MECHANISM OF THE EFFECTS OF GLUCOCORTICOIDS AND MINERALOCORTICOIDS ON VASCULAR SMOOTH-MUSCLE CONTRACTILITY

We have previously demonstrated chat receptors to both mineralocorticoids (MC) and glucocorticoids (GC) exist in the arterial wall and that treatment with GC markedly increases Na+ and Ca2+ influx in cultured ed aortic vascular smooth muscle (VSM) cells, whereas treatment with MC increases only Na+ influx. We now report the results of the study aimed at the elucidation of the mechanism(s) of these effects. Unidirectional influx of Na+ and Ca2+ was measured in cultured cells of rabbit aortic media, using Na-22 and Ca-45 as tracers, in the presence of ouabain. The cells were treated for different periods with dexamethasone (DEX) or aldosterone (ALDO) in physiologic or supraphysiologic concentrations, in the presence or absence of competitive inhibitors of GC-receptor binding, RU 486, or MC-receptor binding, K-prorenoate. DEX in 50 nM concentration increased Na+ influx by 98 +/- 18% and Ca2+ influx by 100 +/- 20%, and the maximum effect was seen after 48 hour cell-treatment. ALDO in 5 nM concentration increased Na+ influx by 90 +/- 12% and 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 ALDO on the influx rate of Na+ was prevented by actinomycin D and by cycloheximide. RU 486 completely inhibited DEX from exercising its enhancing effect on Na+ influx, but diminished influx rate of Na+ increased by ALDO only by 25%. Prorenoate (PRN) did not have any effect on DEX-increased Na+ influx, but completely inhibited ALDO from exercising its effect. At 50 nM concentration, ALDO increased Na+ influx after only 24-48 hours treatment; this effect was similar to that of DEX and was inhibited by RU 486, but not by PRN. The effects of selective blockers of various Na+ and Ca2+ transport systems were examined in separate experiments: 1) amiloride (AMIL), 1.5 mu M; 2) ethylisopropyl amiloride (EIP-A); 3) dichlorobenzamil (DCB); 4) bumetanide (BUM); and 5) nifedipine (NIF). AMIL (almost exclusively Na+ channel blocker at the low concentration used) and BUM (inhibitor of Na+, KC+, 2Cl(-) cotransport) diminished each by 65% and 35%, respectively, the increase in Na+ influx elicited by DEX; the two effects were additive. In contrast, BUM had no effect on Na+ influx increased by 5 nM ALDO, but AMIL inhibited 98% of this influx. NIF inhibited 70% of Na+ transport increased by DEX, but did not affect that increased by 5 nM ALDO. We therefore conclude that: 1) increases in Na+ influx induced by GC and MC in VSM cells are mediated through intracellular vascular receptors for these steroids; 2) in physiologic concentrations, GC and MC act through distinct GC- and MC-receptors; 3) both GC and MC are physiologic modulators of intracellular Na+ concentrations in VSM, and consequently, the contractility of VSM; 4) the described mechanisms may play a major role in the pathogenesis of GC- and MC-induced hypertension.