Extracellular Ca2+, force, and energy state in cardiac tissue of rainbow trout

The importance of ATP and phosphocreatine (PCr) concentrations for myocardial force development was examined in electrically paced ventricular strips from rainbow trout. Three metabolic situations were studied involving either an aerobic block (N2, 5 mM NaCN), both an aerobic and a glycolytic block (1 mM iodoacetate), or no metabolic inhibitors. Increasing extracellular Ca2+ from 1.25 to 5 mM enhanced twitch-force development in all of these situations but caused no change in resting force or ATP and PCr content, except for the noninhibited preparations for which PCr increased. Anaerobiosis caused a decrease in the PCr concentration together with a fall in twitch force and an increase in resting force. Notably the changes in the contractile system associated with a given reduction in PCr were smaller in the absence than in the presence of iodoacetate. The results show that an increased Ca2+ availability stimulates the twitch-force development also at markedly lowered levels of high-energy phosphates. A maintained Ca2+ regulation appears to be one important reason for this. Furthermore, glycolysis seems to protect contractility in a way not reflected in the level of high-energy phosphates.