatively impotent as inhibitors of radiolabel binding. Kd values from displacement curves were highly correlated with IC50 values for inhibition of SR-PDE (r = 0.92; p less than 0.001). In addition, Kd values correlated well with published ED50 values for increases in cardiac contractility in pentobarbital-anesthetized dogs (r = 0.94; p less than 0.001). The results support the hypothesis that [3H]LY186126 labels the pharmacological receptor for the class of positive inotropic agents characterized as isozyme-selective phosphodiesterase inhibitors. Furthermore, the data suggest that the identity of the site labeled by [3H]LY186126 is SR-PDE, the type IV isozyme of cardiac phosphodiesterase located in the sarcoplasmic reticulum.
Influence of rate-dependent cellular uncoupling on conduction change during simulated ischemia in guinea pig papillary muscles: effect of verapamil.
Authors: Hiramatsu Y , Buchanan J W , Knisley S B , Koch G G , Kropp S , Gettes L S
Published: June 2012

This study was performed to determine if the changes in cellular coupling induced by simulated ischemia were rate-dependent and if they contributed to the rate-dependent conduction slowing that occurs in this setting. We also sought to determine if the known ability of verapamil to prevent ischemia-induced conduction changes might be related to the preservation of cellular coupling. We studied the effects of increasing stimulation frequency from 0.5 to 2.0 Hz on the simultaneous changes in the maximum rate of rise (Vmax) of the action potential upstroke, conduction velocity, and internal longitudinal resistance (ri) determined by the voltage ratio method in superfused guinea pig papillary muscles under conditions of simulated ischemia (SI). When stimulation frequency was 0.5 Hz, 30 minutes of SI caused a 16.5% decrease in Vmax, a 16% increase in ri, and a 12.9% decrease in conduction velocity. When stimulation frequency was increased to 2.0 Hz, 30 minutes of SI caused a 30% decrease in Vmax, a 72.9% increase in ri, and a 21.4% decrease in conduction velocity. Thus, the changes were rate-dependent. Verapamil (1 X 10(-6) M) did not influence the changes in these parameters during SI at 0.5 Hz nor the decrease in Vmax during SI at 2.0 Hz, but it did prevent the rate-dependent increase in ri. Verapamil also prevented the rate-dependent decrease in conduction velocity induced by SI. Our results suggest that during simulated ischemia the rate-dependent component of the increase in Ri contributes to the rate-dependence of the conduction slowing.(ABSTRACT TRUNCATED AT 250 WORDS)
Relaxation of rabbit ventricular muscle by Na-Ca exchange and sarcoplasmic reticulum calcium pump. Ryanodine and voltage sensitivity.
Authors: Bers D M , Bridge J H
Published: June 2012

We studied relaxation during rapid rewarming of rabbit ventricular muscles that had been activated by rapid cooling. Rewarming from 1 degree to 30 degrees C (in less than 0.5 second) activates mechanisms that contribute to the reduction of intracellular calcium concentration and thus relaxation (e.g., sarcoplasmic reticulum [SR] calcium pump and sarcolemmal Na-Ca exchange and calcium pump). Rapid rewarming in normal Tyrode's solution induces relaxation with a half-time (t1/2) of 217 +/- 14 msec (mean +/- SEM). During cold exposure, changing the superfusate to a sodium-free, calcium-free medium with 2 mM CoCl2 (to eliminate Na-Ca exchange) slightly slows relaxation upon rewarming in the same medium (t1/2 = 279 +/- 44 msec). Addition of 10 mM caffeine (which prevents SR calcium sequestration) to normal Tyrode's solution during cold superfusion slows relaxation somewhat more (t1/2 = 376 +/- 31 msec) than sodium-free, calcium-free solution. However, if both interventions are combined (sodium-free + caffeine) during the cold exposure and rewarming, the relaxation is greatly slowed (t1/2 = 2,580 +/- 810 msec). These results suggest that either the SR calcium pump or, to a lesser extent, sarcolemmal Na-Ca exchange can produce rapid relaxation, but if both systems are blocked, relaxation is very slow. If muscles are equilibrated with 500 nM ryanodine before cooling, relaxation upon rewarming is not greatly slowed (t1/2 = 266 +/- 37 msec) even if sodium-free, calcium-free solution is applied during the cold and rewarming phases (t1/2 = 305 +/- 66 msec). This result suggests that ryanodine does not prevent the SR from accumulating calcium to induce relaxation.(ABSTRACT TRUNCATED AT 250 WORDS)
Distribution of conduction system fibers in the developing and adult rabbit heart revealed by an antineurofilament antibody.
Authors: Gorza L , Vitadello M
Published: June 2012

