Patrick J. Mulrow

Stimulation of Aldosterone Biosynthesis in Adrenal Mitochondria by Sodium Depletion

The effect of various factors on the conversion of corticosterone to aldosterone was studied in an isolated mitochondrial system from rat adrenal glands. The adrenal mitochondrial fraction from rats on a low sodium diet has a greater capacity for converting corticosterone to aldosterone than mitochondria from rats fed a normal diet. After 1 day on a low sodium diet the amount converted was 162% and after the 2nd and 4th day the amounts converted were 239 and 242%, respectively, compared to a value of 100% for the control rats. Sodium and(or) potassium added in vitro did not affect the conversion of corticosterone to aldosterone. The specificity of the sodium depletion stimulus on the conversion of corticosterone to aldosterone was established by comparing two other mitochondrial enzymes from glomerulosa cell mitochondria. Succinic dehydrogenase and 11 beta-hydroxylase were measured in normal and sodium-depleted rats and no difference in activity of either enzyme was found. The data are consistent with the view that sodium depletion stimulates the last step in aldosterone biosynthesis by causing a specific enzymatic change in adrenal mitochondria.

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.

THE NATURE OF THE ALDOSTERONE-STIMULATING FACTOR IN DOG KIDNEYS*

Previous studies (2-4) have demonstrated that nephrectomy lowers control aldosterone secretion rates and prevents the increase in aldosterone secretion that follows hemorrhage. Injection of saline extracts of kidneys (3, 4) from normal dogs into hypophysectomized, nephrectomized dogs stimulated aldosterone secretion markedly while also stimulating 17-hydroxycorticoid and corticosterone secretion. A marked pressor response also occurred. These observations suggested that the kidney secreted an aldosterone-stimulating factor. The stimulation of glucocorticoid secretion by the saline kidney extracts raised the possibility that ACTH was present in the kidney extract. This possibility appeared to be supported by the finding of Richards and Sayers (5) that exogenous ACTH accumulated preferentially in the kidney. On the other hand, the pressor response produced by the saline kidney extracts suggested that the extracts contained renin. Considerable indirect evidence (6) suggested that the reninangiotensin system may play an important role in the regulation of aldosterone secretion. To investigate the nature of this kidney factor, the effect of several substances on adrenocortical secretion of hypophysectomized, nephrectomized dogs has been studied. Saline extracts of kidneys from hypophysectomized dogs, and dog anterior pituitaries, commercial ACTH, semipurified renin extracts, and synthetic angiotensin II have been assayed. The results indicate that the aldosteronestimulating factor in the dog kidney is renin.

Effect of Propranolol on Blood Pressure and Plasma Renin Activity in the Spontaneously Hypertensive Rat

Spontaneously hypertensive Wistar rats have normal basal levels of plasma renin activity but do not respond to sodium depletion or the stress of ether anesthesia plus laparotomy. Both age and rat strain influence the development of this hyporesponsive renin system: normal Wistar and spontaneously hypertensive rats but not Sprague-Dawley rats develop the hyporesponsive system as they age. Therefore, these rat strains may serve as useful models for studying the effects of age and hypertension on plasma renin activity. In the present study, propranolol administration up to 18 mg/kg day−1 for 15 days did not lower plasma renin activity but did cause a paradoxical rise in blood pressure in spontaneously hypertensive rats. This paradoxical rise was prevented by placing the rats on a low-sodium diet prior to propranolol administration. The mechanism of the paradoxical rise is unknown, but we suggest that it could be the result of increased alpha-adrenergic activity in response to a fall in cardiac output.

Steroid hydroxylations in rat adrenal mitochondria: I. Some properties of the C-18 and 11-β hydroxylations in tightly coupled mitochondria☆

Abstract Mitochondrial preparations have been isolated from cortex fragments, adrenal capsules, and intact rat adrenal glands and shown to have high ADP O and respiratory control ratios with all substrates tested. It was necessary to exclude Mg2+ because of an apparently nonmitochondrial Mg2+-stimulated ATPase. These preparations also gave high rates of both the C-18 and 11-β steroid hydroxylation. Steroid hydroxylation with β-hydroxybutyrate or α-ketoglutarate plus malonate was energy-dependent. With the latter substrate, conditions were found which made steroid hydroxylation dependent on both the enzymes of oxidative phosphorylation and on energy conserved at the substrate-linked phosphorylation site. Isocitrate supported steroid hydroxylation by a nonenergy-dependent pathway, presumably via the active NADP-linked isocitric dehydrogenase present in these preparations. Steroid hydroxylation supported by malate was decreased by KCN and uncouplers of oxidative phosphorylation. With malate plus either inorganic phosphate or arsenate as substrate, however, an additional KCN and uncoupler insensitive pathway for corticosterone production became evident. In this latter situation, pyruvate accumulated suggesting the activation of a malic enzyme-like activity by phosphate or arsenate in the intact adrenal mitochondria. It is concluded that rat adrenal mitochondria have efficient energy-conserving reactions, and that steroid hydroxylation may proceed by energy-dependent, nonenergy-dependent, or by both pathways simultaneously depending on the substrate utilized.

