bluuebunny/crossref_metadata_2025 huggingface dataset:
Properties of adenosine‐triphosphate‐regulated potassium channels in guinea‐pig ventricular cells.
Authors: Kakei M , Noma A , Shibasaki T
Published: December 2014

A class of K channels in cardiac muscle is reversibly blocked by intracellular adenosine 5'‐triphosphate (ATP). The characteristics of this K channel were studied by recording single‐channel currents in ventricular cells isolated enzymatically from guinea‐pig heart. The reversal potential of single‐channel currents agreed well with the K equilibrium potential. Blockers of other K channels, such as tetraethylammonium and 4‐aminopyridine, decreased the mean open time of the channel. The chord conductance increased as the 0.24th power of the K concentration on the outer surface of the membrane, and showed a marked inward‐going rectification on strong depolarizations. The degree of rectification was larger with increasing Na concentration on the inner side of the membrane. The kinetics of the channel were almost voltage independent, but depended on the concentration of intracellular ATP. The conductance of the channel was not affected by ATP. When channel kinetics were examined in the presence of ATP, the distribution of open times and closed times was fitted well with a sum of two exponential components. When ATP concentration was increased, the time constants obtained from the open‐time histogram decreased and those from the closed‐time histogram increased, resulting in a decrease of the open‐state probability. The channel was blocked by ATP, adenosine 5'‐diphosphate,5'‐adenylylimidodiphosphate, guanosine 5'‐triphosphate and uridine 5'‐triphosphate, but not by adenosine 5'‐monophosphate, creatine phosphate, creatine or adenosine. Plots of the open‐state probability versus the ATP concentration revealed Michaelis‐Menten saturation kinetics with strong co‐operativity of multiple receptor sites (Hill coefficient 3‐4, concentration of half‐saturation 0.5 mM). It was concluded that this K channel has three or four receptor sites selective for triphosphate nucleotide on the inner surface of the membrane, and that the channel is blocked through the binding of agonists to the receptors.
The relationship between intracellular calcium and contraction in calcium‐overloaded ferret papillary muscles.
Authors: Allen D G , Eisner D A , Pirolo J S , Smith G L
Published: December 2014

The photoprotein aequorin was microinjected into cells on the surface of ferret papillary muscles. Tension and aequorin light, a function of intracellular Ca concentration ([Ca2+]i), were monitored. The preparations were exposed to increased concentrations of extracellular Ca ([Ca2+]o). Small increases in [Ca2+]o led to the usual increase in both the systolic light signal and the developed tension. However, high [Ca2+]o led to a fall in developed tension known as Ca overload. This fall of tension was seen at lower [Ca2+]o if the experiment was performed in the presence of strophanthidin. In conditions of Ca overload, the systolic light signal was either similar in amplitude or larger than the systolic light observed under conditions which did not lead to Ca overload. Oscillations of diastolic light were invariably present under conditions of Ca overload. These oscillations were accompanied by after‐contractions which were small in relation to the magnitude of the aequorin light oscillations. During Ca overload, the variance of the amplitude of the systolic light signal was greater than could be accounted for by the random nature of the arrival of photons. Small systolic light signals occurred when there had been an oscillation of light in the diastolic period immediately preceding the systolic light signal. Large systolic light signals occurred when the preceding period (approximately 1 s) was free of oscillations of light. These observations suggest that if the sarcoplasmic reticulum (s.r.) has spontaneously released its contents of Ca as shown by a diastolic [Ca2+]i oscillation, then a stimulated systolic Ca signal occurring within about the next second is smaller, possibly because it takes longer than this period for the s.r. to reload with Ca. If this process occurs randomly in the various cells of the preparation, developed tension will be reduced because those cells which have a small Ca release will act as a compliance in series with cells which have a large Ca release.
Calcium ion, an intracellular messenger of light adaptation, also participates in excitation of Limulus photoreceptors.
Authors: Bolsover S R , Brown J E
Published: December 2014

Photoreceptor cells of Limulus ventral eyes were bathed in artificial sea water (ASW) that contained 10 mM‐EGTA and no added Ca2+ (EGTA‐ASW). Test flashes elicited responses that increased to a maximum size within 10 min in EGTA‐ASW but did not change further when dark‐adapted cells were bathed for an additional 35 min in this solution. Light responses progressively declined from this maximum size if the cells were repetitively illuminated in EGTA‐ASW. In this state of reduced responsiveness, response amplitudes were further reduced by intracellular ionophoretic injection of EGTA; response amplitudes were increased by intracellular ionophoretic injection of Ca2+. Both of these findings are opposite to what is normally observed for cells bathed in ASW. Also, after repetitive illumination in EGTA‐ASW, both the slope of the response versus intensity relationship became steeper and light responses often had a delayed increase in amplitude. The light responses and the response versus intensity relation returned to normal when the bathing medium was changed back to ASW containing 10 mM‐Ca2+. The light‐induced rise in luminescence recorded from photoreceptors injected with the photoprotein aequorin (the 'aequorin response') declined by at most 50% after dark‐adapted photoreceptors were bathed in EGTA‐ASW for 45 min. However, the aequorin response progressively declined by 98% if cells were repetitively illuminated while bathed in EGTA‐ASW. The total intracellular Ca content of whole end‐organs was measured by atomic absorption spectroscopy. Total intracellular Ca content did not change significantly while photoreceptors were bathed in EGTA‐ASW even after repetitive illumination. We suggest that cytosolic Ca2+ is required by one or more steps in the mechanisms that link rhodopsin isomerization to both (i) an increase in the conductance of the cell membrane to Na+ and (ii) a release of Ca2+ from a light‐labile store.
An intracellular analysis of gamma‐aminobutyric‐acid‐associated ion movements in rat sympathetic neurones.
Authors: Ballanyi K , Grafe P
Published: December 2014

