BIOCHEMISTRY: A TEXTBOOK FOR MEDICAL UNIVERSITIES - E. S. Severin - 2004

CHAPTER 11. HORMONAL REGULATION OF METABOLISM AND BODY FUNCTIONS

VI. Regulation of Water and Salt Metabolism

The critical parameters of Water-salt Homeostasis include osmotic pressure, pH, and the volume of intracellular and extracellular fluid. Alterations in these parameters can lead to Blood pressure fluctuations, acidosis or alkalosis, dehydration, and tissue edema. The primary Hormones Involved in the fine-tuning of water-electrolyte balance—acting on the distal convoluted tubules and collecting ducts of the Kidneys—are antidiuretic hormone (ADH), aldosterone, and atrial natriuretic peptide (ANP).

A. Antidiuretic Hormone

Antidiuretic hormone (ADH), also known as vasopressin, is a peptide with a Molecular Weight of approximately 1100 D, consisting of 9 Amino Acids linked by a single disulfide bridge.

1. Synthesis and Secretion of Antidiuretic Hormone

ADH is synthesized in hypothalamic Neurons as a preprohormone precursor, which enters the Golgi apparatus and is converted into a prohormone. Packed into neurosecretory granules, the prohormone is transported to the nerve terminals of the posterior Pituitary Gland (neurohypophysis). During transport, the prohormone undergoes Processing, being cleaved into the mature hormone and a carrier protein known as neurophysin. Granules containing mature antidiuretic hormone and neurophysin are stored in the axonal terminals within the posterior pituitary, from which they are secreted into the bloodstream upon appropriate stimulation.

The stimulus triggering ADH secretion is an elevated sodium ion concentration and increased Osmotic Pressure of the extracellular fluid. In cases of inadequate water intake, excessive sweating, or high salt consumption, hypothalamic osmoreceptors sensitive to fluctuations in osmolarity detect an increase in BLOOD OSMOTIC PRESSURE. Nerve impulses are generated and transmitted to the posterior pituitary gland, stimulating ADH release. ADH secretion is also stimulated by signals from atrial baroreceptors. Even a 1% change in osmolarity leads to noticeable shifts in ADH secretion.

2. MECHANISM OF ACTION

There are two types of receptors for ADH: V₁ and V₂. V₂ receptors, which mediate the primary Physiological effects of the hormone, are located on the basolateral membrane of the collecting duct Cells and distal tubules—the principal target cells for ADH, which are normally relatively impermeable to water molecules. In the absence of ADH, urine remains unconcentrated and can be excreted in volumes exceeding 20 L per day (normal is 1.0 — 1.5 L per day). Binding of ADH to V₂ receptors (Fig. 11-32) stimulates the adenylate cyclase system and activates protein kinase A. In turn, protein kinase A phosphorylates Proteins that stimulate the expression of the Gene encoding the membrane water channel protein, aquaporin-2. Aquaporin-2 translocates to the apical membrane of the collecting ducts and inserts into it, forming water channels. This ensures the selective permeability of The Cell membrane to water, allowing water molecules to freely diffuse into the renal tubular cells and subsequently enter the interstitial space. Because this process results in the reabsorption of water from the renal tubules and The excretion of a small volume of highly concentrated urine (antidiuresis), the hormone is termed the antidiuretic hormone.

Class="center">Fig. 11-32. Biological action of ADH in renal tubular cells. 1 — ADH binds to the V₂ membrane receptor, activating adenylate cyclase (AC) and generating cAMP; 2 — cAMP activates protein kinase, which phosphorylates target proteins; 3 — phosphorylated proteins induce the METABOLISM/31.html">Transcription of the aquaporin gene; 4 — aquaporin is incorporated into the membrane of the renal tubular cell.

