Pressor responsiveness and cardiovascular reflex activity in spontaneously hypertensive and normotensive rates during vasopressin infusion

Arginine vasopressin (AVP), phenylephrine, and noradrenaline were infused intravenously into conscious and unrestrained adult spontaneously hypertensive (SH) rats and the changes in arterial pressure and heart rate were compared to those in Wistar--Kyoto (WKY) rats. The curve expressing the relationship of the increase in arterial pressure to the logarithm of the plasma concentration of AVP for SH rats was shifted to the left of that for WKY rats by a factor of four. In contrast, the dose--response (arterial pressure) curves for phenylephrine and noradrenaline in SH rats were displaced slightly to the right. In WKY rats, heart rate fell more for a given elevation of arterial pressure during infusions of AVP than during infusions of phenylephrine and noradrenaline; in SH rats, the heart rate response were less pronounced than in WKY rats, and the responses to vasopressin, phenylephrine, and noradrenaline were similar. The results are consistent with the interpretation that cardiovascular reflex activity is particularly enhanced during infusions of AVP in normotensive rats. The absence of this phenomenon in SH rats appears to contribute to the enhanced pressor activity of AVP in these rats.