Double‐barrelled ion‐sensitive micro‐electrodes were used to measure the changes of the intracellular activities of Cl‐, K+, and Na+ (aiCl, aiK, aiNa) in neurones of isolated rat sympathetic ganglia during the action of gamma‐aminobutyric acid (GABA). The membrane potential of some of the neurones was manually 'voltage clamped' by passing current through the reference barrel of the ion‐sensitive micro‐electrode. This enabled us to convert the normal depolarizing action of GABA into a hyperpolarization. A GABA‐induced membrane depolarization was accompanied by a decrease of aiCl, aiK and no change in aiNa, whereas a GABA‐induced membrane hyperpolarization resulted in an increase of aiCl, aiK and also no change in aiNa. GABA did not change the free intracellular Ca2+ concentration, as measured with a Ca2+‐sensitive micro‐electrode, whereas such an effect was seen during the action of carbachol. pH‐sensitive electrodes, on the other hand, revealed a small GABA‐induced extracellular acidification. The inward pumping of Cl‐ following the normal, depolarizing action of GABA required the presence of extracellular K+ as well as Na+, whereas CO2/HCO3‐‐free solutions did not influence the uptake process. Furosemide, but not DIDS, blocked the inward pumping of Cl‐. In conclusion, our data show that only changes in intracellular activities of K+ and Cl‐ are associated with the action of GABA. Furthermore, they indicate that a K+/Cl‐ co‐transport, and not a Cl‐/HCO3‐ counter‐transport, may be involved in the homoeostatic mechanism which operates to restore the normal transmembrane Cl‐ distribution after the action of GABA.
Dependence of adenosine triphosphatase activity of rabbit psoas muscle fibres and myofibrils on substrate concentration.
Authors: Glyn H , Sleep J
Published: December 2014

The rate of hydrolysis of adenosine triphosphate (ATP) by chemically skinned rabbit muscle fibres was measured as a function of Mg ATP concentration in the range 5 microM to 5 mM. Pyruvate kinase and lactate dehydrogenase were used to link adenosine diphosphate formation to oxidation of nicotinamide adenine dinucleotide which was followed by the change in absorption at 340 nm. The ATPase rate of a fully activated fibre (pCa = 4.5) increased monotonically with Mg ATP concentration in a manner that could be readily fitted by a hyperbola. At 15 degrees C, pH 7 and an ionic strength of 0.2 M the rate at saturating Mg ATP (Vm) was 1.78 +/‐ 0.2 s‐1 per myosin head (mean +/‐ S.D.; n = 6) and the Mg ATP concentration needed for half the maximal rate (Km) was 16.6 +/‐ 2 microM. The ATPase of fibres that had been stabilized by cross‐linking with 1‐ethyl‐3‐(3‐dimethyl‐aminopropyl)carbodiimide (EDC) was also investigated. Cross‐linking did not significantly affect the Vm or Km and these fibres proved useful for investigating the adequacy of the pyruvate kinase activity for regenerating hydrolysed ATP. Myofibrils were cross‐linked with EDC or glutaraldehyde to prevent shortening. Their ATPase properties were investigated: the values of Vm were 0.85 +/‐ 0.18 (mean +/‐ S.D.; n = 14) and 0.82 +/‐ 0.05 s‐1 (n = 6) and of Km were 18.0 +/‐ 2.8 and 12.4 +/‐ 2.4 microM respectively. The values of Vm and Km for EDC cross‐linked myofibrils were fairly insensitive to ionic strength, the Km decreasing 40% and the Vm increasing 50% for a change from 0.2 to 0.3 M. This slight dependence on ionic strength is considered in relation to the ionic strength dependence of the elementary rate constants of the actomyosin subfragment‐1 ATPase cycle.
Adaptation of mechanical properties of muscle to high force training in man.
Authors: Davies C T , Dooley P , McDonagh M J , White M J
Published: December 2014

The first dorsal interosseus muscle of the hand was trained for 8 weeks using eighty maximal 10 s voluntary isometric contractions per day. As a result of this training the maximal voluntary force increased by 33%, but electrically evoked tetanic tension increased by only 11%. In other subjects the muscle was trained using electrical stimulation at 60 Hz to evoke eighty maximal 10 s tetani per day for 8 weeks. This training produced no increase in maximum voluntary force. Our results show that the increase in maximal voluntary force under these conditions may be due to a change in the voluntary neural drive to the muscle.
Membrane currents underlying the cholinergic slow excitatory post‐synaptic potential in the rat sympathetic ganglion.
Authors: Brown D A , Selyanko A A
Published: December 2014

