Renal effects of prolonged intrarenal infusions of angiotensin II and atrial natriuretic peptide in sheep

Angiotensin II (AngII) and atrial natriuretic peptide (ANP) are two hormones that have antagonistic effects on volume and pressure regulation. Plasma levels of both hormones are elevated in sheep pregnancy. However, during pregnancy, volume expansion occurs despite elevated plasma ANP, implying an overriding role of AngII. In addition to counteracting the effects of ANP on the physiological level, AngII also may act on the receptor level. Therefore this study was designed to investigate the hemodynamic and renal effects of ANP and AngII separately and to define their selective effects on the renal natriuretic peptide receptor types in the various segments of the nephron. Eight unilaterally nephrectomized nonpregnant sheep received separately for 10 days, low doses of AngII (1 ng/kg/min) and ANP (0.5 ng/kg/min) directly infused into the renal arteries to avoid systemic effects. Intrarenal AngII infusion decreased sodium excretion (UNaV) from 111 +/- 11 to 36 +/- 8 and 45 +/- 6 mmol/day (p < 0.05) on days 3 and 8-10, respectively. Mean arterial pressure (MAP) increased from 94 +/- 6 mm Hg to a maximum of 107 +/- 8 mm Hg on day 5 of infusion and stabilized at 101 +/- 7 mm Hg on days 8-10 (p < 0.05). Intrarenal ANP infusion significantly increased UNaV on day 1 from 93 +/- 9 to 188 +/- 20 mmol/day (p < 0.05), followed by sodium retention on days 4-6 (average, 60 +/- 13 mmol/day; p < 0.05). UNaV again increased above control levels on days 8-10 to an average level of 111 +/- 15 mmol/day. MAP decreased from 99 +/- 4 to 90 +/- 5 mm Hg (p < 0.05) on days 1-3, and remained lower than control throughout the infusion period. The kidneys were collected at control nephrectomy and at the end of infusion. The natriuretic peptide receptors were characterized by competitive-binding radioreceptor assays on glomerular, outer medullary, and inner medullary membranes. AngII infusion increased the dissociation constant (K-d) of inner medullary natriuretic peptide receptors from 186 +/- 11 to 267 +/- 22 pM (p < 0.05), and ANP infusion decreased maximal binding capacity (B-max) of inner medullary receptors from 134 +/- 10 to 89 +/- 15 fmol/mg protein (p < 0.05). Glomerular and outer medullary natriuretic peptide receptors were not affected by either AngII or ANP infusion. In conclusion, AngII stimulates antinatriuresis and counteracts the hemodynamic and renal effects of ANP in part by downregulating the renal inner medullary natriuretic peptide receptors.