Donald P. Island

Role of the Sympathetic Nervous System in Regulating Renin and Aldosterone Production in Man

Several lines of evidence have been developed indicating that the sympathetic nervous system may play a role in mediating the renal and adrenocortical secretory responses to upright posture and sodium deprivation. Despite concurrent increases in arterial blood pressure, the plasma renin activity of normal subjects increased both in response to the infusion of catecholamines (norepinephrine: epinephrine, 10:1) and in response to stimulation of the sympathetic nervous system by cold. Aldosterone excretion was also increased by catecholamine infusion. In normal subjects the stimuli of upright posture and of sodium depletion both resulted in increases in urinary catecholamines, plasma renin activity, and urinary aldosterone. A patient with severe autonomic insufficiency did not experience normal elevations of urinary catecholamines, plasma renin activity, or urinary aldosterone in response to upright posture or sodium deprivation, despite a substantial fall in arterial blood pressure. When orthostatic hypotension was prevented by infusion of catecholamines, however, increases in plasma renin activity and in aldosterone excretion were observed. We suggest that both upright posture and sodium depletion lead to decreases in effective plasma volume and increases in sympathetic nervous system activity. This increase in sympathetic activity is then responsible for an increase in renal afferent arteriolar constriction, leading to an increase in renin secretion and, ultimately, an increase in aldosterone secretion.

A Diurnal Rhythm in Plasma Renin Activity in Man

Although it has been known for many years that the renin-angiotensin system is responsive to decreases in renal perfusion pressure (1-3), it was only recently discovered that angiotensin is an important regulator of aldosterone secretion (4-6). Before it was known that angiotensin had a role in the regulation of aldosterone secretion, a large number of studies indicated that experimental or pathological conditions that resulted in a reduction of effective blood volume would stimulate al-dosterone secretion (7). Studies have already been published indicating that plasma renin activity and aldosterone excretion both increase in response to sodium deprivation (8-10) and assumption of upright posture (11-13). In the course of studies attempting to correlate plasma renin activity with aldosterone excretion we found that the rises in plasma renin activity and in urinary aldosterone that occur after the assumption of upright posture early in the day are both of limited duration and that they diminish late in the day despite continued maintenance of upright posture (14). The observed afternoon fall in plasma renin activity is not readily explained by existing knowledge of the physiology of renin secretion. The present study, therefore, focuses attention for the first time on the question of whether there might be a consistent diurnal rhythm in plasma renin activity. In order to characterize diurnal variations in plasma renin activity that might occur independently of dietary and postural influences , we studied normal subjects during continuous recumbency while they received constant diets in equal portions at regular intervals throughout the day and night. To determine whether the rise in plasma renin activity that occurs in response to upright posture is due to loss of blood into the lower extremities, we observed the effect of bandaging the lower abdomen, hips, and legs before the subjects assumed upright posture. In order to determine whether the afternoon fall in plasma renin activity of upright subjects was a function of the time of day rather than the duration of upright posture, we compared the effect of rising at noon with the effect of rising at 8 a.m. We also performed studies to assess the importance of sodium and water intake and of al-dosterone secretion in bringing about the afternoon decline in plasma renin activity.

Dopamine Inhibits Angiotensin-Stimulated Aldosterone Biosynthesis in Bovine Adrenal Cells

The possibility that dopamine may play a role in the in vivo control of aldosterone production in man was suggested to us by reports from others; (a) that bromocriptine, a dopaminergic agonist, inhibits the aldosterone response to diuresis and to the infusion of angiotensin or ACTH; and (b) that metaclopramide, a dopamine blocking agent, causes elevations in plasma aldosterone levels. To determine whether such effects were direct or indirect, we examined the action of dopamine on aldosterone biosynthesis in isolated, bovine adrenal cells. Dopamine significantly inhibits the aldosterone response to angiotensin (P < 0.001), but does not influence basal aldosterone biosynthesis. It has previously been reported that angiotensin stimulates both the early and late phases of aldosterone biosynthesis. The present experiments demonstrated that the enhancing effect of angiotensin on the conversion of deoxycorticosterone to aldosterone (late phase of aldosterone biosynthesis) was almost completely inhibited by dopamine (P < 0.001). A significant inhibitory effect of dopamine (10 nM) was seen even when aldosterone biosynthesis was stimulated by a grossly supraphysiological concentration of angiotensin II (10 muM). However, these studies did not demonstrate any direct effect of dopamine on the early phase of aldosterone biosynthesis (cholesterol to pregnenolone) basally or when stimulated, or on the late phase of aldosterone biosynthesis under basal conditions. These in vitro studies suggest a direct inhibitory role for dopamine on the late phase of aldosterone biosynthesis, which may account for the in vivo inhibition of the aldosterone response to angiotensin in subjects treated with a dopaminergic agent.

CORRELATION OF PLASMA ACTH CONCENTRATION WITH ADRENOCORTICAL RESPONSE IN NORMAL HUMAN SUBJECTS, SURGICAL PATIENTS, AND PATIENTS WITH CUSHING'S DISEASE.