Non‐nicotinic slow synaptic currents were recorded from voltage‐clamped neurones in isolated rat superior cervical ganglia bathed in a solution containing d‐tubocurarine and (usually) 1 microM‐neostigmine. Three components of slow synaptic current could be detected following repetitive preganglionic stimulation: a net inward current resulting from inhibition of the voltage‐dependent outward K+ current IM; a net outward current associated with a fall in membrane conductance when IM was deactivated by membrane hyperpolarization or inhibited with external Ba2+ or internal Cs+; and an occasional late inward current associated with an increased membrane conductance. As a result, synaptic current amplitudes showed complex changes with changes in membrane potential. Both the inward current associated with IM inhibition and the outward current were enhanced by neostigmine and blocked by atropine or pirenzepine, and therefore resulted from activation of muscarinic receptors. In unclamped neurones, equivalent stimulation produced a membrane depolarization and induced or facilitated repetitive spike discharges. It is concluded that the principal synaptic response to muscarinic receptor activation is IM inhibition, leading to a net inward current and increased excitability, but that this response may be modified under certain circumstances by other synaptic currents.
Arterial pressure maintenance after haemorrhage in the pregnant rabbit.
Authors: Humphreys P W , Joels N
Published: December 2014

The changes in arterial blood pressure and cardiac output following rapid removal of 10% of the blood volume have been compared in non‐pregnant and pregnant anaesthetized rabbits close to term (28 or 29 days gestation). Immediately after the end of haemorrhage the fall in arterial pressure was greater in the pregnant group (27.7 vs. 17.8 mmHg), but as recovery proceeded the difference diminished and by the tenth minute the reduction from pre‐haemorrhage levels was similar in pregnant and non‐pregnant rabbits. Cardiac output fell by about 30% in both groups and increased only very slightly during the ensuing 10 min. 1 h after section of the carotid sinus and aortic nerves the haemorrhage was repeated. The fall in arterial pressure in the non‐pregnant rabbits was now more than doubled (47.2 vs. 17.8 mmHg). By contrast it was little changed in the pregnant rabbits (32.0 vs. 27.7 mmHg). It is concluded that pregnancy diminishes the role of the baroreceptor reflexes in the response of the rabbit to haemorrhage. Other mechanisms, such as an increased compliance of the resistance vessels and/or a more effective chemically mediated vasoconstriction, probably become of more importance to the maintenance of arterial pressure.
Optical studies of excitation‐contraction coupling in the early embryonic chick heart.
Authors: Hirota A , Kamino K , Komuro H , Sakai T , Yada T
Published: December 2014

Excitation‐contraction coupling at the onset of beating in the 9‐10‐somite embryonic chick heart was studied by means of an optical method together with a voltage‐sensitive merocyanine‐rhodanine dye. Spontaneous optical signals were recorded simultaneously from many areas of the embryonic heart, using a square photodiode matrix array. At time of initiation of the heartbeat, spontaneous optical signals consisting of two components were often detected. The first component (1st signal) is a dye‐related absorption change due to the action potential, and the second component (2nd signal) is a light scattering change due to contraction. When Ca2+ in the bathing solution was partly replaced by Mg2+, the peak size of both signals was reduced. The correlation between the 1st and 2nd signals corresponded to the relationship between excitation and contraction. The formation of excitation‐contraction coupling exhibited a regional non‐uniformity in the developing 9‐10‐somite embryonic hearts: contraction was first generated in the right ventricular region, and then the contractile area spread widely over the whole of the heart. The curves of the excitation‐related 1st signal vs. the contraction‐related 2nd signal obtained from different areas were not superimposable. Decoupling of excitation from contraction was produced by raising the Ca2+ concentration in the bathing solution, by lowering the Na2+ concentration or by inclusion of a Ca2+ ionophore (A23187). Replacement of the bathing solution with D2O or hypertonic solution also suppressed excitation‐contraction coupling. The results suggest that in the early embryonic initial beating chick heart, the contractile system is activated by Ca2+ influx across the sarcolemma accompanying the action potential, and that a Na+‐Ca2+ exchange mechanism participates in the relaxation phase of the heartbeat.
Slow depolarizing response from supporting cells in the goldfish saccule.
Authors: Furukawa T
Published: December 2014

Potentials were recorded intra‐ and extracellularly from the saccular macula of anaesthetized goldfish. Sound‐evoked responses recorded intracellularly consisted of positive microphonic potentials, nerve responses and a slow depolarizing potential. Of these three potentials, only the slow depolarization was peculiar to intracellular records. It rose and fell exponentially with time constants of about 30 and 200 ms, respectively. The amplitude of slow depolarization never exceeded 5‐6 mV, even when a loud sound was applied. It is argued that the impaled cell was a supporting cell and the slow depolarization presumably reflected a rise of extracellular K+ concentration, during sound stimulation. It was also shown that this nerve response, which was resistant to the action of tetrodotoxin, probably represented the excitatory post‐synaptic current that flows into the dendrites of afferent fibres. Efferent stimulation suppressed the nerve response.
Calcium release by noradrenaline from central sarcoplasmic reticulum in rabbit main pulmonary artery smooth muscle.
Authors: Kowarski D , Shuman H , Somlyo A P , Somlyo A V
Published: December 2014

