THE EFFECT OF BILATERAL NEPHRECTOMY AND MIDCOLLICULAR DECEREBRATION WITH PINEALECTOMY AND HYPOPHYSECTOMY ON THE CORTICOSTEROID SECRETION OF SODIUM-DEFICIENT SHEEP

Hypothesis: aldosterone is synthesized by an alternative pathway during severe sodium depletion. 'A new wine in an old bottle'

1. The last three steps of aldosterone biosynthesis, 11 beta-hydroxylation, 18-hydroxylation and 18-oxidation, have been demonstrated to be catalysed by one enzyme, which is the cytochrome P450(11 beta) (CYP11B) in cow, pig, sheep and bullfrog or cytochrome P450aldo (CYP11B2) in rat, human, mouse and hamster. 2. The related enzyme P450(11 beta) (CYP11B1) from rat, human, mouse and hamster adrenals displays 11 beta-hydroxylation and 18-hydroxylation activities, but not 18-oxidation activity in vitro. No such enzyme has been reported in the cow, pig or sheep to date. 3. Data showing the dissociation of aldosterone secretion from plasma angiotensin II (AngII) levels indicate the presence of other factor(s) that regulate aldosterone biosynthesis in response to changes in body sodium status. Thus, we propose the existence of a 'sodium status factor' that regulates aldosterone biosynthesis in addition to AngII, K+, adrenocorticotropic hormone and atrial natriuretic peptide. 4. We propose that during severe sodium deficiency there is a switch in the aldosterone pathway to a pathway using 18-hydroxy-deoxycorticosterone (18-OH-DOC) rather than corticosterone as an intermediate. This switch may be mediated via the putative 'sodium status factor'. 5. Two models of the hypothesis will be discussed in this paper: (i) a 'one-enzyme' model; and (ii) a 'two-enzyme' model. 6. The one-enzyme model proposes that P450aldo (P450(11 beta) as in the case of the cow, sheep and pig) changes its enzymatic activity during severe sodium deficiency (i.e. switching to the alternative aldosterone biosynthesis pathway). 7. The two-enzyme model proposes that, under normal circumstances, P450aldo synthesizes aldosterone from deoxycorticosterone, while during severe sodium deficiency the P450(11 beta) provides the substrate (i.e. 18-OH-DOC) for the P450aldo.

The extrarenal effects of aldosterone on the distribution of extracellular fluid in conscious adrenalectomized-nephrectomized rats

In order to examine the extrarenal roles of aldosterone in cardiovascular homeostasis, the present study compared blood pressure, extracellular fluid volume (ECFV) and plasma volume (PV) in the three following groups: 10 nephrectomized (NX) rats, 10 nephrectomized-adrenalectomized (NX-AX) rats, and 10 NX-AX and aldosterone-treated (NX-AX-A) rats. Two-hundred and fifty micrograms of aldosterone, mixed with sesame oil, was given subcutaneously in the NX-AX-A rats. Mean arterial pressure (MAP) was recorded through previously implanted carotid catheters. ECFV and PV were measured using 35S-Na2SO4 and 131I-RISA, respectively, 24 h after the operation. These measurements were performed in an unanesthetized and unrestricted condition. MAP gradually increased in the NX group, while a gradual decrease was observed in the other groups. However, MAP was significantly higher in the NX-AX-A group than in the NX-AX group 6 h after the operation and thereafter. Changes in body weight were comparable in the three groups 24 h after the operation. ECFV and PV were both reduced in the NX-AX and the NX-AX-A groups as compared to the NX group (P less than 0.001 NX-AX vs NX, and P less than 0.05 and P less than 0.025 NX-AX-A vs NX, respectively). Although ECFV was comparable in the NX-AX and the NX-AX-A groups, PV was significantly greater in the NX-AX-A group than in the NX-AX group (P less than 0.05). There was a significant positive correlation between MAP and PV in the rats as a whole (r = 0.68, P less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Sites of action of sodium depletion on aldosterone biosynthesis in the dog

Secretion of cortisol, corticosterone, and aldosterone was measured in vivo in normal and sodium-depleted hypophysectomized dogs. Biogenesis of steroids was then measured in vitro with outer slices of the adrenals of the same dogs. In some studies, metyrapone or puromycin was added. In vivo, sodium depletion stimulated the production of cortisol, corticosterone, and aldosterone. In vitro, tissues from sodium-depleted animals released more aldosterone, but less corticosterone than those from sodium-replete controls. The results are interpreted to indicate that (a) biosynthesis of aldosterone is regulated at at least two sites in the biosynthetic pathway. The final conversion, that of corticosterone to aldosterone, is stimulated by sodium depletion. This effect persists for at least 3 hr while slices from sodium-depleted dogs are incubated in vitro. Stimulation at this site is thus relatively stable in vitro; its activation by sodium depletion is not inhibited by puromycin in the dog. Stimulation at this site can explain, at least in part, the increased effectiveness of adrenocorticotropin (ACTH) on aldosterone biogenesis during sodium depletion.(b) the earlier site at which sodium depletion stimulates the secretion of aldosterone is "above" the position of desoxycorticosterone in the pathway; it is probably at the conversion of cholesterol to pregnenolone. Stimulation at this site is quickly lost during incubation of adrenal slices. It is thus relatively unstable in vitro; its activation by sodium depletion is inhibited by puromycin in the dog.

Control of aldosterone secretion by the pituitary gland

In the rat, the pituitary gland is essential for the stimulation of aldosterone secretion by sodium depletion. Hypophysectomy abolishes the response to sodium depletion, whereas whole pituitary gland injections partially restore it. The response cannot be restored by injections of either adrenocorticotropin or growth hormone, nor by adrenocorticotropin plus thyroxin. The pituitary gland must secrete a hormone or possibly several hormones which are necessary for the adrenal gland to respond to sodium depletion.