Aldosterone secretion regulation

In an attempt to conserve sodium, the kidney secretes renin increased plasma renin activity increases the release of aldosterone, which regulates the absorption of potassium and leads to kafluresis and hypokalemia. Hypokalemia is responsible in part for decreased glucose intolerance (82). Hyponatremia, postural hypotension, and pre-renal azotemia are considered of tittle consequence. Hypemricemia and hypercalcemia are not unusual, but are not considered harmful. However, hypokalemia, progressive decreased glucose tolerance, and increased semm cholesterol [57-88-5] levels are considered... 

The mechanism by which potassium regulates aldosterone secretion is unclear however, this ion appears to have a direct effect on the adrenal cortex. An increase in the level of potassium in the blood stimulates the release of aldosterone. The effect of aldosterone on the kidney then decreases the level of potassium back to normal. 

There is another system involved in blood pressure regulation the renin-angiotensin-aldosterone system (Fig. 2). The arterial blood pressure in the kidney influences intrarenal baroreceptors which together with the sodium load at the macula densa lead to renin liberation, angiotensin formation and aldosterone secretion, which by influencing the sodium balance changes the blood volume and influences the arterial blood pressure. 

Without ACTH, aldosterone secretion falls to about half the normal rate, indicating that other factors, eg, angiotensin, are able to maintain and perhaps regulate its secretion (see Chapter 17). Independent variations between cortisol and aldosterone secretion can also be demonstrated by means of lesions in the nervous system such as decerebration, which decreases the secretion of cortisol while increasing the secretion of aldosterone. 

In addition to the angiotensin II effects, aldosterone secretion is regulated by increased plasma potassium levels.75,83 Presumably, elevated plasma potassium serves as a stimulus to increase aldosterone release, thus causing increased potassium excretion and a return to normal plasma levels. Finally, there is evidence that ACTH may also play a role in aldosterone release. Although ACTH is primarily involved in controlling glucocorticoid secretion, this hormone may also stimulate mineralocorticoid release to some extent.75... 

It was concluded from these and other studies in hypophysectomized animals that ACTH had no effect on aldosterone release. The volume-sensitive renal mechanism appears to be mainly responsible for postoperative aldosterone changes (S4), but it would now appear that ACTH also plays a part in regulating aldosterone secretion (S4). Removal of the pituitary leads to an immediate fall in aldosterone levels in adrenal venous blood (H9). A linear dose response relationship exists between the infusion rate of ACTH and aldosterone secretion rates (H9). Volume receptors in the right atrium and in the vascular tree respond to minor reductions in blood volume and play an important part in stimulating the aldosterone response (Bl, FI). Patients with suppression of cortisol production due to prolonged administration of steroids continue to secrete aldosterone and are able to increase their output after stress indicating the presence of another trophic factor as well as ACTH (T3). 

The renin-angiolensin system is a hormonal system that plays a central role in (he control of sodium excretion and body fluid volume. It interacts clasely with (he sympathetic nervous system and aldosterone secretion in (he regulation of blood pressure. Figure 19-13 shows Ihe relationship of Ihe component parts of (he renin-angiotensin system and (heir main physiological cffccis. 

The major regulators of aldosterone secretion are the renin-angiotensin system and extracellular potassium ions (K+). The former is sensitive to changes in intravascular volume and arterial pressure, while the latter is an aldosterone-regulated substance that feeds back to reduce aldosterone synthesis (simple negative feedback). Aldosterone secretion is also influenced (but not regulated) by ACTH and, directly and indirectly, by atrial natriuretic factor (ANF). 

A Singular Hormonal Mechanism. I suppose one can debate whether or not angiotensin should be called a hormone. It is an internal secretion from the kidneys and functions, at least in part, as a regulator not only of smooth muscle but of aldosterone secretion. If the catecholamines are considered hormones, why not angiotensin ... 

The demonstration that angiotensin is the chief regulator of aldosterone secretion, by Genest (4), Laragh (II, 12), and later Davis (I) and several others, created a sensation in cardiovascular and endocrine fields. In retrospect, it is easy to see why this makes sense from the body s point of view. 

A. Aldosterone The major natural mineralocorticoid in humans is aldosterone, which has already been mentioned in connection with hypertension (see Chapter 11) and control of its secretion by angiotensin II (see Chapter 17). The secretion of aldosterone is regulated by ACTH and by the renin-angiotensin system and is very important in the regulation of blood volume and blood pressure (see Figure 6 ). Aldosterone has a short half-life and little glucocorticoid activity (Table 39-1). Its mechanism of action is the same as that of the glucocorticoids. 

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