Evidence against a central pressor mechanism for adrenocortical steroid hypertension in sheep

Cardiovascular actions of CRH and urocortin: an update

Urocortin is a potent regulator of cardiac function, with actions that are prolonged in experimental animals. These changes are mediated via binding to CRH receptors found in peripheral tissues. The diversity of actions of urocortin on behaviour, appetite, inflammation and the cardiovascular system suggest that this peptide may be an endogenous factor mediating actions previously attributed to CRH. The present review will focus on the recent understanding of mechanisms mediating the cardiovascular actions of urocortin and CRH reported to date.

Glucocorticoid-induced renal vasodilatation is mediated by a direct renal action involving nitric oxide

Glucocorticoids increase renal blood flow (RBF) and glomerular filtration rate, but the mechanisms are unclear. We investigated whether the cortisol-induced increment in RBF is a direct renal action or secondary to its systemic effects and whether nitric oxide (NO) plays a role in this response. In conscious sheep, cortisol infused intravenously (5.0 mg/h) or into the renal artery (1.3 mg/h) for 5 h increased RBF by 66 +/- 8 and 53 +/- 11 ml/min, respectively. Plasma glucose was increased by intravenous cortisol (0.4 +/- 0.1 mmol/l) but not by intrarenal cortisol. Renal vein plasma cortisol levels were similar at the end of each infusion (193 +/- 31 intravenously; 151 +/- 25 nmol/l intrarenal), but systemic levels were different (277 +/- 31 intravenous; 69 +/- 10 nmol/l intrarenal). Inhibition of NO synthesis by N omega-nitro-L-arginine infused intravenously (10 mg/kg followed by 5 mg.kg-1.h-1) or intrarenally (2 mg.kg-1.h-1) significantly reduced the cortisol-induced renal vasodilatation. In contrast, constriction of the renal vasculature with intrarenal angiotensin (0.3 microgram/h) did not prevent the cortisol-induced renal vasodilatation. These findings demonstrate that cortisol acts directly on the kidney to cause renal vasodilatation and to increase RBF and suggest that this response involves the endothelium-derived relaxing factor NO.

Central hypertensive effects of aldosterone

The soluble mineralocorticoid receptor bound to an agonist acts as a transcription factor for several genes relevant to ion transport by kidney and colon epithelial cells and is a major regulator of electrolyte and fluid homeostasis. Mineralocorticoids, the most prominent of which is aldosterone, also influence the activity of nonepithelial target cells, including vascular smooth muscle cells, by altering intracellular ion transport and content. Evidence is summarized for mineralocorticoid modulation of neuronal activity in a center or centers within the brain, probably in the periventricular area of the anterior hypothalamus, where information on electrolyte, fluid, and cardiovascular status is received and integrated, resulting in alterations in central sympathetic efferent activity. These functions are distinct from central aldosterone effects on salt appetite and peripheral trophic effects on cardiovascular tissue. The isolated mineralocorticoid receptor binds several adrenal steroids, including aldosterone and the major glucocorticoids, with equal affinity. Ligand specificity for the mineralocorticoid receptor differs between tissues, including different organs in the brain. Specificity is conferred extrinsically by the 11-beta-hydroxysteroid dehydrogenase enzymes in transport epithelia, but mechanisms for mineralocorticoid ligand specificity have not been completely defined in the brain. The functional interaction between the mineralocorticoid receptor bound to different ligands and between the mineralocorticoid and glucocorticoid receptors is complex and as yet unresolved. Evidence is presented for the de novo synthesis of adrenal corticosteroids in the brain which may, by paracrine regulation of central control mechanisms, be relevant for certain clinical and experimental forms of hypertension characterized by low circulating levels of mineralocorticoids which respond to mineralocorticoid receptor antagonists.