RECEPTORS FOR ARG8-VASOPRESSIN, ANGIOTENSIN-II, AND ATRIAL-NATRIURETIC-PEPTIDE IN THE MESENTERIC VASCULATURE OF PREGNANT RATS

Blood pressure and sensitivity of blood vessels to vasoconstrictors are decreased in term-pregnant rats (20-21 days). To determine if changes in receptors for vasoactive peptides could account for these observations, receptor kinetics were measured for Arg8-vasopressin (AVP), angiotensin II (Ang II), and atrial natriuretic peptide (ANP) in the mesenteric vascular bed of the rat throughout pregnancy. Receptors for AVP were statistically similar in the five groups of animals (nonpregnant; pregnant 9, 15, and 21 days; and postpartum). The dissociation constant (K(D)) for [H-3]AVP varied from 0.41 to 0.52 nmol/L (NS), while receptor density (B(max)) varied from 310 +/- 110 to 455 +/- 135 fmol/mg protein for six experimental measurements. Similar observations were made for Ang II receptors where K(D) of I-125-labelled Sar1, Ile8-Ang II was between 0.60 and 0.97 nmol/L and B(max) between 215 +/- 30 and 250 +/- 40 fmol/mg protein in the different groups. I-125-labelled ANP (101-126) receptors were markedly modified in terms of number of sites. B(max) was significantly increased during pregnancy (9 days, 429 +/- 86; 15 days, 541 +/- 54; 20 days, 438 +/- 72) and decreased in the postpartum period (133 +/- 21) by comparison with the nonpregnant group (245 +/- 35 fmol/mg protein), while K(D) was similar in the different experimental groups (57 to 82 pmol/L). Despite these increases in receptor density, the vasorelaxant effects of ANP was only increased at 9 days of pregnancy. These results indicate that receptors for vasoconstrictor peptides (AVP and Ang II) are not altered during pregnancy and that receptor homospecific downregulation does not explain the decrease in sensitivity to vasoconstrictors at the end of pregnancy in the rat. Receptors for ANP are, however, markedly increased in number, but this did not result in an increase in the vascular action of the peptide throughout pregnancy.