CARDIAC BAROREFLEX SENSITIVITY AND SODIUM-EXCRETION ARE REDUCED BOTH BY A DEFICIT AND AN EXCESS OF DIETARY SALT IN THE CONSCIOUS DOG

In 10 conscious, chronically instrumented beagle dogs we studied the e ffects of four different dietary sodium intakes (mmol Na/kg body wt/da y: 14.5 [excess], 7.5 [high], 2.5 [normal], and 0.5 [low] [plus an add itional standardized sodium depletion produced by peritoneal dialysis several days before the experiments]) on cardiac baroreflex sensitivit y and renal response to an acute saline load, Full sigmoid barocurves were produced by intravenous injection of phenylephrine (2.5 to 20 mu g/kg) and nitroglycerine (2.5 to 30 mu g/kg). The gain of this relatio nship was significantly decreased by both an excess and low sodium int ake (8.0 +/- 1.0 and 8.3 +/- 0.8 beats/min/mm Hg, respectively) when c ompared with the 2.5 and 7.5 (12.1 +/- 1.4 and 16.0 +/- 1.7 beats/min/ mm Hg, respectively) mmol Na/kg/day sodium intake. Water and sodium ex cretion in response to saline infusion were lower in the 0.5 and 14.5 mmol/kg/day sodium intake groups in spite of the higher atrial natriur etic peptide and lower plasma renin activity and plasma aldosterone le vels in the latter. Mean arterial blood pressure, heart rate, and cent ral venous pressure increased during saline loading in all groups; hem atocrit and plasma protein concentration decreased similarly in all gr oups. The results suggest that the rapid renal homeostatic response to an acute salt load in animals kept chronically on normal or moderatel y increased dietary sodium intake is regulated by baroreflex control o f the renal homeostatic response. Excess dietary sodium intake attenua tes baroreflex sensitivity and delays sodium and water excretion after acute loading.