The subcellular composition of relaxed and noradrenaline‐contracted rabbit main pulmonary artery smooth muscle cells was measured by electron probe X‐ray microanalysis of cryosections of rapidly frozen tissue. Some of the preparations were made permeable with saponin and exposed to a known free Ca ion concentration, rapidly frozen, freeze‐substituted, and also analysed by electron probe X‐ray microanalysis. 98% of intracellular K could be replaced by Rb. This was done to remove the K peak that partially overlaps the Ca peak in the X‐ray spectra. The final [Rb]i plus residual [K]i was not significantly different from the [K]i of normal tissue. The [Ca]i in Rb‐containing tissue was not significantly different from the [Ca]i in normal, K‐containing tissue. Non‐mitochondrial micro‐regions containing high [Ca] (up to 33 mmol/kg dry wt.) were found at sites 200 nm or more away from the plasma membrane. These micro‐regions also contained high [P]. We consider the identification of these regions containing high [Ca] as sarcoplasmic reticulum (s.r.), validated by: (a) conventional electron micrographs that show no other structures in main pulmonary artery smooth muscle in sufficient quantity and location to account for the frequency of these regions, (b) the previous localization of strontium, a functional calcium analogue, in the central s.r. in these smooth muscles (Somlyo & Somlyo, 1971 a), (c) the present demonstration that the central s.r. in this tissue can accumulate large amounts of calcium oxalate. The proportion of regions containing high [Ca] (greater than 12.0 mmol/kg dry wt.) was significantly higher in relaxed (35 of 330 measurements) than in the contracted (14 of 337) tissues (P less than 0.005), or 26 of 34 vs. 6 of 31 high [Ca] measurements in regions identified as s.r. through their high phosphorus content (P less than 0.006). This difference is thought to represent Ca release from the central s.r. There was no significant difference (P greater than 0.05) between the distributions of P in relaxed and contracted smooth muscle. The total cell [Ca]i in relaxed Rb‐containing tissue, measured with randomly positioned small probes (3.6 mmol/kg dry wt.), was the same as that measured with large defocused probes, indicating the validity of random sampling. A mathematical model was used to estimate the frequency of including s.r. (35 nm diameter and 5% of cell volume) by a randomly positioned electron probe (50 nm), because we could not visualize s.r. in the cryosections.(ABSTRACT TRUNCATED AT 400 WORDS)
Electric current flow in cell pairs isolated from adult rat hearts.
Authors: Metzger P , Weingart R
Published: December 2014

Cell pairs were isolated from ventricles of adult rat hearts so as to study cell‐to‐cell coupling. Both cells of each pair were impaled with micro‐electrodes connected to balanced bridge circuits. Rectangular current pulses were passed and the resulting voltage deflexions monitored. The data were analysed in terms of a delta configuration of three resistive elements, the resistances of the non‐junctional membrane of cell 1 and cell 2 (rm, 1 and rm, 2), and the resistance of the nexal membrane (rn). The nexal membrane resistance was found to be insensitive to voltage gradients across the non‐junctional membrane (range examined: ‐70 to ‐10 mV) and direction of current flow. The mean value of rn was 2.12 M omega ([K+]o = 12 mM). Taking into account morphological parameters, this corresponds to a specific nexal membrane resistance (Rn) of 0.1 omega cm2. Spontaneous uncoupling in which one cell remained polarized while the other one depolarized was never observed. The current‐voltage relationship of the non‐junctional membrane was found to be bell‐shaped. The specific resistance (Rm) at the resting membrane potential (approximately ‐50 mV) was 3.2 k omega cm2 ([K+]o = 12 mM). Comparative studies performed on single cells revealed a similar relationship Rm versus Vm. Rm at the resting membrane potential (Vm approximately ‐50 mV) was 2.5 k omega cm2 ([K+]o = 12 mM). The specific capacitance of the non‐junctional membrane (Cm) was determined from experiments on single cells. Cm was found to be independent of Vm (voltage range: ‐80 to 0 mV). The mean value of Cm was 1.66 microF/cm2 ([K+]o = 12 mM). For comparison, experiments on cell pairs and single cells were also carried out with [K+]o = 4 mM. The values obtained for Rn, Rm and Cm did not deviate significantly from those found with [K+]o = 12 mM.
Slow wave heterogeneity within the circular muscle of the canine gastric antrum.
Authors: Bauer A J , Reed J B , Sanders K M
Published: December 2014

A cross‐sectional preparation was designed in which multiple micro‐electrodes can be precisely positioned to impale smooth muscle cells anywhere from the serosa to the submucosa. Intracellular electrical recordings were obtained from gastric antral circular muscle cells from the myenteric plexus to the submucosa. The resting membrane potential changed linearly as a function of distance from the myenteric plexus to the submucosa. Slow wave upstroke dV/dt, upstroke potential amplitude, and plateau potential amplitude changed linearly as a function of distance from the myenteric plexus to the submucosa. When slow waves were recorded simultaneously from a circular cell near the myenteric plexus and from a cell near the submucosa, the event always occurred first in the cell near the myenteric plexus. Electrical differences did not appear to be caused by electrotonic decay of slow waves as they propagated through the circular muscle. Electrical differences could not be explained on the basis of differences in intrinsic neural activity or prostaglandin synthesis. Membrane polarization could not explain the differences in slow waves between myenteric and submucosal circular muscle cells. The conclusion of this paper is that fundamental differences exist between the excitability mechanisms and/or passive membrane properties of cells near the myenteric plexus and the submucosa. These differences might be manifest in different motor performance of these two muscle cell populations.
Vestibular control of neck muscles in acute and chronic hemilabyrinthectomized cats.
Authors: Dutia M B
Published: December 2014

