Patterns of heart rate responses to hydralazine in normotensive and hypertensive rats

ABSTRACT

Hydralazine (H) induces hypotension acompanied by cardiac stimulation due to activation of ther arterial baroreflex. Both clinical and experimental observations suggest, however, that in certain conditions H hypotension can be accompanied by unchanged or even depressed cardiac performance. The present study determined whether varying petterns of heart rate responses could be detected in large populations of conscious normotensive (n=61) and renal hypertensive (n = 59) rats receiving a single dose of H. These patterns were compared with those of normotensive pentobarbital-anesthetized rats (n = 43). In the three groups, hypotension was accompanied by either tachycardia, unchanged heart rate or bradycardia. Tachycardia was found in 52 percent of normotensive conscious rats, in 51 percent of hypertensives and in only 14 percent of anesthetized animals. Heart rate did not change in 26, 35 and 23 percent, while bradycardia was detected in 22, 14 and 63 percent, respectively . These results were explained by postulating the initiation by H of two reflexes with opposite effects 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 relative sensitivity in a given animal