BARORECEPTOR REFLEX CONTROL OF HEART-RATE DURING DEVELOPMENT OF COARCTATION HYPERTENSION

To study whether resetting of the baroreceptors is accompanied by normal reflex activity to the heart, we analyzed time-course changes of pressure, heart rate, baroreceptor reflex sensitivity, and plasma renin activity during the development of coarctation hypertension. Baseline heart rate was measured daily, and plasma renin activity and reflex changes in heart rate (bolus injections of either phenylephrine or nitroprusside, 0.2-25.6-mu-g/kg i.v.) were recorded at different times in coarcted and sham-coarcted rats. Hypertension was stable (approximately 39% from baseline mean arterial pressure of 112 +/- 3 mm Hg), whereas heart rate (333 +/- 4 beats per minute) showed a biphasic behavior: bradycardia at 6 hours (264 +/- 7 beats per minute) and tachycardia at 5 days (418 +/- 14 beats per minute). On day 10 of hypertension, heart rate was normal. Plasma renin activity was markedly increased only after 6 hours (4.9 times). Reflex bradycardia exhibited a progressive impairment: The slopes of the regression lines between changes in heart rate and changes in mean arterial pressure were not significantly reduced at 6 and 48 hours when the resetting was in development (changes of 17% and 28% from a control of -1.89 +/- 0.20 beats per minute/mm Hg) but were significantly depressed after the resetting had been completed (-51% and -56% at 5 and 10 days, respectively). Reflex tachycardia was significantly reduced in all periods studied (75%, 78%, 52%, and 61% at 6 hours and 2, 5, and 10 days, respectively; -3.92 +/- 0.42 beats per minute/mm Hg in sham rats). We conclude that reflex bradycardia but not reflex tachycardia is preserved at early coarctation, when the resetting of the baroreceptors is developing. At established hypertension, when the complete resetting is present, the marked impairment of both reflects an abnormality in the central mediation of the reflex, possibly triggered by angiotensin II.