Reflex activity evoked in neck extensor muscles by head movements in the sagittal plane (the sagittal vestibulocollic reflex (v.c.r.), Dutia & Hunter, 1985), was studied in decerebrate cats with acute or chronic loss of one vestibular labyrinth. After acute hemilabyrinthectomy, tonic electromyographic (e.m.g.) activity in the biventer cervicis muscle ipsilateral to the lesion was normal, while that in the contralateral muscle was abolished. Sinusoidal head movements in the sagittal plane (0.1‐5 Hz, 1‐10 deg peak to peak) caused reflex modulation of e.m.g. activity in the ipsilateral muscle, but did not evoke any response in the contralateral muscle. The phase (re head position) of the reflex response in the ipsilateral muscle was similar to that in a normal cat with intact labyrinths, while reflex gain was lowered by 2‐8 dB below its value before hemilabyrinthectomy. Removal of the remaining labyrinth in acutely hemilabyrinthectomized animals restored bilaterally symmetrical tonic e.m.g. activity in the neck extensors. There was no e.m.g. modulation during head movements after bilateral labyrinthectomy. In chronic hemilabyrinthectomized cats (four to seven weeks), tonic e.m.g. activity in the neck muscles on both lesioned and intact sides was similar to normal. The gain and phase of the sagittal v.c.r. were also normal over a wide range of frequencies of head movement on both lesioned and intact sides. Interruption of the medial longitudinal bundle approximately 1 mm rostral to the obex did not abolish the bilaterally symmetrical compensated reflex response in either muscle, indicating that the descending axons in the medial vestibulospinal tract are not essential in mediating the normal v.c.r. response in compensated animals.
Activation of the human diaphragm during maximal static efforts.
Authors: Gandevia S C , McKenzie D K
Published: December 2014

Voluntary activation of the human diaphragm was assessed in four subjects by interpolation of supramaximal stimuli (one to four shocks, interstimulus interval 20 ms) to one phrenic nerve during graded static inspiratory and expulsive efforts at functional residual capacity. There was an inverse relationship between the size of the voluntarily generated pressure and the size of the electrically evoked change in pressure. Each subject activated the diaphragm fully during brief (2‐5 s) maximal inspiratory and expulsive efforts, as judged by the failure of supramaximal stimuli to augment the voluntarily maintained pressure. During prolonged inspiratory efforts or following fatigue of the diaphragm produced by a sustained contraction each subject was able to activate the diaphragm maximally but did not do so on all occasions.
Patch‐clamp recordings of the light‐sensitive dark noise in retinal rods from the lizard and frog.
Authors: Bodoia R D , Detwiler P B
Published: December 2014

In cell‐attached recordings from rods in the intact lizard retina, light decreased a standing inward membrane current with a reversal potential approximately 60 mV more positive than the resting potential. The peak amplitude of saturating responses depended upon the area of recorded membrane and varied from cell to cell over approximately 100‐fold range. Small patches of membrane gave variable responses to identical moderately intense flashes. Whole‐cell voltage‐clamp recordings were obtained on isolated frog rods with intact ellipsoids. Peak whole‐cell photocurrent was related to flash intensity by a Michaelis equation with saturating response amplitudes ranging up to 30 pA in 0.1 mM‐Ca2+ Ringer solution. In darkness the steady‐state current‐voltage relation, determined with whole‐cell voltage clamp, showed outward rectification. Photocurrent had nearly constant amplitude between ‐80 and ‐10 mV, a mean reversal potential of +8 mV and recovered from flashes more slowly at positive holding potentials. Although it was not possible to resolve light‐sensitive single‐channel current events, power spectral analysis revealed both low‐ and high‐frequency components of the light‐sensitive noise in both cell‐attached and whole‐cell recordings. The low‐frequency component was described by the product of two Lorentzians using time constants derived from the kinetics of the dim flash response. The high‐frequency component of the light‐sensitive noise was described by a single Lorentzian with a half‐power frequency of 62 Hz in lizard and 212 Hz in frog. The half‐power frequency was not appreciably affected by steady illumination. The Lorentzian nature of the noise suggests that the light‐sensitive channel is a pore rather than a shuttle‐type carrier. In cell‐attached recordings the high‐frequency component declined monotonically with increasing light intensity, suggesting that less than one‐half of the channels are open in darkness. Furthermore, the ratio of the variance of the high‐frequency noise to the mean photocurrent was independent of light intensity. Changing external Ca2+ from 0.1 to 0.5 mM reduced the ratio from 19.7 to 9.0 fA without a significant effect on the cut‐off frequency of the noise. The results support the conclusion that the light‐sensitive pore is opened by an internal transmitter that acts as an agonist and that both open and closed states of the pore may be blocked by external Ca2+. The conductance of the light‐sensitive pore in the absence of external Ca2+ is estimated to be 1.25‐2 pS.(ABSTRACT TRUNCATED AT 400 WORDS)
Augmenting responses evoked in area 17 of the cat by intracortical axon collaterals of cortico‐geniculate cells.
Authors: Ferster D , Lindström S
Published: December 2014

