Effects of temperature and calcium availability on ventricular myocardium from rainbow trout

Abstract

We studied the mechanical and electrophysiological properties of ventricular myocardium from rainbow trout ( Oncorhynchus mykiss) in vitro at 4, 10, and 18°C from fish acclimated at 10°C. Temperature alone did not significantly alter the contractile force of the myocardium, but the time to peak tension and time to 80% relaxation were prolonged at 4°C and shortened at 18°C. The duration of the action potential was also prolonged at 4°C and progressively shortened at higher temperatures. An alteration of the stimulation frequency did not affect contraction amplitude at any temperature. Calcium influx via L-type calcium channels was 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 temperatures. Calcium channel blockers had a reverse-negative inotropic effect. Unexpectedly, the duration of the action potential at 10°C was shortened as [Ca2+]o increased. However, Bay K prolonged the plateau phase at 4°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°C. Our results suggest that contractile force in ventricular myocardium from Oncorhynchus mykiss is primarily regulated by sarcolemmal calcium influx and that ventricular contractility is maintained during exposure to a wide range of temperatures.