Using an immunological approach, we demonstrated previously that a neurofilament-like protein is expressed in rabbit heart conduction tissue myocytes, and we proposed that these specialized cardiac muscle cells are of neuroectodermal origin. In the present study, we used the expression of the neurofilament-like protein as a marker for identifying conduction tissue cells and studying their distribution in the developing heart. In 11-day-old rabbit embryos, myocytes expressing the neurofilament-like protein were localized at the atrioventricular and the sinoatrial junctions and had a ring-like distribution. At embryonic day 12, reactive myocytes were found also in the subendocardial layer of the dorsal ventricular wall, in continuity with labeled myocytes at the atrioventricular junction. Examination of older embryos and of neonatal and adult hearts revealed that the expression of the neurofilament-like protein was not restricted to myocytes of conduction tissue regions, but it was also detectable in myocytes of the sinoatrial ring bundle, in scattered myocytes localized in the left sinal horn wall, and in the right atrium in proximity to atrioventricular sulcus tissue. Thus, using an intracellular marker, we show that precursors of adult atrial conduction tissue are distributed at the sinoatrial and atrioventricular junctions; at variance, ventricular conduction tissue precursors do not have a ring-like distribution but are localized in the subendocardial layer, in continuity with the atrioventricular junctional myocytes.
The effect of acute alterations in left ventricular afterload and beta-adrenergic tone on indices of early diastolic filling rate.
Authors: Zile M R , Blaustein A S , Gaasch W H
Published: June 2012

The effects of an acute increase in left ventricular systolic pressure and the effects of an intravenous isoproterenol infusion on myocardial (segment) lengthening rate and chamber (minor axis dimension) filling rate were examined in 12 anesthetized dogs. Measurements of left ventricular systolic pressure (by micromanometer) and of segment length and chamber dimension transients (by ultrasonic crystals) were made in variably afterloaded beats (three-beat descending aortic cross-clamp) before and during an isoproterenol infusion that raised (+)dP/dt by 40%. During the baseline state, we found an inverse relation between the peak rate of increase in minor axis dimension [(+)dD/dt] and systolic pressure over a wide range of systolic pressures (110-160 mm Hg) and end-systolic dimensions (25-40 mm); peak (+)dD/dt and end-systolic dimension were also inversely related. During isoproterenol infusion, end-systolic dimension fell from 29.7 +/- 3.1 to 28.0 +/- 3.1 mm and (+)dD/dt increased from 79.6 +/- 8.0 to 90.1 +/- 8.7 mm/sec; however, the slope and y intercept of the relation between (+)dD/dt and end-systolic dimension were unchanged. Peak (+)dD/dt at a common end-systolic dimension of 31 mm was nearly equal during baseline and isoproterenol states (64.2 +/- 6.3 vs. 65.1 +/- 6.6 mm/sec). Similar results were found using segment length transients. We interpret these data to indicate that (+)dD/dt is strongly influenced by changes in systolic pressure and dimension and that isoproterenol-induced changes in (+)dD/dt are mediated, at least in part, through changes in systolic pressure and dimension.
The behavior of sonicated albumin microbubbles within the microcirculation: a basis for their use during myocardial contrast echocardiography.
Authors: Keller M W , Segal S S , Kaul S , Duling B
Published: June 2012

