REGULATION OF RENAL ATRIAL-NATRIURETIC-PEPTIDE RECEPTORS IN PREGNANT SHEEP

These studies were designed to characterize the atrial natriuretic pep tide (ANF) receptor subtypes (guanylyl cyclase GC-A and GC-B and ANF-C ) in normal sheep kidneys and to evaluate alterations in receptor kine tics during pregnancy. Kidneys were obtained from 12 nonpregnant and 1 2 pregnant sheep during late gestation and maintained on a 100 mmol/da y salt intake. Competition binding receptor assays using [I-125]human ANF showed that inner medullary membranes are exclusively of the GC-A subtype. The maximum binding capacity (B-max, 109 +/- 12 vs. 89 +/- 18 fmol/mg protein) and dissociation constant (K-d, 240 +/- 70 vs. 324 /- 99 pM) are not altered by pregnancy. Specific binding of glomerular membranes to [I-125]Tyr-C-type natriuretic peptide, which shows the h ighest affinity toward GC-B receptors, was observed, but this binding was abolished when ANF-C receptors were saturated with excess C-ANF-(1 01-121), suggesting that [I-125]Tyr-C-type natriuretic peptide binding was mediated by ANF-C receptors. Binding of [I-125]human ANF to glome rular membranes revealed that glomerular ANF receptor number was reduc ed during pregnancy (1040 +/- 212 vs. 335 +/- 42 fmol/mg protein; P = 0.001), but binding affinity was not changed. The reduced number was m ainly due to a decrease in ANF-C receptor density (832 +/- 213 vs. 260 +/- 31 fmol/mg protein; P = 0.005). Autoradiography of whole kidney f rozen sections produced similar findings. These studies demonstrate th at GC-B receptors are absent from renal glomeruli and inner medulla, a nd that ANF receptor subtypes are differentially regulated in the preg nant sheep kidney, suggesting a role for ANF in the altered volume and pressure homeostasis of pregnancy.