Evoked potentials were recorded in the visual cortex of the cat after electrical stimulation of the lateral geniculate nucleus (l.g.n.). The primary response, mediated by geniculo‐cortical fibres, was depressed at stimulation frequencies above 7 Hz and replaced by a late potential, the incremental response, which gradually increased in amplitude with successive stimuli. The incremental response was a negative‐positive potential in the depth of the cortex with the negative component having maximal amplitude in layer 4. The response reversed polarity in layer 1 to become a positive‐negative potential at the surface. The latency of the negative component of the incremental response was about 3.5‐4 ms in layer 4, compared to about 1.5 and 2.5 ms for the mono‐ and disynaptic components of the primary response. The incremental response could only be evoked from the l.g.n. and the optic radiation, not from the optic tract, superior colliculus or other surrounding structures. Within the l.g.n., the effect was only evoked from stimulation sites in approximate retinotopic register with the recording site in the cortex. Low threshold points were found in the A laminae, completely overlapping with the low threshold points for the primary response. Thresholds increased steeply when the stimulation electrode was lowered into the C laminae. The incremental response could still be evoked ten days after the destruction of all cells in the l.g.n. complex by kainic acid. It is concluded that the described incremental response is identical to the augmenting response of Dempsey & Morison (1943) and is mediated by intracortical axon collaterals of antidromically activated cortico‐geniculate neurones.
Cholinergic agonists suppress a potassium current in freshly dissociated smooth muscle cells of the toad.
Authors: Sims S M , Singer J J , Walsh J V
Published: December 2014

Single micro‐electrode voltage‐clamp and current‐clamp techniques were used to study cholinergic responses in single freshly isolated gastric smooth muscle cells from the toad Bufo marinus. Acetylcholine (ACh) or muscarine caused membrane depolarization, which sometimes gave rise to action potentials and contractions. The agonist‐induced depolarization is due to the suppression of a voltage‐dependent K+ conductance, a conclusion based on the following observations. Depolarization was accompanied by an apparent membrane conductance decrease, seen as the increased size of voltage deflexions in response to constant current pulses. The conductance decrease was confirmed under voltage clamp, where current deflexions in response to constant voltage jumps were smaller in the presence of cholinergic agonists. Muscarine induced net inward currents at potentials positive to the K+ equilibrium potential (EK), and net outward currents at potentials negative to EK. In experiments where external K+ concentration ([K+]o) ranged from 20 to 90 mM the reversal potentials shifted 58 mV positive per tenfold elevation of [K+]o, as expected for a K+ current. The steady‐state current‐voltage relationship revealed that the K+ current inhibited by muscarine was larger at more positive potentials than expected from driving force considerations alone. Therefore, the underlying conductance suppressed by cholinergic agonists was voltage dependent, with almost complete deactivation at potentials more negative than approximately ‐70 mV and exhibiting a sigmoidal activation curve upon depolarization. The deactivation of this voltage‐dependent K+ conductance caused slow current relaxations to occur in response to hyperpolarizing voltage commands from depolarized holding potentials. In experiments where [K+]o ranged from 3 to 30 mM, these current relaxations reversed direction at potentials near EK and the reversal potential shifted 52 mV positive per tenfold elevation of [K+]o, indicating that K ions carry most of the charge. The current relaxations that occurred in response to hyperpolarizing voltage commands were suppressed by ACh, muscarine and oxotremorine. The effects of muscarine persisted in nominally Ca2+‐free solutions containing Mn2+. Ba2+ mimicked the effects of muscarinic agonists. Thus, isolated smooth muscle cells exhibit a K+ current resembling the M‐current of sympathetic and other neurones, which is reversibly suppressed by cholinergic agonists. The existence of a cholinergic K+ conductance decrease is of interest because it has not previously been demonstrated in smooth muscle.
A transient outward current in isolated cells from the crista terminalis of rabbit heart.
Authors: Giles W R , van Ginneken A C
Published: December 2014

Voltage‐clamp experiments were carried out with the objective of identifying and characterizing the time‐ and voltage‐dependent properties of a transient outward current recorded in single myocytes from the crista terminalis region of the rabbit heart. A collagenase enzymic dispersion procedure similar to that described by Desilets & Horackova (1982) was used to obtain these viable individual myocytes. Transmembrane ionic currents were recorded using a single micro‐electrode voltage‐clamp technique. In experiments aimed at studying a tetrodotoxin‐resistant transient inward current, (ICa); a transient outward current was consistently recorded following blockade of ICa with Cd2+ (5 X 10(‐4) M). The time and voltage dependence of the activation and inactivation of this current were measured. Its steady‐state inactivation curve spans the voltage range ‐70 to ‐10 mV, and it is activated between ‐20 and +10 mV. The reversal potential of this transient outward current is approximately ‐75 mV in [K+]O 5 mM, suggesting that it is carried mainly by K+. This transient outward current can be inhibited completely by external application of 4‐aminopyridine (4‐AP, 3 mM). The time‐ and voltage‐dependent properties, the reversal potential, and the sensitivity to 4‐AP of this transient outward current are all very similar to those of a transient outward current first identified in molluscan neurones. Hence, we have labelled it, IA. Selective inhibition of IA and knowledge of its voltage‐ and time‐dependent properties yield specific predictions concerning its role in the action potential of isolated crista terminalis cells. Consistent with these predictions, a decrease in stimulus rate is found to decrease the duration of the action potential and vice versa; and application of effective doses of 4‐AP results in a substantial lengthening of the action potential. These results are discussed in terms of the possible physiological role of IA in subsidiary or follower pace‐maker tissue, and the anatomical and physiological heterogeneity of the sino‐atrial node region of the rabbit heart.
Responses of precentral cells during cooling of post‐central cortex in conscious monkeys.
Authors: Brinkman J , Colebatch J G , Porter R , York D H
Published: December 2014

