CARDIAC PHYSIOLOGY IN TUNAS .1. INVITRO PERFUSED HEART PREPARATIONS FROM YELLOWFIN AND SKIPJACK TUNAS

An in situ heart preparation perfused with oxygenated saline was used to examine cardiac performance at 25-degrees-C in yellowfin tuna (Thunnus albacares) and skipjack tuna (Katsuwonus pelamis). Heart rates (91-172 bpm in skipjack tuna and 101 - 157 bpm in yellowfin tuna) were comparable to those measured in vivo, and physiological stroke volumes were possible in yellowfin tuna with subambient filling pressures. In yellowfin tuna, maximum stroke volume and cardiac output were similar to the values obtained in vivo with spinally blocked animals; mean output pressures (up to 145 cmH2O, 1 cmH2O = 0.098 kPa) could exceed in vivo values without a major decrease in the resting cardiac output (homeometric regulation). In contrast, saline-perfused skipjack tuna hearts could not develop physiological output pressures without compromising cardiac output, with cardiac output being only 63% of the in vivo value at an output pressure near the in vivo ventral aortic pressure. The poor performance of the skipjack tuna heart is attributed to limited oxygen diffusion through the thicker walled ventricle. We conclude that the tuna heart is more dependent on its coronary circulation for normal function than the hearts of other fishes examined thus far. The coronary circulation was perfused with saline at various flow rates in isolated hearts from skipjack tuna to develop a pressure-flow relationship for the intact circulation. Coronary resistance reached a minimum of 24 cmH2O . min . g ventricular mass/mL at a flow rate of 2 mL/(min . g ventricular mass) with perfusion pressure about 40 cmH2O. In vivo coronary blood flow was estimated from the pressure-flow relationship as 0.67 mL/(min . g ventricular mass). Injections of adrenaline, noradrenaline, and phenylephrine into coronary circulation under constant flow conditions increased perfusion pressure, indicating the possibility of alpha-adrenergic vasoconstriction.