MECHANISM OF THE EFFECTS OF GLUCOCORTICOIDS AND MINERALOCORTICOIDS ONVASCULAR SMOOTH-MUSCLE CONTRACTILITY

We have previously demonstrated chat receptors to both mineralocortico ids (MC) and glucocorticoids (GC) exist in the arterial wall and that treatment with GC markedly increases Na+ and Ca2+ influx in cultured e d aortic vascular smooth muscle (VSM) cells, whereas treatment with MC increases only Na+ influx. We now report the results of the study aim ed at the elucidation of the mechanism(s) of these effects. Unidirecti onal influx of Na+ and Ca2+ was measured in cultured cells of rabbit a ortic media, using Na-22 and Ca-45 as tracers, in the presence of ouab ain. The cells were treated for different periods with dexamethasone ( DEX) or aldosterone (ALDO) in physiologic or supraphysiologic concentr ations, in the presence or absence of competitive inhibitors of GC-rec eptor binding, RU 486, or MC-receptor binding, K-prorenoate. DEX in 50 nM concentration increased Na+ influx by 98 +/- 18% and Ca2+ influx b y 100 +/- 20%, and the maximum effect was seen after 48 hour cell-trea tment. ALDO in 5 nM concentration increased Na+ influx by 90 +/- 12% a nd had no effect on Ca2+ influx, and the maximum effect was seen after 7-10 days of cell-treatment. The enhancing effect of both DEX and ALD O on the influx rate of Na+ was prevented by actinomycin D and by cycl oheximide. RU 486 completely inhibited DEX from exercising its enhanci ng effect on Na+ influx, but diminished influx rate of Na+ increased b y ALDO only by 25%. Prorenoate (PRN) did not have any effect on DEX-in creased Na+ influx, but completely inhibited ALDO from exercising its effect. At 50 nM concentration, ALDO increased Na+ influx after only 2 4-48 hours treatment; this effect was similar to that of DEX and was i nhibited by RU 486, but not by PRN. The effects of selective blockers of various Na+ and Ca2+ transport systems were examined in separate ex periments: 1) amiloride (AMIL), 1.5 mu M; 2) ethylisopropyl amiloride (EIP-A); 3) dichlorobenzamil (DCB); 4) bumetanide (BUM); and 5) nifedi pine (NIF). AMIL (almost exclusively Na+ channel blocker at the low co ncentration used) and BUM (inhibitor of Na+, KC+, 2Cl(-) cotransport) diminished each by 65% and 35%, respectively, the increase in Na+ infl ux elicited by DEX; the two effects were additive. In contrast, BUM ha d no effect on Na+ influx increased by 5 nM ALDO, but AMIL inhibited 9 8% of this influx. NIF inhibited 70% of Na+ transport increased by DEX , but did not affect that increased by 5 nM ALDO. We therefore conclud e that: 1) increases in Na+ influx induced by GC and MC in VSM cells a re mediated through intracellular vascular receptors for these steroid s; 2) in physiologic concentrations, GC and MC act through distinct GC - and MC-receptors; 3) both GC and MC are physiologic modulators of in tracellular Na+ concentrations in VSM, and consequently, the contracti lity of VSM; 4) the described mechanisms may play a major role in the pathogenesis of GC- and MC-induced hypertension.