EFFECTS OF ANTIHYPERTENSIVE AGENTS ON ARTERIAL BARORECEPTOR REFLEXES IN CONSCIOUS RATS

The effects of antihypertensive treatment with four currently used agents (trichlormethiazide, atenolol, nicardipine, and enalapril) on the arterial baroreceptor reflex control of renal sympathetic nerve activity and heart rate were investigated in 45 conscious spontaneously hypertensive rats and 37 age-matched Wistar-Kyoto rats. Antihypertensive agents were administered for 2 weeks beginning at 8 weeks of age to treat and prevent the development of hypertension. Blood pressure was reduced to a similar level (-13+/-3 mm Hg, p<0.05) by each antihypertensive agent. Blood pressure, heart rate, and renal sympathetic nerve activity were recorded in the conscious state during phenylephrine and nitroglycerin ramp infusion. The gain in the baroreceptor reflex was determined from the maximum slope of logistic function curves. Untreated spontaneously hypertensive rats exhibited decreased sensitivity of reflex control of renal sympathetic nerve activity and heart rate (-1.78+/-0.07% of control/mm Hg and -2.16+/-0.05 beats per minute/mm Hg, respectively) compared with untreated Wistar-Kyoto rats (-3.62+/-0.18% of control/mm Hg, p<0.01, and -3.46+/-0.11 beats per minute/mm Hg, p<0.05, respectively). The gains in baroreceptor reflex control of renal sympathetic nerve activity and heart rate were greater in trichlormethiazide- (-2.99+/-0.15%/mm Hg and -3.05+/-0.13 beats per minute/mm Hg, respectively), atenolol- (-3.22+/-0.22%/mm Hg and -3.31+/-0.08 beats per minute/mm Hg, respectively), nicardipine- (-3.48+/-0.23%/mm Hg and -3.17+/-0.06 beats per minute/mm Hg, respectively), and enalapril- (-3.11+/-0.08%/mm Hg and -3.54+/-0.17 beats per minute/mm Hg, respectively) treated spontaneously hypertensive rats (all p<0.05) than in untreated spontaneously hypertensive rats. The antihypertensive agents affected neither blood pressure nor the baroreceptor reflex function curves in Wistar-Kyoto rats. These results indicate that attenuation of the development of hypertension by the antihypertensive agents is responsible for the restoration of impaired baroreceptor reflex control of renal sympathetic nerve activity and heart rate in conscious early hypertensive spontaneously hypertensive rats, although different antihypertensive mechanisms may mediate the blood pressure reduction.