Regulation of angiotensin II receptors and steroidogenic responsiveness in cultured bovine fasciculata and glomerulosa adrenal cells

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Abstract
The aim of the present study was to investigate the effect of several effectors on angiotensin II (A-II) receptors and steroidogenic responsiveness in cultured bovine fasciculata cells. Treatment of adrenal cells for 24 h with A-II (0.1 μM), corticotropin (1 nM), phorbol ester (PMA 0.1 μM), calcium ionophore A23187 (0.1 μM) and cyclic 8-bromoAMP (1 mM) produced a loss of A-II receptors whereas the A-II antagonist [Sar1-Ala8]A-II (0.1 μM) led to a small but significant increase. The extent of the down-regulation of receptors following maximal concentrations of A-II was greater than that produced by the other agents. The effects of A-II were dose-dependent with a ID50 of 3 nM. Since cycloheximide and actinomycin blocked the down-regulation of receptors, it seems likely that the effectors lead to the synthesis of certain proteins which inhibit the recycling of internalized receptors. Pretreatment of adrenal cells with A-II induced both homologous (90% decrease) and heterologous (corticotropin 83, PMA and ionophore 76% decrease) steroidogenic desensitization. However, the cAMP response to corticotropin of A-II-pretreated cells was higher (P < 0.001) than for control cells. Pretreatment with PMA and A23187 also resulted in both homologous and heterologous steroidogenic refractoriness but to a lesser degree than that induced by A-II. In contrast, corticotropin-pretreated cells responded normally to further stimulation with corticotropin or A-II. Similarly pretreatment of bovine adrenal glomerulosa cells with A-II (1 nM and 0.1 μM) and corticotropin (1 nM) also induced A-II receptor loss and steroidogenic refractoriness. The present findings indicate that, in contrast to the results reported in vivo in the rat, where A-II leads to up-regulation of its own receptors on glomerulosa cells and increases steroidogenic responsiveness, this peptide results in both down-regulation and desensitization in cultured bovine fasciculata and glomerulosa cells. Our results also emphasize the absence of correlation between A-II receptor loss and steroidogenic responsiveness.