V₁-type receptors are localized in the membranes of vascular smooth Muscle cells (VSMCs). The interaction of ADH with V₁ receptors activates phospholipase C, which hydrolyzes phosphatidylinositol 4,5-bisphosphate to produce Inositol trisphosphate and diacylglycerol. Inositol trisphosphate triggers the release of Ca²⁺ from The Endoplasmic reticulum (ER). The functional result of hormone action via V₁ receptors is the contraction of the vascular smooth muscle layer. The vasoconstrictive effect of ADH manifests at high hormone concentrations. Because the affinity of ADH for V₂ receptors is higher than for V₁ receptors, its antidiuretic effect predominates at physiological concentrations.

3. Diabetes Insipidus

ADH deficiency, caused by posterior pituitary dysfunction or impairments in hormone signaling pathways, leads to The Development of diabetes insipidus. This condition is characterized by unregulated water excretion, with systemic dehydration being its most dangerous consequence.

The term "diabetes insipidus" encompasses disorders of diverse etiologies. For instance, the primary causes of central diabetes insipidus may include Genetic Defects in hypothalamic prepro-ADH synthesis, defects in pro-ADH processing and transport (hereditary form), or damage to the Hypothalamus or neurohypophysis (e.g., resulting from traumatic Brain injury, tumors, or ischemia). Nephrogenic diabetes insipidus occurs due to Mutations in the V₂-type ADH receptor gene (hereditary form), rendering the kidneys unresponsive to the hormone. The primary manifestation of diabetes insipidus is hypotonic polyuria, i.e., the excretion of large volumes of low-density urine. Reduced ADH secretion also leads to excessive water intake (polydipsia). Diagnostic criteria for diabetes insipidus include marked polyuria (up to 20 L per day, urine density <1.010, compared to the normal 1.020).

B. Aldosterone

Aldosterone is the most potent mineralocorticoid, synthesized in the adrenal cortex from Cholesterol.

Synthesis and secretion of aldosterone by Cells of the zona glomerulosa are directly stimulated by low plasma Na⁺ and high K⁺ concentrations. Aldosterone secretion is also influenced by Prostaglandins and ACTH. However, The most significant regulation of aldosterone secretion is exerted by the Renin-Angiotensin System.

Aldosterone lacks specific transport proteins, but through weak interactions, it can form complexes with albumin. The hormone is rapidly cleared by the Liver, where it is converted into tetrahydroaldosterone-3-glucuronide and excreted in the urine.

1. Mechanism of Action of Aldosterone

In target cells, the hormone interacts with receptors localized in either The Nucleus or the Cytosol. The resulting hormone-receptor complex binds to specific DNA sequences and alters the transcription rate of target genes. The physiological outcome of aldosterone action includes the induced synthesis of: a) Na⁺ transporter proteins that facilitate sodium entry from the tubular lumen into the renal epithelial cell; b) Na⁺, K⁺-ATPase, which pumps sodium ions out of the renal tubular cell into the extracellular space while importing potassium ions from the extracellular space into the cell; c) potassium ion transporter proteins facilitating K⁺ efflux from renal tubular cells into the primary urine; d) mitochondrial Enzymes of the TCA cycle, particularly citrate synthase, which stimulate The production of ATP molecules required for Active ion transport (Fig. 11-33).

Fig. 11-33. Mechanism of action of aldosterone. Aldosterone interacts with intracellular receptors and stimulates Protein Synthesis: 1 — increases Na⁺ reabsorption from urine; 2 — induces the synthesis of TCA cycle enzymes, whose activity ensures ATP production; 3 — activates Na⁺, K⁺-ATPase, maintaining a low intracellular sodium ion concentration and a high potassium ion concentration.

The net biological effect of aldosterone-induced proteins is an increased reabsorption of sodium ions in the nephron tubules, leading to renal retention of NaCl and increased potassium excretion.

2. The Role of the Renin-Angiotensin-Aldosterone System in Water-Salt Balance Regulation

The renin-angiotensin system serves as the primary mechanism regulating the synthesis and secretion of aldosterone.

