Angiotensin II binding to renal glomeruli from sodium-loaded and sodium-depleted rats

Abstract

125I-labeled angiotensin II (125I-labeled AII) and [3H]angiotensin II ([3H]AII) bind specifically to isolated rat glomeruli. Three groups of receptor sites could be defined by the KD value (7.1 +/- 0.3 X 10(-11, 3.4 +/- 0.2 X 10(-10), and 1.6 X 10(-9) M, respectively) and the number of receptor sites (11.6 +/- 1.2, 29.4 +/- 3.9, and 113.8 +/- 3.8 fmol/mg glomerular protein, respectively). Both association and dissociation constants for 125I-labeled AII were greater than those for [3H]AII, but their ratio (KD) remained unchanged. Specificity of binding to these three groups of receptor sites was demonstrated by the following: 1) inhibition of binding of labeled AII by unlabeled hormone or by antagonists; and 2) reversibility of binding, independent of either hormone or receptor degradation. Binding was increased in glomerular preparations from acutely and chronically sodium-loaded rats, compared with glomerular preparations from acutely and chronically sodium-depleted rats. This change in binding resulted from both a change in the number of receptor sites and modification of the affinity of AII for its receptors. KD was higher in preparations from sodium-depleted rats (12.9 +/- 3.3 and 14.6 +/- 3.9 X 10(-11) M in chronically and acutely depleted rats, respectively) than in those from sodium-loaded rats (2.7 +/- 0.2 and 3.9 +/- 1 X 10(-11) M in chronically and acutely sodium-loaded rats, respectively). Changes in the binding of AII to its glomerular receptors could play a role in the adaptation of glomerular filtration rate to the sodium balance.