CHOLINERGIC AND ADRENERGIC REGULATION OF HEART-RATE AND VENTRAL AORTIC PRESSURE IN 2 SPECIES OF TROPICAL TUNAS, KATSUWONUS-PELAMIS AND THUNNUS-ALBACARES

Tonic cholinergic and adrenergic control of heart rate and ventral aor ta pressure was examined in two species of tropical tunas, the skipjac k 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 ra te by 143 and 58% in skipjack and yellowfin tunas, respectively. Ventr al aortic pulse pressure was significantly decreased and mean ventral aortic pressure was slightly increased. Blockade of beta-adrenergic re ceptors with propranolol produced small (< 6%) decreases in both heart rate and mean ventral aortic pressure, indicating a low level of toni c beta-adrenergic stimulation. The small magnitude of the drop, howeve r, suggests that tonic adrenergic regulation of heart rate and pressur e is of less importance in tunas than in other teleosts, despite compa rable circulating levels of adrenaline and noradrenaline. The alpha-ad renergic blocker phentolamine did not affect either heart rate or pres sure. 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 report ed values for any teleost to date.