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

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 degrees C, which is the body temperature of burbot for most of the year, and after myocytes were acutely warmed to 11 degrees 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. I-K1 conductance was small (39.2 +/- 5.4 pS pF(-1) at 4 degrees C and 71.4 +/- 1.7 pS pF(-1) at 11 degrees C) and I-Kr was large (199 +/- 27 pS pF(-1) at 4 degrees 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 degrees 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 (I-Ca, 0.81 +/- 0.13 pA pF(-1) at 4 degrees C, and 1.35 +/- 0.18 pA pF(-1) at 11 degrees C) and the charge (QCa, 0.24 +/- 0.043 pC pF(-1) at 4 degrees C and 0.21 +/- 0.034 pC pF(-1) at 11 degrees 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.