The Tissue Effects of Mineralocorticoids

Abstract Aldosterone is the major mineralocorticoid secreted by the human adrenal cortex and the kidney is its main target organ. Micropuncture investigations indicate an enhancement of sodium reabsorbtion and potassium excretion in the distal tubule by mineralocorticoids. Whether they affect the proximal tubule is controversial. Prolonged administration of mineralocorticoids to normal man results in transient sodium retention but persistent kaliuresis and the development of hypertension. The mechanism of this hypertension is still an enigma. In addition to an effect on sodium and potassium, aldosterone can influence the excretion of hydrogen, calcium and magnesium ions and the secretion of renin. Many extrarenal tissues are affected by aldosterone: the sweat and salivary glands, the gastrointestinal tract, skeletal and smooth muscle, heart, bone and red blood cells.

Evidence for a direct effect of angiotensin-II on adrenal cortex of the dog.

Summary In 6 nephrectomized hypophysectomized dogs, the increase in aldosterone and 17-hydroxycorticoid secretion produced by infusion of angiotensin-II into the arterial blood supplying the adrenal gland was greater than the increase produced by infusion of the same dose into the jugular vein. It is concluded that angiotensin-II can stimulate adrenocortical secretion by a direct action on the adrenal gland.

Role of the Kidney and the Renin-Angiotensin System in the Response of Aldosterone Secretion to Hemorrhage

In summary, these data show that in hypophysectomized dogs: 1. Hemorrhage stimulates mainly aldosterone secretion. 2. Nephrectomy prevents stimulation of aldosterone secretion by hemorrhage. 3. Nephrectomy lowers basal secretion of aldosterone. 4. Crude saline extracts of dog kidneys, semipurified renin extracts of dog kidneys, and synthetic angiotensin can stimulate aldosterone secretion. These findings suggest that in hypophysectomizeddogs, the kidney, via the renin-angiotensin system, is an important mechanism by which hemorrhage stimulates aldosterone secretion.

Corticosteroid production in vitro by the interrenal tissue of killifish, Fundulus heteroclitus (Linn).

Conclusions With the above reservations in mind, the following tentative conclusions can be drawn. 1. It seems probable that cortisol, in plasma obtained from Fundulus heteroclitus, is synthesized by interrenal tissue of head kidney. 2. Production of cortisol and probably cortisone by both left and right head kidneys suggests that interrenal tissue is present in both, although in lesser amounts on the left. If this is so, no explanation can be given at present for slightly larger amounts of aldosterone produced by left head kidneys. 3. Interrenal tissue of Fundulus possesses enzyme systems, capable of converting progesterone-16-H3 to tritiated cortisol, cortisone, and aldosterone through biosynthetic pathways not known.

SERUM DOPAMINE β-HYDROXYLASE AS AN INDEX OF SYMPATHETIC NERVOUS SYSTEM ACTIVITY IN MAN

Serum dopamine beta-hydroxylase (DBH) activity was comparable in eight adrenalectomized patients and in 41 normal volunteers (37.8 +/- 3.6 vs. 40.0 +/- 4.0 IU), but were very low in four patients with severe diabetic autonomic neuropathy (6.7 +/- 4.2). These observations suggest that the major source of DBH activity in human serum is probably the sympathetic nervous system. Serum DBH activity was also significantly lower in seven patients with low renin essential hypertension than in 17 patients with normal renin hypertension (19.9 +/- 4.3 vs. 43.9 +/- 5.0 IU, P less than 0.05). Dietary sodium depletion, a stimulus for catecholamine secretion, produced only a small increase in serum DBH activity in supine normal volunteers (26.5 +/- 10.4 to 33.9 +/- 11.4, P less than 0.05; n = 7) and in five adrenalectomized patients (33.0 +/- 12.7 to 44.1 +/- 15.2, P = 0.06). Plasma renin activity increased more than 3-fold in these same subjects, and did not correlate with serum DBH activity. Similarly, 4 hours of ambulation produced only a small increase in serum DBH activity which did not correlate with plasma renin activity. The small magnitude of these responses to physiological stimuli of the sympathetic nervous system diminishes the potential clinical usefulness of measurement of serum DBH activity. Further studies utilizing direct correlation of plasma and urine catecholamines with serum DBH activity are needed to establish its relationship to sympathetic nervous system activity.