The kidneys as a target for hormonal influences
The tubular apparatus of the kidneys responds sensitively to external signals, adjusting the processes of reabsorption.
- Aldosterone. An adrenal cortex hormone whose site of action is the distal tubules. It stimulates the synthesis of plasma membrane transport proteins, leading to the active reabsorption of sodium and chloride ions in exchange for the secretion of potassium and hydrogen ions.
- Antidiuretic hormone (vasopressin). Produced in the hypothalamus and acts primarily on the collecting ducts (to a lesser extent on the distal tubules). The hormone facilitates passive water reabsorption in two ways: it stimulates the synthesis of the aquaporin-2 transport protein and decreases the degree of polymerization of glycosaminoglycans in the extracellular matrix.
- Atrial natriuretic peptide (atriopeptin). Released by atrial cardiomyocytes during high blood pressure or blood volume. Its effect is opposite—it decreases the reabsorption of sodium and water.
- Calcium metabolism regulators. Parathyroid hormone from the parathyroid glands increases calcium reabsorption, while thyroid calcitonin decreases it.
Renin-angiotensin system
The best-known product of the kidney's own endocrine activity is renin. This is an enzyme (protease) produced in the juxtaglomerular apparatus (JGA), located near the vascular pole of the renal corpuscle.
The operation of the system is a biochemical cascade:
- Renin acts on circulating angiotensinogen—an inactive peptide synthesized by the liver.
- Cleavage results in the formation of angiotensin I.
- In the lung capillaries, under the action of angiotensin-converting enzyme (ACE), it is converted into active angiotensin II.
Angiotensin II has a potent physiological effect. First, it causes vasoconstriction (narrowing of small vessels), which directly increases blood pressure. Second, it stimulates the adrenal cortex to release aldosterone. Result: aldosterone enhances renal reabsorption, and in combination with vasoconstriction, this leads to a sustained increase in blood pressure.
Antagonistic systems: kallikrein and prostaglandins
To balance the pressor effect of renin, systems with a vasodilatory effect function in the kidneys.
The kallikrein-kinin system operates on a similar cascade principle but produces the opposite effect. Distal tubule cells contain the enzyme kallikrein. It acts on the blood precursor protein (kininogen), converting it into the peptide bradykinin. Bradykinin causes pronounced vasodilation and reduces sodium reabsorption—either directly or by inhibiting aldosterone.
Renal prostaglandins (specifically the E2 fraction) possess similar properties. They are formed from polyunsaturated fatty acids containing a five-carbon ring. Their site of synthesis is the interstitial cells and collecting ducts. Prostaglandins act as functional antagonists of renin, exerting a vasodilatory effect and lowering blood pressure.
Regulation of erythropoiesis and calcium-phosphorus metabolism
The endocrine functions of the kidney are not limited to hemodynamics regulation.
Renal interstitial cells produce erythropoietin, a hormone that travels to the red bone marrow and stimulates erythropoiesis (the formation of new red blood cells).
Additionally, the synthesis of calcitriol—a steroid hormone that is the active form of vitamin D3—is completed in the kidneys. The mechanism of its formation involves two hydroxylation steps: the first occurs in the liver, and the second in the cells of the proximal convoluted tubules of the kidneys. This process is stimulated by parathyroid hormone and (presumably) inhibited by calcitonin. Finished calcitriol performs a crucial function: it enhances calcium ion absorption in the intestine and promotes calcium salt deposition in bone tissue.
Briefly: answers to topical questions
Which cells make up the juxtaglomerular apparatus (JGA) of the kidney?
The juxtaglomerular apparatus (JGA) of the kidney consists of three components.
- Macula densa (macula densa)—a specialized region of the distal convoluted tubule wall.
- Juxtaglomerular cells—located in the tunica media of the arterioles, predominantly the afferent one.
- Juxtavascular cells (Goormaghtigh cells) — located in the space between the two arterioles and the macula densa.
Which specific enzyme catalyzes the formation of calcitriol in the proximal convoluted tubules?
The formation of calcitriol in the proximal convoluted tubules of the kidneys is catalyzed by the enzyme 1α-hydroxylase. In the kidneys, 25(OH)D3 undergoes 1α-hydroxylation to form calcitriol, the active form of vitamin D3.
How to remember the antagonism of systems: Renin — Raises blood pressure (vasoconstriction, sodium retention). Bradykinin — Blocks hypertension (vasodilation, sodium excretion).
Class and exam questions
Where exactly in the kidney is renin produced?
Renin is synthesized in the juxtaglomerular apparatus (JGA), which is localized in the region of the vascular pole of the renal corpuscle.
How does vasopressin (ADH) affect water reabsorption?
It acts on the collecting ducts: it stimulates the synthesis of the aquaporin-2 transport protein and reduces the polymerization of glycosaminoglycans, which facilitates passive water reabsorption.
In which nephron segment does the synthesis of active vitamin D3 occur?
The second, final step of hydroxylation resulting in calcitriol formation takes place in the cells of the proximal convoluted tubules.
What else to study on this topic
- Structure and cellular composition of the juxtaglomerular apparatus (JGA)
- Molecular mechanism of plasma membrane transport proteins (aquaporins)
- Histophysiology of renal interstitial cells
- Cascade enzymatic reactions in blood plasma
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