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

Click for Abstract : 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 (ICa), density, and steady-state and maximal sarcoplasmic reticulum (SR) Ca2+�content (ssSRload�and maxSRload). Current�voltage relations, peak ICa�density and charge density of�ICa�were greatest in mackerel and yellowfin at all temperatures tested.�ICa�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. SRload�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).�ICa�and SRload�were particularly small in bonito, suggesting the Na+/Ca2+�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.