Mineralocorticoid treatment attenuates activation of oxytocinergic and vasopressinergic neurons by icv ANG II.

Central oxytocin (OT) neurons limit intracerebroventricular (icv) ANG II-induced NaCl intake. Because mineralocorticoids synergistically increase ANG II-induced NaCl intake, we hypothesized that mineralocorticoids may attenuate ANG II-induced activation of inhibitory OT neurons. To test this hypothesis, we determined the effect of deoxycorticosterone (DOCA; 2 mg/day) on icv ANG II-induced c-Fos immunoreactivity in OT and vasopressin (VP) neurons in the supraoptic (SON) and paraventricular (PVN) nuclei of the hypothalamus and also on pituitary OT and VP secretion in male rats. DOCA significantly decreased the percentage of c-Fos-positive (%c-Fos+) OT neurons in the SON and PVN, both in the magnocellular and parvocellular subdivisions, and the %c-Fos+ VP neurons in the SON after a 5-ng icv injection of ANG II. DOCA also significantly reduced the %c-Fos+ OT neurons in the SON after 10 ng ANG II and tended to attenuate 10 ng ANG II-induced OT secretion. However, the %c-Fos+ OT neurons in DOCA-treated rats was greater after 10 ng ANG II, and DOCA did not affect the %c-Fos+ OT neurons in the PVN nor VP secretion or c-Fos immunoreactivity in either the SON or PVN after 10 ng ANG II. DOCA also did not significantly alter the effect of intraperitoneal (ip) cholecystokinin (62 microg) on %c-Fos+ OT neurons or of ip NaCl (2 ml of 2 M NaCl) on the %c-Fos+ OT and VP neurons. These findings indicate that DOCA attenuates the responsiveness of OT and VP neurons to ANG II without completely suppressing the activity of these neurons and, therefore, support the hypothesis that attenuation of OT neuronal activity is one mechanism by which mineralocorticoids enhance NaCl intake.

Estradiol attenuates angiotensin-induced aldosterone secretion in ovariectomized rats.

Estrogen replacement therapy significantly reduces the risk of cardiovascular disease in postmenopausal women. Previous studies indicate that estradiol (E2) decreases angiotensin II (AT) receptor density in the adrenal and pituitary in NaCl-loaded rats. We used an in vivo model that eliminates the potentially confounding influence of ACTH to determine whether the E2-induced decrease in adrenal AT receptor expression affects aldosterone responses to angiotensin II (Ang II). Female rats were ovariectomized, treated with oil (OVX) or E2 (OVX+E2; 10 microg, s.c.) for 14 days, and fed a NaCl-deficient diet for the last 7 days to maximize adrenal AT receptor expression and responsiveness. On days 12-14 rats were treated with dexamethasone (DEX; 25 microg, i.p., every 12 h) to suppress plasma ACTH. On day 14 aldosterone secretion was measured after a 30-min infusion of Ang II (330 ng/min). Ang II infusion increased the peak plasma aldosterone levels to a lesser degree in the OVX+E2 than in the OVX rats (OVX, 1870 +/- 290 pg/ml; OVX+E2, 1010 +/- 86 pg/ml; P < 0.05). Ang II-induced ACTH and aldosterone secretion was also studied in rats that were not treated with DEX. In the absence of DEX, the peak plasma aldosterone response was also significantly decreased (OVX, 5360 +/- 1200 pg/ml; OVX+E2, 2960 +/- 570 pg/ml; P < 0.05). However, E2 also reduced the plasma ACTH response to Ang II (P < 0.05; OVX, 220 +/- 29 pg/ml; OVX+E2, 160 +/- 20 pg/ml), suggesting that reduced pituitary ACTH responsiveness to Ang II contributes to the effect of E2 on Ang II-induced aldosterone secretion. Adrenal AT1 binding studies confirmed that E2 significantly reduces adrenal AT1 receptor expression in both the presence and absence of DEX in NaCl-deprived rats. These results indicate that E2-induced decreases in pituitary and adrenal AT1 receptor expression are associated with attenuated pituitary ACTH and adrenal aldosterone responses to Ang II and suggest that estrogen replacement therapy may modulate Ang II-stimulated aldosterone secretion as part of its well known cardioprotective actions.