Renin is a proteolytic enzyme produced by juxtaglomerular cells located along the terminal portion of the afferent arterioles entering the renal corpuscles (Fig. 11-34).

Fig. 11-34. The renin-angiotensin-aldosterone system. Renin, a proteolytic enzyme, catalyzes The conversion of angiotensinogen (a glycoprotein) into angiotensin I (a decapeptide). 1 — renin, a proteolytic enzyme, catalyzes the conversion of angiotensinogen (a glycoprotein) into angiotensin I; 2 — angiotensin I is converted into angiotensin II by ACE, which cleaves two amino acid residues from the decapeptide; 3 — angiotensin II stimulates the synthesis and secretion of aldosterone; 4 — angiotensin II causes vasoconstriction of peripheral Arteries; 5 — aldosterone stimulates Na⁺ reabsorption and K⁺ excretion; 6, 7, 8, 9 — feedback inhibition of renin and aldosterone secretion via a negative feedback mechanism. Dashed lines indicate feedback regulation.

Juxtaglomerular cells are particularly sensitive to decreases in renal perfusion pressure. A drop in blood pressure (due to Hemorrhage, fluid loss, or decreased NaCl concentration) is accompanied by a fall in perfusion pressure within the afferent arterioles of the glomerulus, triggering a corresponding stimulation of renin release.

Angiotensinogen serves as the substrate for renin. Angiotensinogen is an α₂-globulin containing more than 400 amino acid residues. It is synthesized in the liver, a process stimulated by glucocorticoids and estrogens. Renin hydrolyzes a peptide bond within the angiotensinogen molecule, cleaving off the N-terminal decapeptide (angiotensin I), which lacks biological activity.

Under the action of carboxydipeptidyl peptidase, or angiotensin-converting enzyme (ACE)—found in endothelial cells, the Lungs, and Blood Plasma—Two amino acids are removed from the C-terminus of angiotensin I, yielding the octapeptide angiotensin II.

Binding to specific receptors located On the surface of cells in the zona glomerulosa of the adrenal cortex and vascular smooth muscle cells, angiotensin II induces changes in intracellular concentrations of diacylglycerol and inositol trisphosphate. Inositol trisphosphate stimulates the release of Calcium Ions from the endoplasmic reticulum, which together activate protein kinase C, thereby mediating the specific cellular biological response to angiotensin II.

With the participation of aminopeptidases, angiotensin II is converted into angiotensin III, a heptapeptide exhibiting The activity of angiotensin II. However, the concentration of this heptapeptide in blood plasma is four times lower than that of the octapeptide, meaning most effects are mediated by angiotensin II. Further degradation of angiotensin II and angiotensin III proceeds via specific proteases (angiotensinases).

Angiotensin II exerts a stimulatory effect on the production and secretion of aldosterone by the cells of the adrenal cortex zona glomerulosa. Aldosterone, in turn, promotes the retention of sodium ions and water, thereby restoring body fluid volume. Additionally, when present in high concentrations in the blood, angiotensin II exerts a powerful vasoconstrictive effect, thereby increasing blood pressure.

3. Restoration of Blood Volume during Dehydration

A decrease in total fluid volume—resulting from, for example, blood loss, severe vomiting, or diarrhea—triggers the release of renin. This process is also facilitated by a reduction in afferent signaling from atrial and arterial baroreceptors caused by diminished intravascular fluid volume. Consequently, there is an increase in the production of angiotensin II, the most potent stimulus for aldosterone secretion. Elevated blood concentrations of aldosterone induce sodium retention, which serves as a signal for hypothalamic osmoreceptors and stimulates the release of antidiuretic hormone (ADH) from nerve endings in the anterior pituitary, promoting water reabsorption from the collecting ducts. By exerting a potent vasoconstrictive effect, angiotensin II increases blood pressure and additionally enhances thirst. The water ingested is retained in the body to a greater extent than under normal conditions. The increase in fluid volume, along with the rise in blood pressure, eliminates the stimulus

that originally activated the renin-angiotensin system and aldosterone secretion, ultimately restoring blood volume (Fig. 11-35).

