Vasopressin (ADH) and Vascular Tone Control
Antidiuretic hormone (ADH, vasopressin) plays a critical role in maintaining blood pressure. One of the pathways for its effects involves acting on vascular smooth muscle via specific $V_1$ receptors.
Mechanism of action via $V_1$ receptors:
- The hormone binds to $V_1$ receptors located on the membranes of vascular smooth muscle cells.
- The resulting complex activates the enzyme phospholipase C.
- An intracellular cascade is triggered, leading to the release of $Ca^{2+}$ ions from the endoplasmic reticulum.
- Increased intracellular calcium concentration causes contraction of the smooth muscle layer, leading to vasoconstriction (blood vessel narrowing).
Pathology: Diabetes Insipidus Disruption of the ADH system leads to diabetes insipidus. Etiologically, it is divided into two types:
- Central (hormone deficiency due to posterior pituitary dysfunction).
- Nephrogenic (impaired hormonal signal transmission at the renal receptor level).
The key clinical manifestation of the disease is polyuria, which is the pathological excretion of very large volumes of urine with a characteristically low specific gravity.
Aldosterone and Sodium Retention
Aldosterone is the most potent hormone in the mineralocorticoid group. Its primary function is the retention of $NaCl$ in the body, which directly impacts blood volume. Synthesis and Stimulation: The hormone is synthesized in the cells of the zona glomerulosa of the adrenal cortex, with cholesterol serving as its precursor. Aldosterone secretion increases sharply in response to:
- decreased $Na^+$ ion concentration in the blood;
- increased $K^+$ ion concentration;
- activation of the renin-angiotensin system (RAS).
Hypersecretion of this hormone by the adrenal cortex leads to a condition known as hyperaldosteronism.
Molecular Mechanism of Action: Upon reaching the kidneys via the bloodstream, aldosterone enters the renal tubular cells. There, it binds to its specific receptor (which may reside in the cytoplasm or nucleus). This hormone-receptor complex induces the synthesis of new functional proteins.
Role of Induced Proteins:
- They form channels that mediate $Na^+$ ion reabsorption.
- They establish conditions for $K^+$ ion excretion.
- They increase the number of membrane pumps—$Na^+, K^+$-ATPase.
- Some synthesized proteins act as enzymes of the tricarboxylic acid (TCA) cycle. This is necessary for enhanced ATP generation, as active ion transport requires substantial energy expenditure.
Atrial Natriuretic Peptide (ANP)
While the RAAS (the main regulator of water-electrolyte balance) aims to raise blood pressure and retain fluid, Atrial Natriuretic Peptide (ANP) acts as its direct physiological antagonist, specifically antagonizing angiotensin II.
Characteristics and Secretion: ANP is a peptide hormone. It is synthesized and stored as an inactive prohormone directly within cardiomyocytes. The primary stimulus for its release into the bloodstream is mechanical stretch of the atria, which occurs during increased blood volume and elevated blood pressure.
Mechanism of Action: The hormone targets cells in the kidneys, adrenal glands, and peripheral arteries. The ANP receptor on the plasma membrane is unique because it is catalytic—possessing guanylyl cyclase activity. Upon binding ANP, the receptor catalyzes the formation of the second messenger cyclic guanosine monophosphate (cGMP) from GTP.
Main Physiological Effects of ANP:
- Direct inhibition of renin and aldosterone production and secretion.
- A sharp increase in urinary excretion of $Na^+$ ions and water.
- A vasodilatory effect and, consequently, a reduction in blood pressure.