Effects of temperature and calcium availability on ventricular myocardium from rainbow

We studied the mechanical and electrophysiological properties of ventricula r myocardium from rainbow trout (Oncorhynchus mykiss) in vitro at 4, 10, an d 18 degrees C from fish acclimated at 10 degrees C. Temperature alone did not significantly alter the contractile force of the myocardium, but the ti me to peak tension and time to 80% relaxation were prolonged at 4 degrees C and shortened at 18 degrees C. The duration of the action potential was al so prolonged at 4 degrees C and progressively shortened at higher temperatu res. An alteration of the stimulation frequency did not affect contraction amplitude at any temperature. Calcium influx via L-type calcium channels wa s increased by raising extracellular calcium concentration ([ Ca2+](o)) or including Bay K 8644 (Bay K) and isoproterenol in the bathing medium. These treatments significantly enhanced the contractile force at all temperature s. Calcium channel blockers had a reverse-negative inotropic effect. Unexpe ctedly, the duration of the action potential at 10 degrees C was shortened as [Ca2+](o) increased. However, Bay K prolonged the plateau phase at 4 deg rees C. Caffeine, which promotes the release of sarcoplasmic reticulum (SR) calcium, increased contractile force eightfold at all three temperatures, but the SR blocker ryanodine was only inhibitory at 4 degrees C. Our result s suggest that contractile force in ventricular myocardium from Oncorhynchu s mykiss is primarily regulated by sarcolemmal calcium influx and that vent ricular contractility is maintained during exposure to a wide range of temp eratures.