Cholinergic and adrenergic regulation of heart rate and ventral aortic pressure in two species of tropical tunas, Katsuwonus pelamis and Thunnus albacares

Tonic cholinergic and adrenergic control of heart rate and ventral aorta pressure was examined in two species of tropical tunas, the skipjack tuna (Katsuwonus pelamis) and the yellowfin tuna (Thunnus albacares). Unlike that of many other teleosts, the basal heart rate in spinally blocked tunas (at 25 degrees C) was dominated by a cholinergic rather than an adrenergic tonus. Infusion of atropine increased the heart rate by 143 and 58% in skipjack and yellowfin tunas, respectively. Ventral aortic pulse pressure was significantly decreased and mean ventral aortic pressure was slightly increased. Blockade of beta-adrenergic receptors with propranolol produced small (< 6%) decreases in both heart rate and mean ventral aortic pressure, indicating a low level of tonic beta-adrenergic stimulation. The small magnitude of the drop, however, suggests that tonic adrenergic regulation of heart rate and pressure is of less importance in tunas than in other teleosts, despite comparable circulating levels of adrenaline and noradrenaline. The alpha-adrenergic blocker phentolamine did not affect either heart rate or pressure. The intrinsic heart rate (i.e., the heart rate in the absence of cholinergic or adrenergic stimulation) was 180 beats/min in skipjack tuna and 119 beats/min in yellowfin tuna; these are the highest reported values for any teleost to date.