DIFFERENTIAL REGULATION OF ANGIOTENSIN-II AND LOSARTAN BINDING-SITES IN GLOMERULI AND MESANGIAL CELLS

The aim of the present report was to examine the effect of several age nts on angiotensin II (ANG II) and losartan receptors using I-125-[Sar (1),Ala(8)]ANG II and [H-3]losartan as radiolabeled ligand, respective ly. ANG II receptors were downregulated in glomeruli from rats infused with ANG II during 3 wk or rats receiving losartan orally during 1 wk . The number of sites (B-max) was reduced, but the dissociation consta nt (K-d) value was unchanged. Losartan receptors were downregulated in glomeruli from rats receiving losartan, but remained unchanged in glo meruli from rats infused with ANG II. Since in vivo administration of losartan results in increase of plasma ANG II and formation of metabol ites, in vitro studies using human mesangial cells were performed to b etter analyze the present findings. Treatment of mesangial cells durin g 4 days by ANG II, losartan, or its metabolite, EXP-3174, also produc ed downregulation of I-125-[Sar(1),Ala(8)]ANG II binding sites with a decreased B-max and unchanged K-d value. Only treatment of mesangial c ells by ANG II or EXP-3174 produced downregulation of [H-3]losartan bi nding sites. In contrast, exposure of these cells to losartan resulted in upregulation of [H-3]losartan binding sites. Under all conditions, only B-max was modified. Whereas internalization of [H-3]losartan in mesangial cells was negligible under all experimental conditions, ther e was an increase of the percentage of internalized I-125-[Sar(1),Ala( 8)]ANG II after exposure of the cells to ANG II or AT(1) antagonists. No change was observed in mesangial cell ATL receptor mRNA levels. Thi s study demonstrates that 1) AT(1) mRNA is expressed in human mesangia l cells; 2) the characteristics of I-125-[Sar(1),Ala(8)]ANG II and [H- 3]losartan binding sites in rat glomeruli and human mesangial cells ar e different, with K-d and B-max values greater in both preparations wh en [H-3]losartan was utilized; 3) both types of binding sites obey dif ferent regulations, and the effects of losartan in vivo are due in par t to the associated increase in plasma ANG II levels and the transform ation of the drug into its metabolite, EXP-3174; 4) downnregulation of AT(1) receptors does not depend on changes in mRNA expression but is associated with increased relative internalization.