The role of ACTH in various clinical disorders has been difficult to ascertain because the available assay methods have lacked the sensitivity necessary for valid quantitation of the hormone in the plasma of normal subjects (1-4). Even the method of Lipscomb and Nelson (5), the most sensitive practical bioassay procedure now available, usually requires the injection of at least 0.05 mUof ACTHper rat, if responses are to be elicited that will be statistically significant without the use of a prohibitive number of animals. It is usually impractical to inject more than 5 ml of crude human plasma into a single rat. Therefore, in order to be accurately measurable by this procedure, the concentration of ACTH in the plasma must be at least 0.05 mUper 5 ml, or 1 mUper 100 ml. Numerous studies indicate that normal plasma levels of ACTHare well below this concentration. Byr the adrenal ascorbic acid depletion assay method, Sydnor, Sayers, Brown, and Tyler (1) were unable to detect ACTH in plasma of normal subjects, even after attempting to extract the hormone with oxvcellulose in preparation for the bioassay. These workers concluded that blood ACTH concentrations of normal human subjects were less than 0.5 mUper 100 ml. Using a similar procedure, Fujita (3) estimated the normal level of ACTHto be about 1 mUper L, i.e., 0.1 mUper 100 ml. Cooper and Nelson (6) were able to detect ACTH in the plasma of only 3 of 10 patients before surgery, by a method that they

Normal and abnormal regulation of β-MSH in man

A B S T R A C T The regulation of plasma 8-melanocyte-stimulating hormone (,8-MSH) in man has been studied utilizing a radioimmunoassay previously described (1). In normal subjects plasma p-MSH values ranged from 20 to 110 pg/ml. Metyrapone increased and dexamethasone decreased plasma P-MSH levels. Surgical stress stimulated f-MSH secretion. Plasma P-MSH levels were elevated in patients with untreated Addisons disease and untreated congenital adrenal hyperplasia, and these levels fell to normal during glucocorticoid therapy. In patients with Cushings syndrome due to pituitary adrenocorticotropic hormone (ACTH) excess, plasma fi-MSH was slightly elevated before treatment. In those patients who developed pituitary tumors and hyperpigmentation after bilateral adrenalectomy, plasma P-MSH was greatly elevated. In patients with Cushings syndrome due to adrenal tumor, plasma j9-MSH was subnormal. In patients with the ectopic ACTH syndrome, the levels of plasma f8-MSH were high. Plasma fi-MSH had a diurnal variation in normal subjects, patients with Addisons disease, and patients with congenital adrenal hyperplasia; but the normal diurnal variation was lost in patients with Cushings disease. In patients with high plasma 8-MSH, simultaneous determinations of plasma ACTH showed close correlation between the degree of elevation of ACTH and that of 8-MSH. In extracts of tumors from patients with the ectopic ACTH-MSH syndrome the quantities of the two hormones were roughly equivalent. In patients with hyperpigmentation due to a variety of disorders other than pituitary-adrenal abnormalities, plasma 8-MSH was normal. It is concluded that the secretion of P-MSH is regulated by the same factors that regulate ACTH.

Radioimmunoassay of β-MSH in Human Plasma and Tissues

: A radioimmunoassay method for beta-melanocyte-stimulating hormone (beta-MSH) has been developed and utilized in the identification and quantification of this hormone in human plasma and tissues. The concentration of beta-MSH in two human pituitary glands was found to be approximately 350 mug/g. beta-MSH was identified in the tumor tissue of all 11 patients with the ectopic ACTH syndrome who were studied; concentrations in individual cases ranged from 3 to 1600 ng/g. In plasma of chronically hyperpigmented patients with Addisons disease, Cushings disease (after bilateral adrenalectomy), and the ectopic ACTH syndrome, beta-MSH concentrations of 0.5-6 ng/ml were found. The degree of clinical hyperpigmentation was well correlated with the quantity of beta-MSH in the plasma. beta-MSH concentrations in the plasma of normal subjects were less than 0.09 ng/ml. In all of these circumstances, bioassays for MSH were also performed, and it was found that most of the biologic MSH activity of the plasma and tissues could be accounted for by beta-MSH.

Biologic and Immunologic Characterization and Physical Separation of ACTH and ACTH Fragments in the Ectopic ACTH Syndrome

Extracts of tumors from 32 patients with the ectopic ACTH syndrome were subjected to simultaneous bioassay and radioimmunoassays for ACTH. Radioimmunoassays were performed using three antisera, one of which reacts with the extreme N-terminal 1-13 amino acid sequence of ACTH, the second with the N-terminal 1-23 sequence of the ACTH molecule, and the third with the C-terminal 25-39 amino acid sequence of ACTH. There was, in general, good correlation between bioactivity and N-terminal ACTH immunoreactivity. However, there were large excesses of both extreme N-terminal and C-terminal immunoreactive materials in most tumor extracts, which were not found in extracts of three human pituitaries. Three tumor extracts were subjected to molecular sieve chromatography on Sephadex G-50 fine resin. The bioactive ACTH eluted in the same fractions as pituitary ACTH (mol wt approximately 4,500 daltons) and reacted equally in all three ACTH radioimmunoassay systems. The bioactive tumor ACTH was neutralized by incubation with the C-terminal antiserum, indicating it has an intact C-terminal sequence of amino acids. The next several fractions from the Sephadex column contained a material, mol wt approximately 3,100, which was biologically inactive and had C-terminal immunoreactivity but no N-terminal or extreme N-terminal immunoreactivity. Incubation with the N-terminal 1-23 ACTH antiserum did not adsorb these C-terminal fragments, indicating they lacked an intact sequence of amino acids in this region. A smaller ACTH fragment (mol wt approximately 1,800 daltons) eluted in still later fractions and reacted with the extreme N-terminal antiserum but not with the N-terminal or C-terminal antisera. It had no steroidogenic activity, but appeared to have significant melanocyte-stimulating activity. It is concluded that, in addition to an ACTH similar, if not identical, to pituitary ACTH, tumors of patients with the ectopic ACTH syndrome contain both N-terminal and C-terminal ACTH fragments.