The mas oncogene enhances angiotensin-induced [Ca2+]i responses in cells with pre-existing angiotensin II receptors.

The proposal that the mas oncogene is an angiotensin receptor was evaluated in Xenopus oocytes injected with human and rat mas RNA transcripts, and during transient expression of mas in several cell lines. No evidence of mas-induced angiotensin II (AII) receptors or [Ca2+]i responses was observed in Xenopus oocytes or in most of the transfected cells. However, Cos-1 cells, which showed a small endogenous [Ca2+]i response to AII, exhibited a modest but reproducible enhancement of this response after mas transfection. Such responses were inhibited by [Sar1, Ala8]AII and [Sar1, Ile8]AII, but not by [D-Arg1, D-Pro2, D-Trp7,9, Leu11] substance P, an antagonist reported to inhibit mas-induced responses to AII in oocytes. These findings are not compatible with the proposal that the mas oncogene is an angiotensin receptor, but suggest that expression of mas leads to increased responsiveness of the endogenous AII signaling system.

Interaction between serotonin and other regulators of aldosterone secretion in rat adrenal glomerulosa cells.

Although serotonin (5HT) is a recognized stimulator of aldosterone secretion in vivo and in vitro, its physiological role as a regulator of mineralocorticoid secretion and its mechanism of action in the adrenal glomerulosa have not been elucidated. To address these questions we studied the interaction of 5HT with other aldosterone regulators in isolated rat adrenal glomerulosa cells. 5HT stimulated aldosterone production 14-fold, with an ED50 of 20 +/- 5 nM, and stimulation was maximal at 0.8 microM. The stimulation of aldosterone production by 5HT was accompanied by a 5-fold increase in cAMP production, with an ED50 of 1 microM. Threshold levels of 5HT (1 nM) potentiated the effect of submaximal concentrations of angiotensin-II (AII), decreasing the ED50 from 1.3 to 0.46 nM and increasing the maximum response in an additive manner. In contrast, the stimulatory effect of 5HT was purely additive to that of submaximal ACTH concentrations. 5HT had no effect on aldosterone secretion stimulated by maximal ACTH concentrations, despite full additivity on cAMP accumulation. Stimulations of steroidogenesis by potassium and 5HT were fully additive at submaximal concentrations, but only partially additive at-maximal levels. To determine the mechanism of the synergistic effects of AII and 5HT, we analyzed the interaction of both stimuli on cAMP accumulation, intracellular calcium, and inositol phosphate formation. Consistent with the inhibitory effect of AII on adenylate cyclase, in the presence of AII the stimulation of cAMP by 5HT was reduced by 18 +/- 3%. 5HT alone had no effect on cytosolic calcium, but significantly enhanced the peak and later phases of the AII-stimulated increase (P less than 0.005). This effect of 5HT was due to calcium influx and not to release from intracellular pools, as shown by suppression of the potentiation in the absence of extracellular calcium and the lack of effect of 5HT on basal or AII-stimulated inositol phosphate formation. The ability of low concentrations of 5HT to potentiate the stimulatory effect of AII on aldosterone secretion suggests that under some physiological conditions, 5HT may play a role in regulating the adrenal sensitivity to AII.