AGE-RELATED WEAKENING OF BAROREFLEX-MEDIATED SYMPATHETIC ACTIVITY IN SPONTANEOUSLY HYPERTENSIVE RATS IN RESPONSE TO BLOOD-PRESSURE REDUCTION

Nicardipine, a dihydropyridine type calcium channel blocker, was infus ed into 4-, 6-, and 23-wk-old spontaneously hypertensive(SH) and age-m atched normotensive Wistar-Kyoto (WKY) rats (under sodium thiobutabarb ital anesthesia and ventilation, n = 4) through the left femoral vein, resulting in the reduction of blood pressure. In each rat, mean arter ial blood pressure, heart rate, and the concentration of plasma catech olamines (CAs), norepinephrine (NE), and epinephrine (E) were concomit antly determined, and the correlations between these three variables w ere studied. During the infusion of nicardipine, the plasma concentrat ion of CAs was measured with an automatic detection system in blood sa mples collected from the right femoral artery of each rat. The reducti on in blood pressure induced by nicardipine brought about an increase in plasma CA levels. The blood pressure correlated well with the logar ithm of plasma NE or E concentration according to the formula Y= -a lo g (X) + m (Y, blood pressure; X, concentration of plasma NE or E; a, s lope; and m, intercept). The slopes las) of 6-wk-old and 23-wk-old SE rats were significantly greater than those of aged-matched WKY rats, m eaning that the increment in plasma CAs in response to a decrease in b lood pressure was smaller in SH than in WKY rats of similar ages. Howe ver, no significant differences were found between the as of 4-wk-old SH and WKY rats. We conclude that the increment in the baroreflex-medi ated sympathetic activity in response to a drop in blood pressure indu ced by nicardipine is similar or greater in prehypertensive SH than in normotensive WKY 4-wk-old rats, while the increment becomes smaller i n SH rats with the onset of hypertension (6-wk-old rats), and is much less in fully hypertensive adult (23-wk-old) SH rats than in age-match ed WKY rats. On the basis of these findings and previous data obtained by neurography, we conclude that plasma CAs can be used to evaluate b aroreflex-mediated sympathetic activity countering the blood pressure reduction caused by calcium antagonists.