Angiotensin binding to rat adrenal capsular cell suspensions

Physiological and pathological effects of steroids on the function of the adrenal cortex

The adrenal cortex is the site of the synthesis of steroid hormones such as the glucocorticoid cortisol and the mineralocorticoid aldosterone. The pathway of biosynthesis of these steroids from cholesterol involves a sequence of transformations using cytochrome P-450 enzymes which varies within the adrenal cortex as a result of the differential localization of enzymes within the zones. The hypothesis presented here is that as a result of the arrangement of the vasculature in the adrenal gland, high concentrations of steroids may be expected to accumulate and may have autoregulating effects. These may include the following: (1) the normal morphological and functional zonation of the adrenal cortex may be regulated by gradients of steroids in the adrenal cortex; (2) destruction of cytochrome P-450 enzymes on interaction with certain steroids which act as pseudosubstrates may form part of the pathogenesis of some steroidogenic enzyme deficiencies. Under normal conditions, the individual cytochrome P-450s are not rate-limiting for steroidogenesis. Under some pathological conditions, individual cytochrome P-450 enzyme activities may become rate-limiting, with consequent overproduction of precursor steroids, leading to mineralocorticoid or androgen excess.

Angiotensin receptors in vascular tissue

The biologic effect of angiotensin II is triggered by its interaction with components of target organs, which specifically recognize the hormone. These receptors have been studied with the use of radioactive angiotensin and, as for other peptidic hormones, have been localized in the plasma membrane of target cells. Such angiotensin receptors have been characterized in three target organs: vascular tissue, uterus and adrenal cortex. The binding characteristics differ in contractile tissue and in adrenal glands, the N and C terminal ends of angiotensin being involved in the former, whereas the N terminus does not appear to have the same importance in the latter. Numerous factors, including ionic composition, seem to be able to modify angiotensin-receptor interaction in vascular smooth muscle. However, the molecular mechanisms responsible for angiotensin binding and for the transmission of the signal determined by receptor-angiotensin interaction are not yet understood. As observed with other peptidic hormones, the number of angiotensin receptors seems to be susceptible to variation under certain conditions. In uterine smooth muscle, it was shown that the number of receptors increased after nephrectomy, a phenomenon which was prevented by the prolonged infusion of angiotensin. The significance of such a variation remains unknown, but it may be partially responsible for the inverse relationship that exists between the endogenous angiotensin level and the pressor effect of exogenous angiotensin. In the near future, investigation of the angiotensin-receptor mechanism will probably answer whether the variation in angiotensin receptors is similar in all target tissues and whether an angiotensin-receptor mechanism is involved in the pathogenesis of certain varieties of hypertension. In addition, a precise understanding of the angiotensin-receptor interaction with help the development of new angiotensin antagonists.

Variations in the number of uterine angiotensin receptors following changes in plasma angiotensin levels

3H-labelled angiotensin II binding to receptor sites was studied in plasma membranes isolated from myometrial homogenates of uterine horns. Removal of the kidneys, which results in the disappearance of plasma angiotensin II, was followed 19 h after nephrectomy by an increase in the number of uterine receptor sites without significant variation in the apparent dissociation constant. Acute pressor i.v. injection of angiotensin II into nephrectomized rats immediately before removing uteri, did not affect the number of uterine angiotensin receptors, whereas long-lasting angiotensin infusion did reduce the number of receptors. These changes cannot be accounted for by variations in the occupancy of receptor sites. These results demonstrate that the number of angiotensin receptors, at least in uterine contractile cells, is affected by chronic variations of endogenous angiotensin levels. The relation between the specific supersensitivity to angiotensin II observed in uteri from nephrectomized rats and the variations at the receptor level is discussed.

Selective inhibition by des-1-Asp-8-lle-angiotensin ii of the steroidogenic response to restricted sodium intake in the rat

Angiotensin III (des-1-Asp-angiotensin II) is a potent steriodogenic agent in many species. The effects of the heptapeptide in the adrenal zona glomerulosa are resistant to blockade by C-terminally substituted analogues of angiotensin II (1-Sar-8-Ile- or 1-Sar-8-Ala-octapeptides). For this reason, the effects of 7-Ile-angiotensin III, a C-terminally substituted analogue of the heptapeptide, and 1-Sar-8-Ile-angiotensin II on aldosterone biosynthesis in rabbit adrenal cortical cell suspensions and on urinary aldosterone excretion in sodium-deprived rats were studied. In the vitro studies, 7-Ile-angiotensin III was a better antagonist of angiotensin II- or angiotensin III-induced steroidogenesis than was 1-Sar-8-Ile-angiotensin II. In the rats, subcutaneously administered 1-Sar-8-Ile-angiotensin II (0.9 mumoles/kg) produced prolonged blockade of the pressor responses to exogenous angiotensin II. 70Ile-angiotensin III (0.9 mumoles/kg) had no effect on resting blood pressure or on blood pressure responses to angiotensin II infusions. At the doses studied, however, 7-Ile-angiotensin III caused a marked decrease (50%) in aldosterone excretion in sodium-deprived rats, but 1-Sar-8-Ile-angiotensin II had no effect on aldosterone excretion. In the sodium-deprived rats, the administration of 7-Ile-angiotensin Ile was not associated with an acute increase in plasma renin activity, but treatment with 1-Sar-8-Ile-angiotensin II resulted in a sixfold increase in plasma renin activity, but otensin III was not associated with an acute increase in plasma renin activity, but treatment with 1-Sar-8-Ile-angiotensin II resulted in a sixfold increase in plasma renin activity. These results are consistent with a role for angiotensin III in the control of aldosterone biosynthesis.

Angiotensin II- and angiotensin 3-induced aldosterone release vivo in the rat

The potential role of angiotensin II and its heptapeptide metabolite, des-aspartyl-angiotensin II, was studied in the conscious unanesthetized rat. Aldosterone release was induced by both peptides at physiologic doses (0.72 nanogram per minute). [I-Sarcosyl-8-alanyl]-angiotensin II (P-113 inhibited angiotensin II more effectively than des-aspartyl-angiotensin II (101 percent as compared to 82 percent). These results indicate that angiotensin controls aldosterone release in the rat and that des-aspartyl-angiotensin II (that is, angiotensin III) may be important in this sequence.