Cardiovascular responses to hypoxia in the Adriatic sturgeon (Acipenser naccarii)

The in vivo cardiovascular responses to hypoxia, and the intrinsic function al characteristics of the heart in vitro, were determined, and compared, in the Adriatic sturgeon (Acipenser naccarii). During exposure to hypoxia in vivo, blood oxygen content (Cao,) declined as water O-2 partial pressure (P wo(2)) was reduced, despite an increase in haematocrit. The main cardiovasc ular response was a reduction in dorsal aortic blood pressure, with a sligh t bradycardia, while cardiac output remained constant. Reduced oxygen conte nt of the perfusate had significant inhibitory effects on the intrinsic per formance of the heart in vitro, causing a reduction in the heart rate; a re duction in the sensitivity of responses to increased preload (Frank-Starlin g response), and a more rapid decline in power output and stroke volume whe n afterload was increased. Overall, the in vitro results suggest that hypox ia depresses the contractility of the heart (i.e. its inotropic responses). The reduction in dorsal aortic pressure in vivo may, therefore, counteract the depressive effects of hypoxia on heart contractility, and thereby avoi d a hypoxic depression of cardiac output.