Electrophysiological properties of the L-type Ca2+current in cardiomyocytes from bluefin tuna and Pacific mackerel

Abstract

Tunas are capable of exceptionally high maximum metabolic rates; such capability requires rapid delivery of oxygen and metabolic substrate to the tissues. This requirement is met, in part, by exceptionally high maximum cardiac outputs, opening the possibility that myocardial Ca²⁺delivery is enhanced in myocytes from tuna compared with those from other fish. In this study, we investigated the electrophysiological properties of the cardiac L-type Ca²⁺channel current ( I_Ca) to test the hypothesis that Ca²⁺influx would be larger and have faster kinetics in cardiomyocytes from Pacific bluefin tuna ( Thunnus orientalis) than in those from its sister taxon, the Pacific mackerel ( Scomber japonicus). In accordance with this hypothesis, I_Cain atrial myocytes from bluefin tuna had significantly greater peak current amplitudes and faster fast inactivation kinetics (-4.4 ± 0.2 pA/pF and 25.9 ± 1.6 ms, respectively) than those from mackerel (-2.7 ± 0.5 pA/pF and 32.3 ± 3.8 ms, respectively). Steady-state activation, inactivation, and recovery from inactivation were also faster in atrial myocytes from tuna than from mackerel. In ventricular myocytes, current amplitude and activation and inactivation rates were similar in both species but elevated compared with those of other teleosts (Vornanen M. Am J Physiol Regul Integr Comp Physiol 272: R1432-R1440, 1997). These results indicate enhanced I_Cain atrial myocytes from bluefin tuna compared with Pacific mackerel; this enhanced I_Camay be associated with elevated cardiac performance, because I_Cadelivers the majority of Ca²⁺involved in excitation-contraction coupling in most fish hearts. Similarly, I_Cais enhanced in the ventricle of both species compared with other teleosts and may play a role in the robust cardiac performance of fishes of the family Scombridae.