Pharmacology and Physiology of Ovine Corticosteroid Receptors

Abstract

The aim of these studies was to characterize the ovine corticosteroid receptors (MR, mineralocorticoid receptors and GR, glucocorticoid receptors) in ovine hippocampus and brainstem. Adrenal-intact and adrenalectomized ewes were studied; adrenalectomized ewes were killed 47 ± 9 h after steroid withdrawal, when symptoms of hypotension and/or hyperkalemia became evident. RT-PCR, immunoblotting and pharmacologic studies indicated the presence of both MR and GR in hippocampus and brainstem. Competitive binding studies using ³H-cortisol in brain tissue showed that the ovine MR binds cortisol, aldosterone and progesterone with equal affinity. Differences in receptor availability in intact and adrenalectomized ewes, along with determination of the binding affinity (K_d) of MR and GR, suggested that MR occupancy is about 90%, whereas GR occupancy is about 30%, in normal animals. There was a significant increase in protein level of MR in brainstem, and the appearance of a higher molecular weight band for MR in hippocampus following steroid withdrawal, however no significant change in mRNA was detected by semiquantitative RT-PCR for either MR or GR in hippocampus or brainstem following steroid withdrawal. These studies suggest that physiological ligands of MR in the sheep brain include progesterone and cortisol, and that, as in other species, affinity of MR for cortisol is greater than that of GR.

Differential effects of mineralocorticoid blockade on the hypothalamo-pituitary-adrenal axis in pregnant and nonpregnant ewes

“…We have found previously that progesterone is a physiological ligand for MR in sheep (35) as well as in humans (37). Progesterone shows a high affinity for MR, with only weak transactivational activity (7,32,37,41).…”

“…In the nonpregnant ewe, MR in the hippocampus are ϳ90% occupied at basal plasma levels of cortisol (35); occupancy appears to be reduced in pregnant ewes. This finding suggests reduced MR effects despite the higher plasma cortisol levels that occur with pregnancy.…”

“…In sheep, both receptors are expressed within key regulatory areas of the HPA axis: the hypothalamus, hippocampus, brainstem, and pituitary (25,35,36). Activation of central and pituitary GR leads to feedback inhibition of stress-induced HPA axis activation, thereby reducing ACTH and cortisol secretion (24).…”

Cardiac corticosteroid receptors mediate the enlargement of the ovine fetal heart induced by chronic increases in maternal cortisol

“…This is consistent with the ability of cortisol to alter myocyte growth in cultured myocytes. We also hypothesized that blockade of MR would have a greater effect in inhibiting the effect of cortisol than would blockade of GR, because MR has been shown to have greater affinity for cortisol than GR (Reul & DeKloet 1985, Richards et al 2003. Indeed, this is what we observed in the present study: in the cortisol group, there was a 14% increase in heart weight relative to body weight when compared with the control group; this enlargement was completely blocked when MR antagonist was administered to the heart, whereas there was only 44% blockade of the increase in weight after administration of the GR antagonist.…”

“…9 nM in the fetuses of the cortisol-infused ewes. Based on previous studies of cortisol binding at ovine MR and GR (Richards et al 2003), we would predict that these free concentrations would result in w65% occupancy of MR and 35% occupancy of GR in the control fetuses, and 85% occupancy of MR and 60% occupancy of GR in the cortisol-infused fetuses. Thus, these levels would be expected to exert more effects via MR than via GR activation if both act at GRE to induce genes responsible for cardiac growth.…”