Renal Hormones

A. Renal hormones
In addition to their involvement in excretion and metabolism, the kidneys also have endocrine functions. They produce the hormones erythropoietin and calcitriol and play a decisive part in producing the hormone angiotensin II by releasing the enzyme renin. Renal prostaglandins .have a local effect on Na+ resorption. Calcitriol (vitamin D hormone, 1,25-dihydroxycholecalciferol) is a hormone closely related
to the steroids that is involved in Ca2+ homeostasis . In the kidney, it is
formed fromcalcidiol by hydroxylation at C-1.
The activity of calcidiol-1-monooxygenase [1] is enhanced by the hormone parathyrin (PTH). Erythropoietin is a peptide hormone that is formed predominantly by the kidneys, but also by the liver. Together with other factors known as “colony-stimulating factors” , it regulates the differentiation of stemcells in the bone marrow. Erythropoietin release is stimulated by hypoxia (low pO2). Within hours, the hormone ensures that erythrocyte precursor cells in the bone marrow are converted to erythrocytes, so that their numbers in the blood increase. Renal damage leads to reduced erythropoietin release, which in turn results in anemia. Forms of anemia with renal causes can now be successfully treated using erythropoietin produced by genetic engineering techniques.


The hormone is also administered to dialysis patients. Among athletes and sports professionals, there have been repeated cases of erythropoietin being misused for doping purposes. B. Renin–angiotensin system _ The peptide hormone angiotensin II is not synthesized in a hormonal gland, but in the blood. The kidneys take part in this process by releasing the enzyme renin. Renin [2] is an aspartate proteinase . It is formed by the kidneys as a precursor (prorenin), which is proteolytically activated into renin and released into the blood. In the blood plasma, renin acts on angiotensinogen, a plasma glycoprotein in the 2-globulin group , which like almost all plasma proteins is synthesized in the liver.
The decapeptide cleaved off by renin is called angiotensin I. Further cleavage by peptidyl dipeptidase A (angiotensin-converting enzyme, ACE), a membrane enzyme located on the vascular endothelium in the lungs and other tissues, gives rise to the octapeptide angiotensin II [3], which acts as a hormone and neurotransmitter. The lifespan of angiotensin II in the plasma is only a few minutes, as it is rapidly broken down by other peptidases (angiotensinases [4]), which occur in many different tissues.
The plasma level of angiotensin II is mainly determined by the rate at which renin is released by the kidneys. Renin is synthesized by juxtaglomerular cells, which release it when sodiumlevels decline or there is a fall in blood pressure. Effects of angiotensin II. Angiotensin II has effects on the kidneys, brain stem, pituitary gland, adrenal cortex, blood vessel walls, and heart via membrane-located receptors. It ncreases blood pressure by triggering vasoconstriction (narrowing of the blood vessels). In the kidneys, it promotes the retention of Na+ and water and reduces potassium secretion. In the brain stem and at nerve endings in the sympathetic nervous system, the effects of angiotensin II lead to increased tonicity (neurotransmitter effect).
In addition, it triggers the sensation of thirst. In the pituitary gland, angiotensin II stimulates sopressin release (antidiuretic hormone) and corticotrophin (ACTH) release. In the adrenal cortex, it increases the biosynthesis and release of aldosterone, which promotes sodium and water retention in the kidneys. All of the effects of angiotensin II lead directly or indirectly to increased blood pressure, as well as increased sodium and water retention. This important hormonal system for blood pressure regulation can be pharmacologically influenced by inhibitors at various points:
Using angiotensinogen analogs that inhibit
renin.
Using angiotensin I analogs that competitively inhibit the enzyme ACE [3].
sing hormone antagonists that block the binding of angiotensin II to its receptors