Influence of cations on kinetics of angiotensin II binding to adrenal, renal, and smooth muscle receptors

Potassium depletion and regulation of angiotensin II receptors in vascular smooth muscle cells

Mesenteric artery smooth muscle cells were grown in culture media containing high, normal, or low concentrations of potassium to study the effects on angiotensin II (Ang II) receptor regulation. Cell growth was similar among cells grown in the different culture media. Cells grown in high potassium media (K = 5.8 mEq/L) had an equilibrium dissociation constant, Kd, of 1.59 +/- 0.2 nM, whereas those grown in normal potassium media (K = 4.1 mEq/L) had a Kd of 1.79 +/- 0.2 nM and those grown in a low potassium media (K = 2.9 mEq/L) had a Kd of 1.19 +/- 0.12 nM (not significantly different, NS). Binding capacity of smooth muscle cells grown in high potassium media was 81 +/- 16.7 fmol/mg prot, 95.1 +/- 12.4 fmol/mg prot in those grown in normal potassium media and those grown in low potassium media 86.4 +/- 24.1 fmol/mg prot (NS). Binding of radiolabelled Ang II was reduced by approximately 70% in cells exposed to unlabelled Ang II for 30 or 60 minutes. However, this effect of exposure to Ang II to reduce subsequent binding of Ang II was identical in cells grown in high and low potassium medium. Therefore, we were unable to identify a direct effect of low potassium to induce changes in Ang II receptor binding affinity or binding capacity. Previously observed changes in these Ang II binding parameters in potassium-depleted rats was probably a consequence of other factors which were simultaneously altered by potassium deficiency.

Subpressor infusions of angiotensin II alter glomerular binding, prostaglandin E2, and cyclic AMP production

Angiotensin II (ANG II) has been postulated to have pathogenetic role in diminished glomerular function in a number of animal models of acute renal failure. The present studies were designed to test the hypothesis that modest elevations in circulating ANG II potentiate the ability of ANG II to reduce glomerular capillary surface area through an effect on ANG II binding to glomerular mesangial cells and/or influences on other modulators of function. Rat glomeruli isolated by a sieving technique were employed in vitro in an ANG II radioreceptor assay. Subpressor infusion of ANG II for 36 hours in rats increases the affinity and number of ANG II binding sites of isolated glomeruli. The ability of ANG II to influence function was tested by assessing its effect on glomerular surface area in vitro by image-analysis microscopy, a method of measuring mesangial cell contractility. The sensitivity and magnitude of ANG II-induced decrements in glomerular surface area were increased. ANG II infusion diminished glomerular prostaglandin E2 (PGE2) production, increased basal cyclic adenosine 3',5'-monophosphate (cAMP) production, and enhanced ANG II-induced decrements in cAMP production. In control glomeruli, only pharmacological concentrations of ANG II inhibited cAMP, but after ANG II infusion, physiological concentrations of ANG II were capable of inhibiting cAMP by as much as 57% (below basal values). In conclusion, continuous infusion of subpressor concentrations of ANG II in rats enhances the contractile response of the glomerular mesangial cell through effects on the cell's surface receptor for ANG II and on prostaglandin and cAMP production. These actions may be important mediators of the effects of ANG II on glomerular function associated with a number of experimental models of kidney disease.