Sarcolemmal ion currents and sarcoplasmic reticulum Ca2+content in ventricular myocytes from the cold stenothermic fish, the burbot(Lota lota)

Abstract

SUMMARY: The burbot (Lota lota) is a cold stenothermic fish species whose heart is adapted to function in the cold. In this study we use whole-cell voltage-clamp techniques to characterize the electrophysiological properties of burbot ventricular myocytes and to test the hypothesis that changes in membrane currents and intracellular Ca2+ cycling associated cold-acclimation in other fish species are routine for stenothermic cold-adapted species. Experiments were performed at 4°C, which is the body temperature of burbot for most of the year, and after myocytes were acutely warmed to 11°C, which is in the upper range of temperatures experienced by burbot in nature. Results on K+ channels support our hypothesis as the relative density of K-channel conductances in the burbot heart are similar to those found for cold-acclimated cold-active fish species. IK1 conductance was small (39.2±5.4 pS pF-1 at 4°C and 71.4±1.7 pS pF-1 at 11°C)and IKr was large (199±27 pS pF-1 at 4°C and 320.3±8 pS pF-1 at 11°C) in burbot ventricular myocytes. We found high Na+-Ca2+ exchange(NCX) activity (35.9±6.3 pS pF-1 at 4°C and 58.6±8.4 pS pF-1 at 11°C between -40 and 20 mV),suggesting that it may be the primary pathway for sarcolemmal (SL)Ca2+ influx in this species. In contrast, the density(ICa, 0.81±0.13 pA pF-1 at 4°C, and 1.35±0.18 pA pF-1 at 11°C) and the charge(QCa, 0.24±0.043 pC pF-1 at 4°C and 0.21±0.034 pC pF-1 at 11°C) carried by the l-type Ca2+ current was small. Our results on sarcolemmal ion currents in burbot ventricular myocytes suggest that cold stenothermy and compensative cold-acclimation involve many of the same subcellular adaptations that culminate in enhanced excitability in the cold.