L-type Ca2+ current in fish cardiac myocytes: effects of thermal acclimation and beta-adrenergic stimulation.

A patch-clamp analysis of L-type Ca2+ current in ventricular myocytes of cold- and warm-acclimated rainbow trout (Oncorhynchus mykiss) and crucian carp (Carassius carassius) hearts was performed. Trout were acclimated at 4 and 17 degrees C and carp at 4 and 24 degrees C for a minimum of 4 weeks. Ventricular myocytes were isolated by enzymatic dissociation using collagenase and trypsin. Marked species-specific differences were noted in Ca2+ current density and its ss-adrenergic regulation. The density of basal Ca2+ current in crucian carp (6.9-7.4 pA pF-1) was almost double that of trout (4.2-4.5 pA pF-1) ventricular myocytes. Maximal beta-adrenergic stimulation increased Ca2+ current by approximately 2.3-fold in trout but by only 1.4-fold in crucian carp, so that Ca2+ current densities in the presence of 10 micromol l-1 isoprenaline were almost equal in trout (8.6-10.5 pA pF-1) and carp (9.6-10.4 pA pF-1) cardiac cells. Direct activation of adenylate cyclase by forskolin (10 micromol l-1) was also associated with similar interspecies differences in the stimulation of Ca2+ current. Thermal acclimation did not change either the density or the kinetics of L-type Ca2+ current in crucian carp ventricular myocytes. In trout cardiac cells, thermal acclimation had no effects on the density of Ca2+ current, but the rate of current inactivation was accelerated after acclimation to cold temperature. As a consequence of faster current decay, the contribution of sarcolemmal Ca2+ current to total cellular [Ca2+] was smaller in cold-acclimated than in warm-acclimated trout. The responses of Ca2+ current to maximal beta-adrenergic stimulation by isoprenaline or to direct activation of adenylate cyclase by forskolin were not changed by thermal acclimation in either species. It is concluded (1) that the density of sarcolemmal Ca2+ current is not increased after acclimation to cold, (2) that sarcolemmal Ca2+ influx through L-type Ca2+ channels can make a significant contribution to contractile [Ca2+] in both teleost species studied and (3) that ss-adrenergic stimulation of Ca2+ current is more important in modulating cardiac contractility in trout than in carp.