Baroreflex control of heart interval in conscious renal hypertensive dogs

The purpose of this study is to determine if baroreflex control of heart interval is abnormal in conscious dogs with renal hypertension. Changes in heart interval in response to transient nitroglycerin-induced decreases in arterial pressure and transient phenylephrine-induced increases in arterial pressure were determined in nine normotensive [mean arterial pressure 92 +/- 4 (SE) mmHg] and nine renal hypertensive conscious dogs (mean arterial pressure 139 +/- 10 mmHg). Data were acquired before and after beta-adrenergic blockade with 2 mg/kg iv propranolol and before and after parasympathetic blockade with 0.5 mg/kg iv atropine. Control heart rates for the normotensive and hypertensive dogs were not different (91 +/- 4 and 93 +/- 7 beats/min, respectively). Before autonomic blockade, the responses of normotensive and hypertensive dogs to nitroglycerin were not different. However, prolongation of the heart interval in response to phenylephrine in hypertensive dogs was significantly less than in normotensive dogs. In both groups, atropine nearly abolished the decrease in heart interval in response to nitroglycerin and the increase in heart interval in response to phenylephrine, although there were small but significant residual responses. Propranolol was without significant effect on heart interval responses to nitroglycerin and phenylephrine. We conclude that the baroreflex control of heart interval during transient decreases in arterial pressure is preserved in hypertensive dogs and is mediated mainly by decreases in parasympathetic outflow to the heart. In contrast, baroreflex control of heart interval during transient increases in arterial pressure is impaired in hypertension and is mediated mainly by activation of parasympathetic cholinergic mechanisms.