Effect of renal medullary circulation on arterial pressure

PRESSURE-NATRIURESIS EFFECTS IN HYPERTENSION: Considerable advances have been made in our understanding of pressure-natriuresis and the effects of this mechanism in hypertension. We have shown that in the absence of changes in neural and endocrine factors, sodium and water excretion doubled when arterial pressure was increased by only 10 mmHg. These responses were greatly blunted or obscured by elevations in renal sympathetic tone, infusion of the vasoconstrictors angiotensin and vasopressin or by inhibition of paracrine factors such as eicosanoids and nitric oxide.

The pressure-natriuresis response is closely associated with changes in papillary blood flow as determined by laser-Doppler flowmetry. In volume-expanded rats, papillary blood flow is not well autoregulated, which results in elevations of vasa recta capillary pressure and renal interstitial fluid pressure. The increased interstitial fluid pressure is transmitted from the medulla to the cortex in the encapsulated organ and is associated with inhibition of sodium transport in the proximal tubule and/or the thin descending loop of Henle of deep nephrons. Selective reductions in medullary blood flow by infusion of the nitric oxide inhibitor N6-nitro-L-arginine methylester (L-NAME) into the renal medullary interstitial space resulted in decreased interstitial fluid pressure and reduced sodium excretion. The mechanisms by which small elevations in renal interstitial fluid pressure alter tubular sodium reabsorption remain to be determined. PRESSURE-NATRIURESIS EFFECTS IN HYPERTENSIVE RATS: Our studies have also shown that the pressure-natriuresis response is blunted in spontaneously hypertensive rats (SHR) compared to normotensive Wistar-Kyoto (WKY) rats. This abnormality is associated with shifts in the relationships among papillary flow, renal interstitial pressure and renal perfusion pressure towards higher pressures. The calcium antagonist nisoldipine corrected the defect in vasa recta hemodynamics in SHR and normalized relationships among sodium excretion, renal interstitial pressure and renal perfusion pressure.

These studies indicate that sodium and water excretion is very sensitive to small changes in renal perfusion pressure due to associated changes in papillary blood flow, and that alterations in medullary hemodynamics can have an important effect on the relationship between arterial pressure and sodium and water excretion.