Rat model for investigating ACTH-independent angiotensin-induced aldosterone secretion.

Study of the acute effects of angiotensin II (Ang II) on aldosterone secretion has been hindered by the confounding influence of Ang II-induced adrenocorticotropic hormone (ACTH) secretion on aldosterone secretion, and by the fact that when laboratory rats are fed standard laboratory chows that are high in sodium, the adrenal is only minimally responsive to Ang II. In this study, we report the development of a model of Ang II-induced aldosterone secretion in NaCl-deprived, dexamethasone (DEX)-treated rats. This model allows the observation of (a) a high magnitude of Ang II-induced aldosterone secretion, (b) a return of plasma aldosterone levels to baseline after stimulation, and (c) aldosterone secretion without the potentially confounding influence of ACTH stimulation.

Differential effects of pregnancy on mineralocorticoid and glucocorticoid receptor availability and immunoreactivity in cortisol feedback sites.

The suppression of plasma adrenocorticotropic hormone by very low levels of cortisol is reduced in pregnant adrenalectomized ewes, suggesting that pregnancy reduces the efficacy of the high-affinity corticosteroid receptor. This study was designed to determine the effects of pregnancy on the availability, immunoreactivity, and affinity of both corticosteroid receptors: the high-affinity mineralocorticoid receptor (MR) and the lower-affinity glucocorticoid receptor (GR). Availability was measured in the hypothalamus, pituitary, hippocampus and kidney using a saturation point radioligand binding assay. GR availability was significantly decreased in hippocampal cytosols obtained from pregnant ewes, but did not significantly change in other tissues. This finding is consistent with increased GR activation due to elevated circulating concentrations of cortisol. MR availability significantly increased from undetectable levels in hippocampal cytosols obtained from nonpregnant ewes to 2.8 +/- 1.6 fmol/mg protein in pregnant ewes, suggesting a reduced MR activation in the hippocampus during pregnancy. MR availability tended to be greater in other tissues during pregnancy, but these differences were not significant. The amount of immunoreactive MR (iMR) and GR (iGR) protein was estimated by quantifying Western blots. iGR significantly increased in the pituitary, but did not significantly change in other tissues. In contrast, iMR was significantly increased during pregnancy in all tissues assayed, suggesting that an increased cytosolic MR protein amount contributes to the observed increase in MR availability. Since studies suggest that progesterone is a potent anticorticosteroid, we tested for evidence of endogenous inhibition of binding to MR and/or GR during pregnancy by determining MR and GR affinity in pituitary cytosols obtained from nonpregnant and pregnant ewes. Although there was a tendency towards a decreased affinity of the MR in pregnant ewes, there was no significant change in the K(D) of the pituitary MR or GR during pregnancy. We hypothesize that an alteration in activation and/or autoregulation of the MR during pregnancy, particularly in the hippocampus, may contribute to the observed changes in receptor availability and immunoreactivity and increase basal plasma cortisol levels during pregnancy.

Progesterone rapidly reduces arterial pressure in ewes.

Chronic progesterone treatment decreases mean arterial pressure (MAP) and expands plasma volume. Evidence now suggests that progesterone metabolites have rapid nongenomic actions on the baroreflex. This experiment tests for a rapid effect of progesterone on MAP,Na+, arginine vasopressin, and baroreflex sensitivity. Ewes were studied during 2-h infusions of vehicle or progesterone at 3 and 6 micrograms.kg-1.min-1. Infusion of progesterone at 3 micrograms.kg-1.min-1 resulted in progesterone levels characteristic of ovine pregnancy and significantly reduced MAP by the 17th minute, suggesting a nongenomic mechanism. However, progesterone infusion at 6 micrograms.kg-1.min-1 produced supraphysiological progesterone levels and failed to modify MAP. Overall baroreflex sensitivity was not altered by either dose of progesterone, but the slope of the tachycardic response to hypotension tended to be attenuated after infusion of progesterone at 6 micrograms.kg-1.min-1. We speculate that the lack of a simple, linear dose-response effect of progesterone on blood pressure and baroreflex sensitivity can be explained by progesterone action at multiple receptor populations.