Temperature effects on Ca2+ cycling in scombrid cardiomyocytes: a phylogenetic comparison.

Specialisations in excitation-contraction coupling may have played an important role in the evolution of endothermy and high cardiac performance in scombrid fishes. We examined aspects of Ca²⁺ handling in cardiomyocytes from Pacific bonito (Sarda chiliensis), Pacific mackerel (Scomber japonicus), yellowfin tuna (Thunnus albacares) and Pacific bluefin tuna (Thunnus orientalis). The whole-cell voltage-clamp technique was used to measure the temperature sensitivity of the L-type Ca²⁺ channel current (I_Ca), density, and steady-state and maximal sarcoplasmic reticulum (SR) Ca²⁺ content (SSSR_load and maxSR_load). Current-voltage relations, peak I_Ca density and charge density of I_Ca were greatest in mackerel and yellowfin at all temperatures tested. I_Ca density and kinetics were temperature sensitive in all species studied, and the magnitude of this response was not related to the thermal preference of the species. SR_load was greater in atrial than in ventricular myocytes in the Pacific bluefin tuna, and in species that are more cold tolerant (bluefin tuna and mackerel). I_Ca and SR_load were particularly small in bonito, suggesting the NaiCa²⁺ exchanger plays a more pivotal role in Ca²⁺ entry into cardiomyocytes of this species. Our comparative approach reveals that the SR of cold-tolerant scombrid fishes has a greater capacity for Ca²⁺ storage. This specialisation may contribute to the temperature tolerance and thermal niche expansion of the bluefin tuna and mackerel. [ABSTRACT FROM AUTHOR]