Impaired Sympathetic Baroreflexes in Prehypertensive Dahl Hypertension-Sensitive Rats

Baroreflex responsiveness was assessed in Dahl rats that had been fed a low-salt diet to retard subsequent development of hypertension. After five weeks there were no differences in baseline pressures, but all responses elicited reflexly by elevating blood pressure with phenylephrine were less pronounced in hypertension-sensitive (DS) than in hypertension-resistant (DR) rats. Diminished responses included the following: acceleration in afferent aortic nerve activity, bradycardia, and inhibition of splanchnic and renal sympathetic nerve activity. By contrast, opposite responses elicited by lowering blood pressure with sodium nitroprusside did not differ significantly. Selective baroreflex attenuation (i.e. which was demonstrable only when blood pressure was elevated but not when it was lowered) in DS rats may have been due to differences either in pharmacology of the two drugs, or in sinoaortic vessel wall geometry. The baroreflex impairment shown here could represent a genetic predisposition that eventually elevates blood pressure in DS rats by increasing sympathetic vasomotor tone in the renal and splanchnic vascular beds.