The purpose of this study was to determine whether the behavior of sonicated albumin microbubbles accurately mimics red blood cell flow in the microcirculation and is thus consistent with their use as in vivo tracers of red blood cell flow during myocardial contrast echocardiography. Accordingly, microbubbles prepared from fluorescein-conjugated albumin and fluorescently labeled red blood cells were injected intravascularly in eight golden hamsters. Their intravascular distribution, velocities, arteriolar-to-venular transit and flux ratios at branch points were determined in the microcirculation of the cheek pouch. Albumin microbubbles (mean diameter, 4.9 +/- 3.6 microns) and red blood cells displayed a similar frequency of distribution across the arteriolar lumen (33% in the central 20% of the arterioles), and their arteriolar velocities were also similar (2.5 +/- 0.7 mm/sec and 2.3 +/- 0.7 mm/sec,p = NS). The mean velocities of microbubbles correlated well with those of red blood cells at baseline and after adenosine application (r = 0.97 and r = 0.89, respectively), as did the calculated maximum velocity (r = 0.98 and r = 0.80, baseline and adenosine, respectively). The velocity profiles across the lumen of the vessels for albumin microbubbles and red blood cells were similar at baseline and after adenosine-induced velocity changes. The flux ratios at branch points also correlated well (r = 0.92, p less than 0.001). Arteriolar-to-venular transit times of albumin microbubbles were similar to those of red blood cells in vessels ranging in size from 22 microns to 45 microns. We conclude that the behavior of albumin microbubbles in the microcirculation mimics that of red blood cells and supports their use as intravascular tracers of red blood cell flow during myocardial contrast echocardiography.
Determinants of sensitization of carotid baroreceptors by pulsatile pressure in dogs.
Authors: Chapleau M W , Abboud F M
Published: June 2012

The threshold pressure of single baroreceptor units is decreased after compared with before exposure to pulsatile pressure according to previous studies in our laboratory. The purpose of the present study is to characterize the determinants of sensitization of arterial baroreceptors by pulsatile pressure. Carotid sinus nerve activity was recorded in dogs anesthetized with chloralose. Two indexes of baroreceptor "sensitivity" were obtained by comparing nerve activity before and immediately after exposure of the isolated carotid sinus to pulsatile pressure for periods up to 10 minutes. Sensitization occurred 1) when the threshold pressure of single baroreceptor units determined with a slow nonpulsatile ramp decreased after as compared with before pulsing and 2) when multiple unit activity increased after as compared with before pulsing at various mean levels of static pressure. Sensitization was evident after pulsing at mean pressure of 50 and 100 mm Hg, but not at 150 and 200 mm Hg, and was caused by the pulsatile change in diameter or deformation and not by the pulsatile change in wall tension. The magnitude of the effect was directly related to the duration of the pulsing period and to the frequency and amplitude of the pressure pulses. The sensitization could not be explained by increased diameter (sonomicrometers) or strain of the carotid sinus at the same pressure after pulsing; thus, there was an increase in "strain sensitivity" that outlasted the period of pulsing by up to several minutes. In most experiments the shift from static to pulsatile pressure at 50 and 100 mm Hg caused an increase in nerve activity, yet sensitization occurred after pulsing when one would have expected postexcitatory hyperpolarization or depression of activity upon return to static pressure. The sensitization was not caused by the release of prostacyclin from the endothelium since it was not reduced after endothelial denudation or inhibition of cyclooxygenase with indomethacin (30-80 microM) or ibuprofen (250 microM). We speculate that sensitization of baroreceptors by pulsatile pressure may contribute to the decreased sympathetic activity after periods of elevated pulse pressure (e.g., after exercise). We also propose that the decreased sensitivity of baroreceptors after acute elevation of arterial pressure (acute resetting) may be offset in part by the sensitizing effect of increased pulsatile stretch.
Comparison of the myocardial uptake of a technetium-labeled isonitrile analogue and thallium.
Authors: Leppo J A , Meerdink D J
Published: June 2012

