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ABSTRACT

Stimulation of aldosterone synthesis by angiotensin II (AII) is associated with depolarization of the cell membrane. Since the potential difference of adrenocortical cells is dependent on membrane permeability to potassium ions, the effects of agents which hyperpolarize the cell (by increasing permeability to K⁺) on the control of aldosterone synthesis were investigated further. Basal and AII-stimulated aldosterone synthesis was increased by 20–70% in cells incubated with 1 or 10 nm of the potassium ionophore valinomycin; higher concentrations markedly inhibited AII-stimulated synthesis. Cromakalim, a potential antihypertensive drug which facilitates the opening of K⁺ channels in smooth muscle cells, stimulated basal aldosterone synthesis at 2 μm but had no effect at 40 μm. AII-stimulated aldosterone synthesis was not affected by cromakalim except at 40 μm, which was inhibitory. The inhibitory effects of cromakalim, unlike those of valinomycin, were not reversible. Aldosterone synthesis from added hydroxycholesterol and pregnenolone (but not from deoxycorticosterone and corticosterone) was significantly inhibited by 40 μm cromakalim.

Potassium efflux from cells preloaded with ⁴³K was unaffected by low concentrations of valinomycin, but was markedly increased by concentrations which inhibited AII-stimulated aldosterone production. Small decreases and increases in ⁴³K efflux, caused by 1 and 40 μm cromakalim respectively, corresponded with increases and decreases in basal aldosterone production; cromakalim did not affect ⁴³K efflux from AII-stimulated cells.

We suggest that increasing adrenocortical cell membrane permeability to K⁺ reduces steroidogenesis, but that valinomycin and cromakalim have other actions which complicate the relationship between ⁴³K efflux and aldosterone production. Cromakalim appears to inhibit 21-hydroxylase activity in the biosynthetic pathway and may also affect 3β-hydroxysteroid dehydrogenase activity.

ABSTRACT

Mouse hepatoma × rat hepatocyte hybrids that segregate rat chromosomes were used to determine the chromosomal location of the rat genes encoding 11 β-hydroxylase and aldosterone synthase (Cyp11b1 and Cyp11b2 respectively). By means of species-specific restriction fragments and microsatellite markers both genes were mapped to rat chromosome 7. The Cyp11b1 microsatellite marker was subsequently found to vary in length between and within rat strains. Furthermore, we compared the sequences of Cyp11b1 markers in two genetically hypertensive strains of rat with their normotensive counterparts. Previous studies have indicated that 11β-hydroxylase activities in Milan and Lyon hypertensive strains are different from their respective genetic controls. The Cyp11b1 microsatellite regions from Lyon hypotensive and normotensive strains of rat were similar and were both shorter by 15 bases than that of the Lyon hypertensive strain. The Cyp11b1 marker in Milan hypertensive (MHS) and normotensive (MNS) strains differ from all the Lyon strains and from each other. The MHS marker is 12 bases shorter than that of MNS rats. These differences in microsatellite length may provide useful polymorphic markers in co-segregation studies of genetic hypertension in rats.

ABSTRACT

Dexamethasone administration in vitro has been shown to increase adenylyl cyclase activity in vascular smooth muscle cells (VSMC) from renal arteries and in non-vascular cell lines. To investigate whether G proteins are involved in this response, cultured VSMC from mesenteric arteries of Sprague—Dawley rats were incubated in the presence and absence of 10 nm dexamethasone for 24 and 48 h. Basal and stimulated adenylyl cyclase activities were increased by approximately 50% after treatment with dexamethasone. The changes were neither specifically associated with ligands which stimulate adenylyl cyclase catalytic unit via G_s (isoproterenol and prostaglandin E₁) nor with guanylylimidodiphosphate (0·1 nm), which inhibits the catalytic unit via G_i. This suggests that dexamethasone enhances adenylyl cyclase activity through changes at the level of the catalytic unit, rather than through the G proteins which modulate its activity. No differences were seen in immunoblotting studies of the levels of G_iα2, G_sα, G_iα3 and β subunits. Similarly, dexamethasone had no effect on the expression of mRNA for G_iα2 and G_sα.

The results indicate that glucocorticoid-induced increases of adenylyl cyclase activity are due to changes at the level of the adenylyl cyclase catalytic unit rather than alteration of the levels or turnover of G_sα, G_iα2, G_iα3 and β subunits in the membranes of VSMC.