[Role of the vagus nerve in reflex tachycardia evoked by arterial hypotension in the non-anesthetized dog].

The intravenous rapid-injection of nitroglycerin in the resting conscious dog induced arterial hypotension followed by reflex tachycardia. The experiments were performed before and after cold vagal blockade. When vagus nerve was blocked hypotension was more pronounced than in control conditions, but the rise of heart rate was lower and occurred later. This result suggests that, in the conscious dog, hypotension induced reflex tachycardia is due to a decrease in vagal tone, as well as to an increase in sympathetic activity. It is conceivable that the decrease in vagal tone occurs earlier than the sympathetic activity increase. Therefore the short latency reflex inhibition of vagal tone, through early positive inotropic effects, probably plays a relevant role in buffering rapid decreases in arterial pressure.