The myocardial transmicrovascular transport of thallium-201 (201Tl) and technetium-99m hexakis(2-methoxyisobutylisonitrile) (MIBI) were compared during variable blood flow levels in nine blood-perfused, isolated rabbit hearts. Seventeen injections of radiolabeled albumin and EDTA as well as 201Tl and MIBI were performed by indicator-dilution techniques. When coronary flow was varied from 0.52 to 3.19 ml/g/min, myocardial extraction for MIBI averaged 0.38 +/- 0.09 (SD) whereas 201Tl myocardial extraction averaged 0.73 +/- 0.10 (p less than 0.001). Net extraction, which was calculated using end points of 1.8-4.9 minutes, averaged 0.41 +/- 0.15 for MIBI and was less than the 201Tl net extraction of 0.57 +/- 0.13 (p less than 0.001). The mean capillary permeability-surface area product for MIBI (0.44 +/- 0.13 ml/g/min) was one third of 201Tl (1.30 +/- 0.45 ml/g/min; p less than 0.001). However, parenchymal cell permeability-surface area product for MIBI (47.58 +/- 25.85 ml/g/min) was much higher than 201Tl (6.52 +/- 6.51 ml/g/min; p less than 0.0001), and apparent cellular volume of distribution for MIBI (15.15 +/- 3.31 ml/g) was also higher than 201Tl (10.19 +/- 4.00 ml/g; p less than 0.01). These data suggest that capillary permeability for 201Tl is greater than MIBI, but the reverse is true at the parenchymal cell wall. In addition, a new blood-perfused preparation is used for indicator-dilution techniques, and previously developed modeling analyses are also extended to these experiments.
Arachidonate metabolism in cultured fibroblasts derived from normal and infarcted canine heart.
Authors: Weber D R , Stroud E D , Prescott S M
Published: June 2012

Metabolites of arachidonic acid (eicosanoids) may have an important role in the healing process after myocardial infarction. We examined the ability of cardiac fibroblasts from normal and from healing infarcted ventricle to metabolize arachidonate. We induced myocardial infarction in dogs and then allowed them to recover for 1 week, at which time they were killed, and the heart was removed. Fibroblasts were harvested from normal and from the healing, infarcted areas of the left ventricle. The cells from each source were morphologically indistinguishable. There were 347 +/- 102-fold more fibroblasts cultured from the infarcted area than from the normal area. Interestingly, the infarct-derived cells had a slower doubling time (37.4 +/- 3.7 hours) than the normal cells (22.0 +/- 3.6 hours). The uptake of exogenous arachidonate and its distribution in complex lipids was the same in the cells from each area. When stimulated with the calcium ionophore, free exogenous arachidonate, bradykinin, or histamine the cells produced prostaglandin E2 and prostaglandin I2. In each case the infarct-derived cells produced from twofold to fivefold more prostaglandin than the normal cells. We also found that prostaglandin synthesis was highly dependent on the growth state of the cells with a marked decrease a confluence. Finally, in experiments designed to mimic the early state of infarction, we confirmed that isolated cardiac myocytes release arachidonate and showed that normal fibroblasts can incorporate it. The production of eicosanoids by cardiac fibroblasts may be substantial during the healing of myocardial infarction due to their dramatic proliferation and the increased prostaglandin production per cell.
Ionic basis and analytical solution of the wenckebach phenomenon in guinea pig ventricular myocytes.
Authors: Delmar M , Glass L , Michaels D C , Jalife J
Published: June 2012

The ionic mechanisms of slow recovery of cardiac excitability and rate-dependent activation failure were studied in single, enzymatically dissociated guinea pig ventricular myocytes and in computer simulations using a modified version of the Beeler and Reuter model for the ventricular cell. On the basis of our results, we developed a simplified analytical model for recovery of cell excitability during diastole. This model was based on the equations for current distribution in a resistive-capacitive circuit. A critical assumption in the model is that, in the voltage domain of the subthreshold responses, the sodium and calcium inward currents do not play a significant role, and only the two potassium outward currents, the delayed rectifier (IK) and the inward rectifier, are operative. The appropriate parameters needed to numerically solve the analytical model were measured in the guinea pig ventricular myocyte, as well as in the Beeler and Reuter cell. The curves of recovery of excitability and the rate-dependent activation patterns generated by numerical iteration of the analytical model equations closely reproduced the experimental results. Our analysis demonstrates that slow deactivation of the delayed rectifier current determines the observed variations in excitability during diastole, whereas the inward rectifier current determines the amplitude and shape of the subthreshold response. Both currents combined are responsible for the development of Wenckebach periodicities in the ventricular cell. The overall study provides new insight into the ionic mechanisms of rate-dependent conduction block processes and may have important clinical implications as well.
Synergistic disaggregation of platelets by tissue-type plasminogen activator, prostaglandin E1, and nitroglycerin.
Authors: Stamler J S , Vaughan D E , Loscalzo J
Published: June 2012

