Salom, Miguel G.

To evaluate the participation of nitric oxide (NO) on pressure-induced natriuresis in pentobarbital-anesthetized dogs, renal perfusion pressure (RPP) was increased twice from 100 to 150 mmHg before and during the intrarenal administration of an NO-synthesis inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), while determining changes in glomerular filtration rate (GFR), renal blood flow (RBF), and urine sodium and water excretion. Before the inhibition of NO, the increase in RPP induced diuresis (5-fold) and natriuresis (4.2-fold) with no change in RBF or GFR. However, the intrarenal infusion of L-NAME (1 microgram.kg-1.min-1) blunted the diuretic and natriuretic responses without altering RBF or GFR. The infusion of the NO synthesis precursor L-arginine prevented the inhibitory effect that L-NAME exerted on the diuretic and natriuretic responses to the increase in RPP. These results indicate that the increase in RPP stimulates NO synthesis and suggest that NO might play an important role in the control of sodium and water excretion during acute changes in RPP.

Acetylcholine (ACh) stimulates the endothelial release of prostacyclin and endothelium-derived relaxing factor (EDRF). However, the relative participation of these substances in mediating the renal effects of ACh remains undefined. To elucidate this issue, we studied the modifications of renal responses to intra-renal ACh infusion (25 ng.kg-1.min-1) produced by blocking the synthesis of EDRF and/or prostaglandins (PG) in anesthetized dogs. ACh induced a significant increase in renal blood flow (RBF) (34%), urine volume (UV) (450%), and urinary sodium excretion (UNaV)(259%), which remained unaltered after blocking the synthesis of EDRF [NG-monomethyl-L-arginine (LNMMA), 50 micrograms.kg-1.min-1 intrarenal] or PG (meclofenamate, 5 mg/kg iv). However, the simultaneous administration of meclofenamate and LNMMA prevented the ACh-induced increase in RBF and UV but not in UNaV. The concomitant infusion of L-arginine but not D-arginine prevented these blocking effects of LNMMA. It was concluded that the ACh-induced increases in RBF and UV, but not UNaV, are mediated by both PG and EDRF. The hemodynamic and diuretic effect of either one of these mediators can be fully compensated during the blockade of the other.

Intrarenal infusion of acetylcholine in meclofenamate-treated dogs significantly increased renal blood flow, diuresis, and natriuresis. Intrarenal infusions of either NG-monomethyl-L-arginine (inhibitor of endothelium-derived relaxing factor formation), or L-arginine (precursor of endothelium-derived relaxing factor formation) did not modify basal levels of those parameters. However, the infusion of NG-monomethyl-L-arginine inhibited the acetylcholine-induced increases in renal blood flow and diuresis without affecting natriuresis, which increased significantly. The infusion of L-arginine failed to further enhance hemodynamic and excretory effects elicited by acetylcholine. The concomitant infusion of L-arginine and NG-monomethyl-L-arginine did not change renal blood flow, urine flow, or sodium excretion rate. L-Arginine administration prevented the inhibitory effect of NG-monomethyl-L-arginine on acetylcholine-induced renal vasodilatation and diuresis. Glomerular filtration rate and mean arterial pressure did not change throughout the experiment. The results indicate that the vasodilatory and diuretic responses to intrarenal acetylcholine in meclofenamate-treated dogs are largely dependent on endothelium-derived relaxing factor.