| Abstrakt: |
The inhibitory action of atrial natriuretic (ANPs) on mammalian aldosterone synthesis is well documented. In addition, other work indicates that ANP and an analogue of its second messenger, 8-Br-cGMP, inhibit aldosterone production by chicken adrenal steroidogenic cells. However, the interaction between angiotensin II (AII) and ANP in the regulation of avian aldosterone production is poorly understood because chicken adrenal steroidogenic cells, the commonly used in vitroavian model, are comparatively unresponsive to AII. By contrast, turkey (Meleagris gallopavo) adrenal steroidogenic cells are sensitive to AII. Thus, in the present study, the action of ANPs and related peptides and their interaction with other stimulators of aldosterone production were investigated using freshly isolated and briefly cultured turkey adrenal steroidogenic cells. Surprisingly, several ANPs [rat (r), human (h), chicken (c)], and rat brain natriuretic peptide (rBNP) were as efficacious as [Ile5]AII for stimulating aldosterone production (2 hr) in freshly isolated cell suspensious but were less potent than [Ile5]AII (ED50of ANPs -5-10 nM; (Ile5]AII ED50-0.1nM). In addition, chicken ANP enhanced maximal aldosterone production induced by [Ile5]AII (1 nM). K+(25 nM), and hACTII-(1-39) (I nM: maximal enhancement of the action of these secretagogues was +49% +137% and +15%, respectively (P< 0.05; n= 3). Furthermore, other ANPs and related peptides [rBNP and bovine aldosterone secretion inhibiting factor (bASIF) enhanced maximal [He5]AII-induced aldosterone production; the order of maximal enhancement was rBNP (I 180%) > hANP/rANP (+50%) > bASIF (+25%) (P< 0.05; n= 3). Moreover, 8-Br-cGMP was also effective (+ 31%). In addition, ANPs and related peptides stimulated cGMP production (2 hr) with relative potencies and efficacies that were similar to their relative potencies and efficacies for stimulating aldosterone production, thus suggesting that cGMP mediated the action of ANPs and related peptides in turkey adrenal steroidogenic cells. However,like [Ile5]AII and K+ANPs and related peptides and 8-Br-cGMP did not stimulate corticosterone production, thus suggesting that these agents stimulated the conversion of the existing intracellular pool of corticosterone to aldosterone. Evaluation of specific [125I-Tyr28]rANP binding to intact, freshly isolated cells indicated the presence of high-affinity ANP receptors [dissociation constant (Kd) = 1.46 ± 0.23 nM; cellular concentration = 89.400 ± 2600 sites/cell (means ± SEM; n= 3)]. Competitive binding studies with briefly cultured (12 hr) cells indicated that these receptors were specific for ANPs and related peptides. In addition, the order of competition potencies was consistent with the order of ED50s for the stimulation of cGMP and aldosterone production. Collectively, these findings suggest that ANP, through the generation of cGMP positively modulates aldosterone production by turkey adrenal steroidogenic cells. In addition, the present study suggests that the turkey adrenal steroidogenic cell may be a unique model to elucidate the effector molecules of ANP action on aldosterone synthesis. |
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