Renal and endocrine response to saline infusion in essential hypertension

To assess the contribution of the renin-angiotensin-aldosterone system and renal hemodynamics to acute renal sodium handling in essential hypertension we studied 21 subjects who had essential hypertension (16 with normal renin, 5 with low renin) and 9 normal subjects. All were in balance on a 10 mEq sodium intake before receiving a small sodium load, 60 mEq intravenously over 1 hour. Hypertensive subjects with low renin showed the anticipated exaggerated natriuresis, which was transient and occurred without a rise in blood pressure. Natriuresis in hypertensive subjects with normal renin was either normal or blunted; delayed sodium excretion occurred in a subset, along with delayed suppression of the renin-angiotensin-aldosterone system by the saline load. Neither renal plasma flow nor glomerular filtration rate changed during the saline load. After 72 hours of converting enzyme inhibition with enalapril, renal plasma flow increased substantially more in the subjects with a blunted renin response and their natriuretic response to the sodium load returned to normal. These results indicate that when prior sodium intake is controlled, large sodium loads are avoided, and low renin hypertension is removed as a confounding variable, blunted rather than exaggerated natriuresis is the common feature of essential hypertension. This abnormality is reversed by angiotensin converting enzyme inhibition, perhaps because of converting enzyme inhibition-induced renal vasodilatation.