Endothelial cells produce at least three substances that can attenuate the platelet aggregation response: tissue-type plasminogen activator; the platelet inhibitory prostaglandins I2 and E1; and endothelium-derived relaxing factor, one form of which exhibits properties of nitric oxide. Since platelet aggregates formed in vivo are involved in the initiation of many clinically important occlusive vascular syndromes, we tested the hypothesis that these endothelial products act synergistically to disperse platelet aggregates. Our data reveal that tissue-type plasminogen activator, prostaglandin E1, and nitroglycerin (an organic nitrate activator of guanylate cyclase analogous to endothelium-derived relaxing factor) act synergistically to disaggregate platelets and do so in part by modulation of platelet cyclic nucleotides. These data suggest a potential mechanism by which the endothelium protects against the formation of platelet aggregates in vivo and offer a potential strategy for improving the efficacy of thrombolytic therapy.
Vascular responses to leukocyte products in atherosclerotic primates.
Authors: Lopez J A , Armstrong M L , Harrison D G , Piegors D J , Heistad D D
Published: June 2012

Little is known about the possible role of leukocytes in the pathogenesis of vasospasm. We hypothesized that vasoactive products released by leukocytes might produce constriction of atherosclerotic arteries. To test this hypothesis, we infused fmet-leu-phe (fMLP), a peptide that activates leukocytes to release their vasoactive products, into the perfused hind limb of normal and atherosclerotic cynomolgus monkeys. Infusion of fMLP did not change resistance of large arteries in normal monkeys. In contrast, fMLP produced pronounced constriction of large arteries in atherosclerotic monkeys. To determine whether leukotrienes, platelet-activating factor, or prostaglandin E2 (PGE2), which are released by leukocytes, may contribute to leukocyte-induced vasoconstriction in atherosclerotic monkeys, we injected leukotriene D4, platelet-activating factor, and PGE2 intra-arterially into the perfused hind limb. Leukotriene D4 and platelet-activating factor had minimal effects on large arteries in both normal and atherosclerotic monkeys. PGE2 produced marked constriction of large arteries in atherosclerotic, but not normal, monkeys. Thus, pronounced constriction in atherosclerotic, but not normal, arteries during infusion of fMLP suggests that products released by leukocytes may mediate vasoconstriction in atherosclerotic vessels. Vasoconstrictor responses to PGE2 are profoundly potentiated by atherosclerosis, which suggests that PGE2 may contribute to leukocyte-induced vasoconstriction.
Metabolic oxidation of glucose during early myocardial reperfusion.
Authors: Renstrom B , Nellis S H , Liedtke A J
Published: June 2012

We have previously studied the relation between long-chain fatty acid and pyruvate metabolism in reperfused myocardium and noted a rapid return of fatty acid oxidation to at least preischemic values accompanied by a marked decrease in pyruvate oxidation. The purpose of the present report is to further characterize carbohydrate metabolism during reflow by describing rates of glucose oxidation using [6-14C]glucose. Oxidative performance was determined with and without preserved fatty acid utilization; the latter condition was effected by oxfenicine, which inhibits palmitoylcarnitine transferase I. In the main protocol, two groups of working swine hearts (n = 18) were perfused aerobically for 30 minutes, rendered regionally ischemic (-60 delta % in anterior descending coronary flow) for 45 minutes, and reperfused at control flows for a final 50 minutes of perfusion. An emulsion of Intralipid with heparin was administered systemically throughout the studies to augment serum fatty acids (average fatty acid values, 1.05 +/- 0.05 mumol/ml for both groups). Serum glucose was monitored and maintained at or about 100 mg/dl with additional infusions of glucose as needed. Oxfenicine (33 mg/kg) was administered systemically by bolus injection at time 0 and 60 minutes of perfusion in nine animals. Decreased mechanical performance, that is, stunning, during reflow was evident in both groups (-50 delta % in regional systolic shortening, p less than or equal to 0.05 compared with aerobic values in the control group, and -32 delta %, p less than or equal to 0.05 compared with aerobic values in treated hearts).(ABSTRACT TRUNCATED AT 250 WORDS)
F(ab')2 fragments of anti-Mo1 (904) monoclonal antibodies do not prevent myocardial stunning.
Authors: Schott R J , Nao B S , McClanahan T B , Simpson P J , Stirling M C , Todd R F , Gallagher K P
Published: June 2012

