INFLUENCE OF DIETARY-SODIUM RESTRICTION ON ANGIOTENSIN-II RECEPTORS IN RAT ADRENALS

We studied the distribution of angiotensin II (AII) receptors type 1 ( AT(1)) and type 2 (AT(2)) and the effects of a low sodium intake on th ese two subtypes of receptors in male rat adrenals. Binding studies on adrenal slices, on cell membranes and on cell suspensions were perfor med using [I-125]AII and specific analogs for AT(1) (Losartan) and AT( 2) (PD 123319) receptors. The distribution of AT(1) was also studied b y immunofluorescence. Complementary approaches were necessary to reach our goal. Indeed, by autoradiography on adrenal slices, [I-125]AII wa s shown to bind to the zona glomerulosa (ZG) and to the medulla (M). W hen coincubated with [I-125]AII, PD 123319 displaced [I-125]AII from t he medulla and from the ZG, indicating the presence of AT(2) receptors in both zones. Losartan partially displaced [I-125]AII from the ZG, i ndicating the presence of AT(1) receptors in that zone. Furthermore, t he labeling intensity of the medulla (AT(1) receptors) was much strong er in adrenal sections from rats kept on a low sodium regimen than fro m controls. Immunofluorescence microscopy revealed that AT(1) receptor s were located mainly in the ZG of control rats. After sodium restrict ion, AT(1) receptors appeared to be uniformly distributed within an en larged ZG; furthermore AT(1) receptor-positive cells were found to a l imited degree in the zona fasciculata and possibly in the zona reticul aris, and a greater number of these positive cells appeared in these z ones under sodium restriction. Cell suspensions from rats fed a low so dium diet showed a 2.7- and 2.1-fold increase in total AII receptors i n adrenal ZG and ZFR + M cells when compared with controls. Based on L osartan displacement, we calculated that [I-125]AII bound to AT(1) and to AT(2) receptors was increased in both ZG and ZFR + M cell preparat ions under sodium restriction. Results of binding studies on cell memb ranes were also indicative of an increasing effect of sodium restricti on on AT(1) and AT(2) receptors binding capacity. Furthermore, Norther n blotting analysis revealed 3.0- and 2.5-fold increases in the level of AT(1) receptor mRNA in the ZG and the ZFR + M of rats fed a low sod ium diet as compared with those fed a normal diet. The low sodium inta ke resulted in a weaker increase (1.5-fold) in the level of AT(2) rece ptor messenger RNA in the ZG, with no changes in the ZFR + M preparati ons. In conclusion, in this study complementary approaches were needed to determine the localization of AT(1) and AT(2) receptors in the rat adrenal, and to show the increasing effects of a low sodium regimen o n the adrenal level of these receptors. Immunofluorescence studies rev ealed AT(1) receptors mainly in the ZG and also in some cells of the i nner adrenal cortex zones; in adrenals of rats kept on a low sodium di et the ZG was markedly enlarged, and an increased number of immunoreac tive cells with AT(1) receptors were observed throughout that zone; al so more immunoreactive cells were present in the inner zones of the ad renal cortex. Furthermore in the adrenals of rats kept on a low sodium diet, we observed: 1) an increased number of AT(1) and AT(2) receptor s in cell suspensions from the ZG, and in cell suspensions of the ZFR + M; 2) an increased level of AT(1) and AT(2) receptor mRNAs in the ZG ; 3) an increased level of AT(1) receptor mRNA, with no changes in the AT(2) mRNA level in the ZFR + M. These results suggest a role for AT( 1) as well as for AT(2) receptors in controlling adrenal function and differentiation under normal as well as under physiological stimulatio n of AII production following sodium restriction.