RENAL-FUNCTION IN ONE-KIDNEY, ONE-CLIP HYPERTENSION AND LOW RENIN ESSENTIAL-HYPERTENSION

One-kidney, one-clip hypertension (1-K, 1-C HT) is initiated by increased preglomerular resistance which decreases nephron perfusion and causes several intrarenal changes that lead to increased mean arterial pressure (MAP). Elevated MAP serves to return nephron perfusion and sodium excretion to normal, so that fluid intake and output are balanced. Increased MAP usually occurs through volume homeostasis mechanisms that initially raise cardiac output and later elevate total peripheral vascular resistance via autoregulatory adjustments. However, if adequate volume is unavailable because of sodium restriction, sustained activation of the renin-angiotensin system increases blood pressure sufficiently to restore nephron perfusion. Thus, depending upon the availability of volume, renal perfusion and sodium balance can be restored either by volume retention or by increased angiotensin II (ANGII) formation and peripheral vasoconstriction. Similarities exist between 1-K, 1-C HT and low-renin essential hypertension (LRHT). In both cases, renal-pressure natriuresis is shifted to higher levels and there are marked increases in preglomerular resistance that necessitate increased MAP to maintain sodium balance. However, in 1-K, 1-C HT, there is a parallel shift of pressure natriuresis with little or no change in the slope of this curve, similar to that found in the normal-renin essential hypertension. In LRHT the slope of pressure natriuresis is decreased, indicating that blood pressure is much more salt sensitive than normal. Another difference is that PRA is low compared to normal PRA in 1-K, 1-C HT after compensatory increases in MAP. There is also no indication of glomerular membrane damage in 1-K, 1-C HT, whereas LRHT may have significant glomerulopathy, especially as hypertension progresses. These differences suggest that there may be additional factors besides preglomerular vasoconstriction involved in the etiology of LRHT. One possible factor is a reduction in nephron number in LRHT. Decreased functional nephrons would lead to glomerular hyperfiltration and increased distal tubular flow rate in the remaining nephrons, causing decreased PRA and eventually glomerular damage. Increased fractional sodium reabsorption, particularly in distal tubular segments, could also contribute to decreased PRA and cause blood pressure to be salt sensitive. These abnormalities, along with preglomerular vasoconstriction, may explain many of the characteristics of LRHT.