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1. Effects of carbidopa, a dopa (3,4-dihydroxy-phenylamine) decarboxylase inhibitor, on the renal, haemodynamic and hormonal responses to acute volume expansion were examined in six healthy mongrel dogs which were infused intravenously with 0.9% sodium chloride solution (saline; 30 ml h−1 kg−1) over 2 h. 2. Saline infusion studies were performed in the absence (control) and in the presence of carbidopa given by nasogastric tube in a dose of 1 mg/kg every 8 h beginning 24 h before the infusion. 3. Saline infusion resulted in an increase in renal excretion of dopamine (3,4-dihydroxy-phenylethylamine) and a decrease in renal excretion of noradrenaline. 4. Carbidopa treatment decreased urinary sodium excretion and eliminated the increase in renal production of dopamine in response to saline infusion without affecting renal or haemodynamic response to acute vascular volume expansion with saline. 5. Carbidopa treatment obliterated the suppression of aldosterone produced by saline infusion. 6. Thus, dopamine appears to play a significant role in mediating both the natriuretic and aldosterone response to acute volume expansion.

1. To investigate factors regulating blood pressure in obesity we have compared Na+ transport mechanisms and plasma noradrenaline (NA) responses to upright posture and isometric hand grip exercise in obese patients with transport mechanisms and NA responses in age, sex and race matched non-obese subjects. In the obese subjects we examined the effects of caloric restriction and weight reduction over a 12 week period on intracellular erythrocyte cation concentrations, Na+,K+-dependent ATPase activity and 86Rb uptake as well as responses of NA, plasma renin activity (PRA), aldosterone and mean arterial pressure to posture and isometric exercise. 2. Obese patients had greater (P < 0.05) basal supine plasma NA as well as enhanced NA responses to upright posture and exercise. Supine plasma NA as well as NA responses were reduced (P < 0.05) after 4 weeks caloric restriction and PRA and aldosterone responses were decreased (P < 0.05) after 8 weeks caloric restriction. Plasma NA at the onset of the diet correlated (r = 0.68, P < 0.01) with blood pressure, and decrements in NA and blood pressure were also related (r = 0.60, P < 0.01). Reductions in supine PRA and supine plasma NA during wieght loss were correlated (r = 0.55, P < 0.05) and the reduction in PRA may therefore be secondary to a decrease in sympathetic nervous system activity. 3. The intracellular Na+/K+ ratio in erythrocytes was greater in the obese patients than in non-obese controls whereas erythrocyte Na+,K+-ATPase activity and 86Rb uptake in obese patients was reduced (P < 0.01). There were significant negative correlations between Na+,K+-ATPase activity and 86Rb uptake and the degree of obesity. Although Na+,K+-ATPase and 86Rb increased and intracellular Na+ decreased during the 12 weeks of weight loss, they remained altered compared with values in non-obese subjects.