Among active fish species, tunas are renowned for high maximum heart rates and high cardiac output (Farrell, 1991). The greater contractile strength and maximum heart rate of tunas suggest that Ca2+ delivery via either trans-sarcolemmal or sarcoplasmic reticulum pathways may be enhanced in tuna myocytes compared with other fish. In this study, we investigated the properties of the L-type Ca2+ channel current (ICa) to test the hypothesis that Ca2+ influx would be larger and have faster kinetics in myocytes from bluefin tuna (Thunnus thynnus) compared with those of its sister taxa the Pacific mackerel (Scomber japonicus).
Bluefin tuna and Pacific mackerel were humanely killed by cervical dislocation, the hearts were removed and atrial and ventricular myocytes were isolated by enzymatic digestion. Myocytes were whole-cell voltage-clamped and the electrophysiological properties of ICa were examined at 20 ± 1 °C, which was the acclimation temperature of the fish. All values are given as means ± S.E.M. and differences between species were considered significant at P < 0.05 using Student’s unpaired t tests.
Atrial myocytes from bluefin tuna had a greater peak current amplitude (-4.8 ± 0.3 pA pF-1; n = 32 cells) compared with those from mackerel (-2.7 ± 0.5 pA pF-1; n = 12 cells). We observed faster fast inactivation kinetics in tuna atrial myocytes compared with mackerel atrial myocytes (tf = 25.9 ± 1.6 vs. 33.6 ± 3.9 ms, respectively) and the slope (k) of steady-state activation and inactivation was greater in tuna (activation, k = 6.19 ± 0.1; inactivation, k = 5.29 ± 0.3) compared with mackerel (activation, k = 7.69 ± 0.7; inactivation, k = 9.31 ± 0.5).
In contrast to atrial myocytes, ventricular myocytes did not differ between species in terms of current density and inactivation kinetics. However, ICa for tuna ventricular myocytes activated at significantly more depolarized voltages (tuna, -10.6 ± 0.6 mV, n = 19; mackerel, -14.3 ± 1.0 mV, n = 22).
Our results indicate that atrial myocytes of bluefin tuna have significantly enhanced Ca2+ flux pathways in comparison with those of Pacific mackerel. This supports the hypothesis that distinct differences do exist between bluefin tuna and Pacific mackerel, which may be associated with the elevated maximal heart rates observed in tuna. However, further studies are necessary to confirm this possibility, as well as to explain why ventricle myocytes did not show similar differences.
This research is supported by NSERC to A.P.F. and H.A.S. and the Monterey Bay Aquarium Foundation to B.A.B.
All procedures accord with current National and local guidelines.