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Dietary salt loading produces baroreflex impairment and mild hypertension in rats.

Mild hypertension with reduced chronotropic baroreflex sensitivity to phenylephrine occurred in Sprague-Dawley rats fed a high-salt diet for 5 wk. Progressive elevation of systolic and mean pressures, detected initially by indirect tail-cuff measurement, was later verified by direct recording of phasic pressures from indwelling aortic catheters in the same rats. Reflex bradycardia during pressor responses to phenylephrine was consistently less pronounced in salt-loaded than in control rats, whether tested while rats were awake or anesthetized. However, attendant decreases in renal nerve activity were not appreciably altered. Neither central nor carotid baroreceptor mechanisms were considered likely but aortic baroreceptors must have somehow been depressed because increases in afferent aortic nerve activity elicited during intravenous infusion of phenylephrine were invariably smaller in salt-loaded than in control rats. Whatever the underlying mechanisms may be, our results show that when hypertension develops during dietary salt loading, baroreflex chronotropic responses are selectively inhibited while attendant decreases in renal nerve activity are preserved.