PATTERNS OF HEART-RATE RESPONSES TO HYDRALAZINE IN NORMOTENSIVE AND HYPERTENSIVE RATS

Hydralazine (H) induces hypotension accompanied by cardiac stimulation due to activation of the arterial baroreflex. Both clinical and exper imental observations suggest, however, that in certain conditions H hy potension can be accompanied by unchanged or even depressed cardiac pe rformance. The present study determined whether varying patterns of he art rate responses could be detected in large populations of conscious normotensive (n = 61) and renal hypertensive (n = 59) rats receiving a single dose of II,These patterns were compared with those of normote nsive pentobarbital-anesthetized rats (n = 43). In the three groups, h ypotension was accompanied by either tachycardia, unchanged heart rate or bradycardia. Tachycardia was found in 52% of normotensive consciou s rats, in 51% of hypertensives and in only 14% of anesthetized animal s. Heart rate did not change in 26, 35 and 23%, while bradycardia was detected in 22, 14 and 63%, respectively. These results were explained by postulating the initiation by H of two reflexes with opposite effe cts on heart rate: the arterial baroreflex producing tachycardia and a cardiac mechanoreceptor reflex producing bradycardia. These reactions would compete with each other, with results depending on their relati ve sensitivity in a given animal.