A cooling plate was implanted over the forelimb representation in area 2 of the post‐central region of cerebral cortex in two monkeys. Recordings were made of the discharges of thirty‐seven movement‐related neurones (thirty‐four precentral and three post‐central) in the forelimb motor representation of the cerebral cortex during active and passively imposed limb movements before, during and after cooling area 2 and local surrounding regions. Perfusion of the cooling plate with ice‐cooled water for 3‐5 min caused marked clumsiness of the conscious animal's forelimb movement and anaesthesia of the contralateral hand. Cooling of area 2 did not reduce the responses of area 4 cells to passive joint movements, nor did it alter the over‐all pattern of activity of these cells during self‐initiated lever pulling while that could still be performed. Cooling of area 2 did cause a significant increase in background cellular discharge in area 4 while the animal was at rest. Afferent impulses which are generated by passive joint movement and which have been shown to influence cells in area 4 of the conscious monkey at short latencies are probably not transmitted through cortico‐cortical connexions from area 2.
Calcium content and calcium exchange in dark‐adapted toad rods.
Authors: Fain G L , Schröder W H
Published: December 2014

We have used laser‐activated micro mass analysis (l.a.m.m.a.) and energy‐dispersive X‐ray analysis (e.d.x.) to measure Ca content and Ca movements in 'red' rod photoreceptors in the dark‐adapted retina of the toad, Bufo marinus. Measurements with both l.a.m.m.a. and e.d.x. show that intact rod outer segments contain 4‐5 mmol total Ca/l wet tissue volume, or 1‐2 Ca per rhodopsin. We could detect no significant variation in the total Ca as a function of distance across or up and down the outer segment. In the inner segment, Ca could be detected only within the mitochondria‐rich ellipsoid body, where the total Ca concentration was of the order of 100‐400 mumol/l wet tissue volume. To measure the exchange of Ca in outer segments from intact photoreceptors, we exposed the dark‐adapted retina to Ringer containing the stable isotope 44Ca. Since l.a.m.m.a. can measure separately the concentrations of each of the isotopes of the elements, and since native rods contain almost exclusively 40Ca, the increase in 44Ca and decrease in 40Ca could be used as a measure of Ca influx and efflux. Ca exchange in intact rod outer segments in darkness is very slow. The rate of accumulation of 44Ca was only 10(5) Ca/rod.s, or about 10% of the total outer segment Ca/h. This slow rate of exchange is apparently not the result of restricted movement of Ca across the plasma membrane. Ca exchange was also measured in outer segments which were either partially or entirely detached from the rest of the photoreceptor. In broken‐off outer segments, Ca exchange is faster than in the intact organelles, and in 1 h, half of the 44Ca exchanges for 40Ca. When the retina was incubated in Ringer for which all of the Na was substituted with Li or choline, there was an increase in the rate of 44Ca accumulation in intact outer segments, probably due to an inhibition of Na‐Ca counter transport across the plasma membrane. Our measurements indicate that the great majority of the Ca in the rod appears to be inaccessible to exchange under physiological conditions, probably because it is sequestered within the disks which in intact rods appear to be nearly impermeable to Ca in darkness.
Early events in development of electrical activity and contraction in embryonic rat heart assessed by optical recording.
Authors: Hirota A , Kamino K , Komuro H , Sakai T , Yada T
Published: December 2014

Spontaneous action potential and contraction in the early embryonic heart of the rat have been monitored optically using a voltage‐sensitive merocyanine‐rhodanine dye together with a multiple‐element photodiode matrix array, and the onset of rhythmical action‐potential activity in the early phases of rat cardiogenesis was conclusively determined for the first time. Spontaneous rhythmical action potentials were first generated in the central part of the embryonic heart at the middle period of the 3‐somite stage of development, at 91/2 days after copulation. Subsequently, contractions coupled with the action potential also appeared at the end of the 3‐somite stage. Usually, at the 3‐somite stage, spontaneous action signals were synchronized among the different areas in the heart. From this result, it is evident that the paired right and left cardiac primordia are fused completely at the time of initiation of spontaneous electrical activity. In the 3‐somite embryonic heart, excitatory waves were conducted radially over the heart, at a uniform rate (0.4‐0.8 mm/s), from the pace‐making area. However, the regional priority of pace‐making activity is not rigid but is flexible.
The effects of phosphodiesterase inhibitors and lanthanum ions on the light‐sensitive current of toad retinal rods.
Authors: Cervetto L , McNaughton P A
Published: December 2014

