STRESS-INDUCED CHANGES IN THE RATE OF SODIUM-EXCRETION IN HEALTHY BLACK-AND-WHITE MEN

The effects of exposure to competitive mental stressors on rates of excretion of sodium (Na), potassium (K) and fluid, together with cardiovascular responses, were evaluated in 14 black and 14 white normotensive men after 3 days of controlled diet (200 mEq Na, 100 mEq K daily). In the first hour of a 5-hr protocol, subjects ingested a standard dinner and drank 1 liter of water. In hours 2-5, they voided to provide urine collections every 30 min, and drank 200 ml additional water after each collection. Hours 1-2 were for stabilization of excretion, hour 3 was baseline, hour 4 was stress and hour 5 was post-stress rest. During stress, 20 men showed faster natriuresis and 8 showed slower natriuresis than at baseline. These subgroups did not differ in any excretion measure at baseline. During stress, the slow stress natriuresis group showed slower excretion of potassium and fluid as well as sodium; slow natriuresis and kaliuresis persisted 30 min after stress. Creatinine clearance rate was also decreased, but only during the first 30 min of stress. Slow stress natriuresis subjects were found to have higher blood pressures during baseline and stress, greater heart rate and cardiac output increases during stress, and a larger percentage had hypertensive parents (63 vs 37 %) compared to fast stress natriuresis subjects. Slow stress natriuresis was observed in 50 vs 17 % of men with resting diastolic levels above vs below 80 mmHg, and in 43 % of blacks vs 18 % of whites tested. Overall, black subjects tended to excrete sodium more slowly than whites at baseline, and showed significantly lesser increases in sodium excretion rate when pressures rose during stress.