To determine if inhibition of leukocyte adhesion and aggregation could improve postischemic ventricular dysfunction ("stunning"), a monoclonal antibody (904) that binds to the adhesion-promoting Mo1 glycoprotein on the cell surface of leukocytes was administered intravenously (0.5 mg/kg) to open-chest dogs before a 15-minute coronary occlusion. Ultrasonic crystals placed in ischemic and control myocardium were used to measure systolic wall thickening during a 15-minute occlusion of the left anterior descending artery and for 3 hours after reperfusion. Myocardial blood flow was measured with tracer-labeled microspheres before occlusion, after 10 minutes of occlusion, 3 minutes of reperfusion, and at 1 and 3 hours after reperfusion. Six animals receiving anti-Mo1 antibody had antibody excess demonstrated with immunofluorescence techniques at 5 minutes and 3 hours of reperfusion; this finding indicated saturation of binding sites. Five animals served as controls and received an antibody (murine immunoglobulin G) that does not influence neutrophils. The two groups did not differ hemodynamically during ischemia and reperfusion. Risk areas and myocardial blood flow were also not significantly different between the two groups. The main parameter used to define regional myocardial stunning, percentage systolic wall thickening in the ischemic/reperfused area, did not differ significantly between the two groups. Specimens from nonischemic myocardium were compared with ischemic specimens for myeloperoxidase content. There were no significant differences within or between groups. These data indicate that the anti-Mo1 monoclonal antibody (904) is not effective in improving the profound myocardial dysfunction that persists for 3 hours of reperfusion after 15 minutes of ischemia.
One hour of myocardial ischemia decreases the activity of the stimulatory guanine-nucleotide regulatory protein Gs.
Authors: Susanni E E , Manders W T , Knight D R , Vatner D E , Vatner S F , Homcy C J
Published: June 2012

The effect of 1 hour of myocardial ischemia on the function of the stimulatory guanine-nucleotide-binding protein Gs was examined. This study follows our recent finding that myocardial ischemia increases the density of beta-adrenoreceptors in a conscious canine model while having the opposite effect on the activity of adenylate cyclase. Coronary artery occlusion was induced in five conscious dogs and verified by measurement of blood flow using the Doppler and microsphere techniques. Alterations in the level and function of Gs were examined in sarcolemmal membranes prepared from ischemic and nonischemic regions of the left ventricle. After 1 hour of coronary artery occlusion, the functional activity of sarcolemmal Gs, as determined by reconstitution with cyc- membranes, decreased by 27 +/- 7% in the ischemic zone. Cholera toxin labeling performed in parallel with the reconstitution studies demonstrated a similar decrease of 28 +/- 7%. This was associated with decreases in basal activity and decreases in adenylate cyclase activity stimulated by GTP, GTP plus isoproterenol, sodium fluoride, and forskolin. Thus, a defect distal to the beta-adrenoreceptor occurs in the transduction of adrenergic signals to the heart as a consequence of 1 hour of ischemia.
Inhibition of sarcolemmal carbon-centered free radical formation by propranolol.
Authors: Mak I T , Arroyo C M , Weglicki W B
Published: June 2012

The mechanism of propranolol-inhibited sarcolemmal membrane lipid peroxidation was investigated by electron spin resonance spin-trapping technique using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and 2-methyl-2-nitrosopropane (MNP). Highly purified canine myocytic sarcolemma were peroxidized by a superoxide-driven (from dihydroxyfumarate) and Fe3+-catalyzed free radical-generating system. Hydroxyl radicals (.OH), identified by electron spin resonance signals as DMPO-OH adducts, were generated in the aqueous phase. Propranolol up to 500 microM did not effectively reduce the intensity of the DMPO-OH adducts. When the sarcolemma were incubated with MNP before the addition of free radicals, MNP adducts characteristic of carbon-centered radicals were produced. Pretreatment of the membranes with propranolol (3-100 microM) decreased the intensity of the MNP adducts in a log concentration-dependent manner; the EC50 is about 7 microM. D- and L-propranolol were found equally effective. When protein-depleted sarcolemmal lipids were similarly incubated with MNP and the free radical system, identical MNP adducts were observed; this finding suggests that the adducts were lipid-derived products arising from lipid peroxidation. Furthermore, their formation was also inhibited by propranolol pretreatment. Since propranolol is not an effective scavenger of oxygen radicals in the aqueous phase, the data suggest that the antiperoxidative effect of propranolol is due to its lipophilic interaction with the membrane and thus subsequent interruption of the free radical chain reactions.
A novel neurogenic vasodilator mechanism in bovine mesenteric artery.
Authors: Axelsson K L , Ljusegren M E , Ahlner J , Grundström N
Published: June 2012