The light‐sensitive current of isolated toad rods was recorded using the method of Yau, McNaughton & Hodgkin (1981) and the effects of the phosphodiesterase inhibitor 3‐isobutyl‐1‐methylxanthine (IBMX) and of La ions were examined. IBMX caused an increase in the light‐sensitive current and a prolongation of the time course of the response. A small inward current which may reflect the operation of an Na‐Ca exchange pump was also increased in IBMX. With low doses of IBMX the time course of the dim flash response could be mapped onto that in Ringer solution by a linear transformation of the time scale. Light adaptation had opposite effects to those of IBMX on the time course of the dim flash response, and a steady background light could exactly neutralize the effects of IBMX on the time course. Light adaptation had the additional effect of strongly reducing the amplitude of the dim flash response. La ions caused a rapid inhibition (t1/2 less than 1 s) followed by slow inhibition (t1/2 approximately equal to 30 s) of the light‐sensitive current. In low [Ca] the rapid inhibition became more prominent, perhaps because of a competition between La and Ca for a blocking site near the light‐sensitive channel. The time constants of the falling phases of responses to both bright and dim flashes were slowed by La. The dim flash response could be fitted by a model in which a single time constant in the chain underlying the flash response is slowed by La. La reduced the rate of activation of light‐sensitive current in response to a reduction of external [Ca]. A concentration of La sufficient to block the activation of current in low [Ca] did not prevent the activation of current in IBMX. Light‐sensitive currents carried by Mn, Ca or Mg in the absence of Na and in the presence of IBMX were inhibited by La. An outward current observed in the absence of permeable ions was inhibited by La. The effects of La on the time course of the response and on the rate of activation of current when [Ca] is reduced are consistent with an inhibition of the Ca pump. La ions also have a high affinity for the light‐sensitive channel and can block current carried by another ion.
Effects of changes in excitability and intercellular coupling on synchronization in the rabbit sino‐atrial node.
Authors: Delmar M , Jalife J , Michaels D C
Published: December 2014

The mechanisms of synchronization between sino‐atrial pace‐maker cells were studied in biological preparations from rabbit hearts, and in computer simulations of the Hodgkin & Huxley type. For biological experiments, thin strips of sino‐atrial node were placed in a three‐compartment bath. The electrical properties of the tissue in the middle segment (the 'gap') were manipulated pharmacologically to alter electrical coupling and/or excitability of cells in that segment, and to study the patterns of interaction between two pace‐maker centres in the external segments. Superfusion of the gap segment with either verapamil (2 microM) or acetylcholine (10 microM) produced a loss of 1:1 synchrony (entrainment) of spontaneous discharges generated by the external pace‐makers but subharmonic (i.e. 3:2; 5:4; 9:8; etc.) entrainment was always maintained. When the gap segment was superfused with heptanol (3.5 mM), which is known to increase intercellular resistance, the pace‐maker centres in the external chambers beat independently of one another. Progressive loss of synchrony paralleled reductions in amplitude of electrotonic responses to current pulses applied across the gap. Gap superfusion with hypertonic Tyrode solution (600 mosM) produced a major reduction in the degree of synchronization between the external pace‐makers, even though the cells in the central compartment maintained their excitability. Under these conditions, as many as three independent pace‐maker centres, one in each chamber, coexisted in a given preparation. Using computerized simulations based on equations of time‐ and voltage‐dependent membrane currents, three 'cells', each capable of maintaining spontaneous activity, were connected in a linear array through ohmic resistances. When selective parameters (e.g. membrane conductances, coupling resistance) were modified appropriately, the mathematical simulations reproduced very closely the interaction patterns observed in the experimental preparations. Our results show that synchronization in the sinus node results from mutual interactions and entrainment between all the cells in this region. These interactions are of the kind expected for a population of coupled, self‐sustained oscillators, and are mediated through electrotonic propagation of current across low‐resistance junctions.
A fast‐activated inward calcium current in twitch muscle fibres of the frog (Rana montezume).
Authors: Cota G , Stefani E
Published: December 2014

Voltage‐clamp experiments were performed at 18 degrees C in intact twitch muscle fibres of the frog using the three micro‐electrode technique. Membrane currents were recorded in the presence of 120 mM‐tetraethylammonium‐methanesulphonate and 10 mM‐Ca2+. The recording solution was made hypertonic by adding 350 mM‐sucrose to avoid contraction. Two components of inward current in the absence of external Na+ were observed. Depolarization induced a fast‐activated inward current of small amplitude in addition to the well‐known slow, transient Ca2+ current (ICa,s). Both components of inward current persisted in the presence of tetrodotoxin. They practically disappeared on replacing external Ca2+ with Mg2+ and were blocked by millimolar additions of Cd2+ to the bath. Thus, the fast‐activated component of inward current was also carried by Ca2+ (ICa,f). Neither ICa,f nor ICa,s were reduced by 5 microM‐diltiazem. During 400 ms depolarizations ICa,f was detected at approximately ‐60 mV, 30 mV more negative than the membrane potentials at which ICa,s appeared. At about 0 mV the time constant for activation was 5 ms for ICa and 150 ms for ICa,s. ICa,f did not significantly decline during depolarizations up to 2s in duration at membrane potentials between ‐60 and ‐30 mV. ICa,f tended to disappear as a function of time on exposure to the hypertonic recording solution. Its maximum amplitude decreased from about ‐25 microA/cm2 during the first 5 min to about ‐5 microA/cm2 after 25 min while ICa.s remained practically unchanged (maximum peak amplitude of about ‐60 microA/cm2). These results indicate the existence of two types of voltage‐dependent CA2+ channels in intact muscle fibres. The kinetic properties of fast‐activated Ca2+ channels suggest that they significantly activate during a single twitch.