The present study examined the time course of changes in baroreceptor reflex function by means of sigmoidal curvefitting analysis in conscious, unrestrained renovascular one-kidney, one clip (1K1C) rats at 1,3,7,15, 30, and 60 days after renal artery clipping. The reflex heart rate responses were elicited by alternate intravenous bolus injections of phenylephrine (change, +5 to +50 mm Hg) and sodium nitroprusside (change, -5 to -50 mm Hg). Atropine methylnitrate and atenolol were given to evaluate the responses mediated by the cardiac sympathetic or vagal component, respectively. The average baroreceptor reflex gain (sensitivity) decreased progressively (day 1, 3.35±0.3 beats per minute [bpm] per millimeter of mercury), reaching a maximal attenuation in the 30-day 1K1C group (1.83±0.5 bpm/mm Hg) compared with sham rats (approximately 4.60 bpm/mm Hg).The data showed a decreased vagal activity contributing to the attenuation of the baroreceptor gain only in the 30-day 1K1C group. In contrast, the cardiac sympathetic component of the baroreceptor reflex was significantly decreased in all 1K1C groups (from 2.10±0.4 to 0.50±0.2 bpm/mm Hg) compared with the respective sham groups (from 3.80±0.3 to 3.10±0.4 bpm/mm Hg). These results suggest that a reduced contribution of the sympathetic component to the baroreceptor heart rate reflex may be the main cause of the progressive attenuation of the baroreceptor reflex sensitivity observed in conscious 1K1C hypertensive rats. Key Words • hypertension, renovascular • pressoreceptors • blood pressure • heart rate T he moment-to-moment regulation of the function of the heart and blood vessels is achieved through reflex action that detects and corrects changes in arterial pressure. It is now widely acknowledged that the baroreceptor heart rate (HR) reflex activity is diminished in animals with experimentally induced hypertension 1 -4 and in patients with essential hypertension. 57 In hypertensive animals, studies using renovascular models 489 have contributed to the understanding of the baroreceptor reflex function in hypertensive diseases. 12 In a previous study, 13 it was observed that renovascular hypertension is accompanied by an exaggerated cardiac sympathetic tone and a transitory reduction of vagal tone. However, to our knowledge there are no long-term studies analyzing the changes in baroreceptor reflex function during the different phases of one-kidney, one clip (1K1C) hypertension in conscious rats and the relative contribution of the sympathetic and vagal components to the baroreceptor reflex gain.Several methods have been used to assess the baroreceptor HR reflex in normotensive and hypertensive animals and humans. However, the relation between arterial pressure and HR is described satisfactorily by a sigmoidal logistic function 1415 rather than a linear function, as has been commonly assumed. Head and McCarty,16 using the method of sigmoidal-fitting baroreceptor curve analysis, showed for the first time in conscious rats that the average gain can be estima...