The presence of a neurogenic vasodilator mechanism was investigated in isolated bovine mesenteric arteries (BMAs) that were precontracted with phenylephrine. Electrical field stimulation induced tetrodotoxin-sensitive relaxations in guanethidine-pretreated BMAs. The relaxation occurred after a delay of about 5-8 seconds and amounted to 25-35% in different sets of experiments. The relaxation was not affected by classical receptor antagonists such as atropine (1 microM), cimetidine (3.9 microM), clemastine (2.8 microM), naloxone (1.2 microM), 8-phenyltheophylline (1 microM), propranolol (3.4 microM), ritanserin (5 microM), or droperidol (13 microM). The nicotinic acetylcholine-receptor stimulant 1,1-dimethyl-4-phenyl-piperazinium iodide (10 microM) was without effect on the relaxation, and removal of the endothelium of the arteries also had no effect. The bee venom component apamin (1 microM), which has been shown to block the nonadrenergic, noncholinergic relaxation in intestinal and vascular smooth muscle from other species, was also found to be without effect on the relaxation induced by electrical field stimulation in BMAs. Pretreatment of the arteries with capsaicin (1 microM) had no effect per se and did not affect the relaxation induced by a subsequent stimulation. Capsaicin has been suggested to release neurotransmitter and eventually deplete neurons containing substance P and calcitonin gene-related peptide. Furthermore, exogenously applied calcitonin gene-related peptide (1-100 nM), substance P (10 nM-1 microM), and vasoactive intestinal peptide (0.3-30 nM) gave relaxations amounting to less than 10%. It is postulated that electrical field stimulation induces a neurogenic relaxation of a nonadrenergic, noncholinergic nature in BMAs.(ABSTRACT TRUNCATED AT 250 WORDS)
Differential cardiovascular effects of centrally administered vasopressin in conscious Long Evans and Brattleboro rats.
Authors: Harland D , Gardiner S M , Bennett T
Published: June 2012

The cardiovascular effects of arginine vasopressin (AVP) administered into a lateral cerebral ventricle or into the cisterna magna were investigated in conscious Long Evans (control) rats and AVP-deficient Brattleboro rats. The effects of subpressor intracerebroventricular and intracisternal doses of AVP on cardiac baroreflex sensitivities were also determined. Intracerebroventricular and intracisternal AVP increased blood pressure of both strains of rat in a dose-dependent manner. The maximum pressor response produced by intracerebroventricular AVP in Long Evans rats was 13 +/- 2/13 +/- 1 mm Hg (systolic/diastolic, n = 6) after 100 ng AVP. The pressor response to the highest intracerebroventricular dose of AVP tested in Brattleboro rats (30 ng) was 46 +/- 13/21 +/- 6 mm Hg (n = 6). Intracerebroventricular AVP caused a tachycardia in Brattleboro rats but had no effect on heart rate of Long Evans rats. At doses greater than 1 ng, the increases in blood pressure produced by intracisternal AVP in both groups of rats were significantly greater than the increases produced by the same doses given intracerebroventricularly. Heart rate fell in a dose-dependent manner after intracisternal AVP in Long Evans rats but not in Brattleboro rats. Cardiac baroreflex sensitivities of Brattleboro rats were not significantly different from those of Long Evans rats and were not modified by intracerebroventricular (0.3 ng) or intracisternal (0.1 ng) AVP. In Long Evans rats, intracisternal AVP (0.3 ng) increased cardiac baroreflex responses to both increases and decreases in pressure. Intracerebroventricular AVP (0.3 ng) increased the sensitivity of the reflex in response to an elevation but not to a reduction in blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)
Analysis of the characteristics of the flow velocity waveforms in left atrial small arteries and veins in the dog.
Authors: Kajiya F , Tsujioka K , Ogasawara Y , Hiramatsu O , Wada Y , Goto M , Yanaka M
Published: June 2012

