EFFECTS OF ACIDOSIS ON MYOCARDIAL CONTRACTILITY AND METABOLISM*
Authors: Williamson John R. , Safer Brian , Rich Terrell , Schaffer Stephen , Kobayashi Kumpei 
Published: August 2010

The effects of increased H+ concentration and the competition between H+ and Ca2+ on cardiac contractile function and metabolism have been investigated using the perfused rat heart. A working heart preparation was established by cannulating the aorta and left atrium. Fluid ejection from the left ventricle passed into a small closed air space and escaped through the coronary circulation, thereby allowing a minimum dead space between changes of perfusion fluid. Respiratory acidosis (high pCO2,) to pH 6.6 produced a rapid fall of left ventricular pressure with a half time of 5 sec. This effect could be fully counteracted by an increase of the Ca2+ concentration in the perfusion fluid. Calcium titration curves against left ventricular pressure are shown illustrating a shift of the curves towards higher Ca2+ concentrations with decreased pH or verapamil addition and a shift towards low Ca2+ concentrations with epinephrine. In contrast to effects obtained with respiratory acidosis, an extracellular pH of 6.6 induced by metabolic acidosis (low HCO3−) or artificial buffers caused a small and much slower decline of left ventricular pressure development. Under the latter conditions, intracellular pH decreased much less than with respiratory acidosis. Studies with isolated cardiac sarcolemma showed that both high and low affinity Ca2+ binding was inhibited at pH 6.6 relative to pH 7.4. Verapamil inhibited only low affinity Ca2+ binding. From these and other data, it is concluded that increased extracellular H+ in the presence of high pCO2 causes a rapid fall of intra cellular pH and exerts a negative inotropic effect primarily by competing with Ca2+ for intracellular calcium binding sites, although extracellular sites are also involved. It is proposed that H+ interferes with that phase of the excitation‐contraction coupling process whereby activator calcium under the control of the plateau phase of the action potential causes regenerative release of calcium from intracellular stores.

Hearts perfused with 5 mM glucose, 5 mM acetate and 5 × 10‐3 unitslml of insulin were rapidly frozen different times up to 3 min after the pH 7.4 to 6.6 transition with respiratory acidosis, and analyzed for metabolic intermediates. At the half‐time for decreased left ventricular pressure development (5 sec), ATP and creatine phosphate levels increased while ADP levels decreased. Subsequently, creatine phosphate and ATP levels decreased while ADP levels increased, indicating an imbalance between the rates of production and utilization of ATP. These data show that a fall in the rate of energy production is not responsible for the initial negative inotropic effect of Hi. Glycolytic flux, oxygen uptake and citric acid cycle activity decreased rapidly with pH 6.6 perfusion. Acetyl‐CoA levels increased linearly during the first minute while oxalacetate and α‐ketoglutarate levels rapidly decreased. Citrate, malate, aspartate and alanine levels initially increased and subsequently decreased, with malate and alanine levels finally rising again. Changes of glutamate levels were opposite to those of aspartate. Control of citric acid cycle activity is discussed in relation to the coordination of inhibitory interactions at the sites of citrate synthase, isocitrate dehydrogenase and α‐ketoglutarate dehydrogenase. During the transition state, flux through the different steps of the cycle is non‐uniform due mainly to changes of tissue aspartate levels, but the predominant initial interactions appear to be mediated by increases of the NADHNAD and ATP/ADP ratios resulting from the work decrease and resultant decreased rate of energy utilization. During this phase, cycle flux is regulated mainly by oxalacetate availability to citrate synthase. Within about 30 sec after the pH transition, additional interactions due to an increased intramitochondrial H+ concentration cause further inhibitions at citrate synthase and the dehydrogenase sites resulting in a fall of the adenine nucleotide energy balance in the heart.

In further experiments, metabolic and functional changes were measured after Ca2+ addition to hearts perfused under conditions of respiratory acidosis. With glucose, insulin and acetate as exogenous substrates, depressed work performance, respiratory activity and glycolytic flux observed at pH 6.67 were almost fully restored by increased extracellular Ca2+ and remained stable even though intracellular pH, ATP and creatine phosphate levels remained depressed. With glucose alone as substrate in the presence of insulin, cardiac work output was increased for only 5 to 8 min by Ca2+ addition under similar conditions, after which a spontaneous decrease of systolic left ventricular pressure rapidly developed which was associated with irreversible failure, severe hypoxia, depletion of ATP and creatine phosphate and loss of total adenine nucleotides. Hypotheses are discussed concerning the causative factors of irreversible cardiac failure and the above model is presented in the light of its possible usefulness to determine the biochemical basis of irreversible failure.