Fig. 11-35. Diagram illustrating the restoration of blood volume during hemorrhage and dehydration. 1 — decreased fluid volume and lowered blood pressure activate the renin-angiotensin-aldosterone system; 2 — angiotensin II causes vasoconstriction as an emergency measure to maintain blood pressure; 3 — aldosterone stimulates sodium retention, leading to vasopressin release and enhanced water reabsorption; 4 — angiotensin II also induces thirst, which helps increase body fluid levels.

4. Hyperaldosteronism

Hyperaldosteronism is a condition caused by the hypersecretion of aldosterone by the Adrenal Glands. Primary hyperaldosteronism (Conn's syndrome) is caused by an Adrenal Adenoma in approximately 80% of patients; in the remaining cases, it results from diffuse hyperplasia of the aldosterone-producing cells in the zona glomerulosa. In primary hyperaldosteronism, excess aldosterone enhances sodium reabsorption in the renal tubules. The resulting increase in plasma Na⁺ concentration acts as a stimulus for ADH secretion and renal water retention. Furthermore, the excretion of potassium, magnesium, and protons is increased. This leads to hypernatremia—which, notably, causes Hypertension, hypervolemia, and edema—as well as hypokalemia (leading to muscle weakness), magnesium deficiency, and mild metabolic alkalosis.

Secondary hyperaldosteronism is much more common than primary hyperaldosteronism and may be associated with various conditions (such as Heart Failure, Chronic Kidney Disease, or renin-secreting tumors accompanied by Impaired blood supply). In secondary hyperaldosteronism, patients exhibit elevated levels of renin and angiotensin II, which stimulate the adrenal cortex to produce and secrete excessive amounts of aldosterone. Clinical symptoms are less pronounced than in primary aldosteronism. Simultaneous determination of plasma aldosterone concentration and renin activity allows for a definitive Differential Diagnosis between primary (plasma renin activity is decreased) and secondary (plasma renin activity is increased) hyperaldosteronism.

C. Atrial Natriuretic Peptide (ANP)

ANP is a 28-amino-acid peptide containing a single disulfide bridge. It is synthesized primarily in atrial cardiomyocytes and stored as a preprohormone consisting of 126 amino acid residues.

The primary factor regulating the secretion of atrial natriuretic peptide is an increase in blood pressure. Other secretory stimuli include increased plasma osmolarity, elevated heart rate, and elevated blood levels of catecholamines and glucocorticoids.

The main target cells of ANP are the kidneys and peripheral arteries. In the kidneys, ANP stimulates dilation of the afferent arterioles, increases renal blood flow, and raises the Glomerular Filtration rate and sodium ion excretion. In peripheral arteries, ANP decreases smooth muscle tone and consequently dilates arterioles (Fig. 11-36). Thus, the net action of ANP is an increase in Na⁺ excretion and a reduction in blood pressure.

Fig. 11-36. BIOLOGICAL EFFECTS OF ANP. 1 — inhibits renin release; 2 — inhibits aldosterone secretion; 3 — inhibits ADH secretion; 4 — induces vasodilation.

The mechanism of ANP signaling does not involve G-protein activation. The ANP receptor has a domain Structure: a Ligand-binding domain located in the extracellular space, and a single transmembrane domain with guanylyl cyclase activity. In the absence of ANP, its receptor is in a phosphorylated and inactive state. Binding of ANP to the receptor induces conformational changes and an increase in the receptor's guanylyl cyclase activity. As a result, GTP is converted into cyclic GMP (cGMP), which activates protein kinase G (see Section 5).

ANP is generally considered a physiological antagonist of angiotensin II, since its effects lead not to vasoconstriction and sodium retention, but rather to vasodilation and increased renal salt excretion.