To clarify the characteristics of the phasic blood velocity pattern in small arteries and veins on the left atrial surface, we used our newly developed fiber-optic laser Doppler velocimeter. We intended particularly to examine the influence of atrial contraction and relaxation on velocity waveforms to obtain some insight into the nature of the mechanical force acting on the atrial intramyocardial vascular beds. In 14 anesthetized open-chest dogs, the left atrial appendage was gently displaced to expose small branches of the artery and vein. Vessels with an outer diameter of about 150-500 microns were chosen for the measurements because their walls are transparent to laser light. The fiber tip (velocity sensor) was fixed on the vessel surface with a drop of cyanoacrylate when good-quality Doppler signals were consistently observed. Additional experiments with three dogs were performed to observe the blood velocities in the atrial artery and vein during arrhythmia. The blood velocity waveform in the artery was similar to the pattern of aortic pressure during ventricular ejection (peak velocity, 18.8 +/- 7.8 cm/sec) but was characterized by a pronounced dip during atrial contraction. The temporal coincidence between the dip formation and atrial contraction was confirmed during atrial flutter with an atrioventricular block. After isoproterenol administration (2 micrograms i.v.), the acceleration rate of the forward flow velocity increased by 176% (p less than 0.05), and reverse flow appeared during atrial contraction in five cases out of eight (p = 0.013). The blood flow velocity in atrial small veins, on the other hand, was predominant during atrial contraction (peak velocity, 15.6 +/- 5.8 cm/sec). Isoproterenol increased the acceleration rate of this forward flow velocity by 121% (p less than 0.01). Nitroglycerin did not change the blood velocity waveform significantly in atrial arteries or in veins. The phase opposition between arterial inflow into and venous outflow from the atrial myocardium indicates that a large portion of the coronary inflow to the atrial myocardium may be stored due to the presence of atrial myocardial vascular capacitance. We conclude that atrial myocardial contraction impedes atrial inflow and promotes venous outflow from atrial capacitance vessels.
Capillary growth in anemia-induced ventricular wall remodeling in the rat heart.
Authors: Olivetti G , Lagrasta C , Quaini F , Ricci R , Moccia G , Capasso J M , Anversa P
Published: June 2012

To determine whether anemia-induced cardiac hypertrophy affects ventricular size and shape and the component structures of the capillary network of the left and right ventricles, young male rats were fed an iron- and copper-deficient diet for 7 weeks. By that time, blood hemoglobin content fell to 5 +/- 1 g/dl, and packed cell volume fell to 18 +/- 3%. To further characterize the implications of anemia, red blood cell number, hemoglobin corpuscular content, systemic arterial pressure, heart rate, and blood viscosity were measured. Moreover, the changes in ventricular weights were analyzed in terms of the alterations in ventricular wall area and ventricular wall thickness to establish the impact of the elevation in load associated with a high cardiac output state on ventricular remodeling. The quantitative properties of the capillary circulation were also examined biventricularly by low power electron microscopic morphometry to evaluate the adaptive growth potential of the coronary microcirculation in this form of cardiac hypertrophy. Anemia was found to interfere with the production of red blood cells and their mean corpuscular hemoglobin content and resulted in a 40% reduction in blood viscosity and a 12% and 27% decrease in systolic and diastolic blood pressure, respectively. The changes in heart rate were not statistically significant. In comparison with control animals, heart weight increased by 50%, but the enlargement in right ventricular mass (65%) was greater than that of the left ventricle (47%). Ventricular hypertrophy occurred with increases in wall area and wall thickness although the former increased consistently more than the latter in either ventricle. Tissue growth was accompanied by a 60% lengthening of the capillary network, which in combination with an increase in capillary diameter resulted in a 65% and 34% expansion in capillary luminal volume and 56% and 20% larger luminal surface density in the left and right sides of the heart, respectively. In conclusion, hypochromic microcytic anemia leads to eccentric ventricular hypertrophy with a significant amount of capillary proliferation that may tend to protect the myocardium from the increased potential for ischemic injury.