Renin is involved in controlling blood pressure, in sympathetic response
Renin is involved in controlling blood pressure, in sympathetic response
Renin
Juxtaglomerular cells secrete renin. Renin is a peptide with 340 amino acids. Along with angiotensins, renin forms the renin-angiotensin system, which is a hormone system that plays an important role in the maintenance of blood pressure.
Stimulants for renin secretion
Secretion of renin is stimulated by four factors:
i. Fall in arterial blood pressure
ii. Reduction in the ECF volume
iii. Increased sympathetic activity
iv. Decreased load of sodium and chloride in macula densa.
Renin-angiotensin system
When renin is released into the blood, it acts on a specific plasma protein called angiotensinogen or renin substrate. It is the α2-globulin. By the activity of renin, the angiotensinogen is converted into a decapeptide called angiotensin I. Angiotensin I is converted into angiotensin II, which is an octapeptide by the activity of angiotensin-converting enzyme (ACE) secreted
from lungs. Most of the conversion of angiotensin I into angiotensin II takes place in lungs. Angiotensin II has a short half-life of about 1 to 2 minutes. Then it is rapidly degraded into a heptapeptide called angiotensin III by angiotensinases, which are present in RBCs and vascular beds in many tissues.
Angiotensin III is converted into angiotensin IV, which is
a hexapeptide.
Actions of Angiotensins
Angiotensin I
Angiotensin I is physiologically inactive and serves only as the precursor of angiotensin II.
Angiotensin II
Angiotensin II is the most active form. Its actions are:
On blood vessels:
i. Angiotensin II increases arterial blood pressure by directly acting on the blood vessels and causing vasoconstriction. It is a potent constrictor of arterioles. Earlier, when its other actions were not found it was called hypertensin.
ii. It increases blood pressure indirectly by increasing the release of noradrenaline from postganglionic sympathetic fibers. Noradrenaline is a general vasoconstrictor.
On adrenal cortex:
It stimulates zona glomerulosa of adrenal cortex to secrete aldosterone. Aldosterone acts on renal tubules and increases retention of sodium, which is also responsible for elevation of blood pressure.
On kidney:
i. Angiotensin II regulates glomerular filtration rate by two ways:
a. It constricts the efferent arteriole, which causes decrease in filtration after an initial increase
b. It contracts the glomerular mesangial cells leading to decrease in surface area of glomerular capillaries and filtration
ii. It increases sodium reabsorption from renal tubules. This action is more predominant on proximal tubules.
On brain:
i. Angiotensin II inhibits the baroreceptor reflex and thereby indirectly increases the blood pressure. Baroreceptor reflex is responsible for decreasing the blood pressure.
Glomerular mesangial cells are phagocytic in nature. These cells also secrete glomerular interstitial matrix, prostaglandins and cytokines.
JUXTAGLOMERULAR CELLS
Juxtaglomerular cells are specialized smooth muscle cells situated in the wall of afferent arteriole just before it enters the Bowman capsule. These smooth muscle cells are mostly present in tunica media and tunica adventitia of the wall of the afferent arteriole. Juxtaglomerular cells are also called granular cells because of the presence of secretary granules in their cytoplasm.
Polar Cushion or Polkissen
Juxtaglomerular cells form a thick cuff called polar cushion or polkissen around the afferent arteriole before it enters the Bowman capsule.
FUNCTIONS OF JUXTAGLOMERULAR APPARATUS
Primary function of juxtaglomerular apparatus is the secretion of hormones. It also regulates the glomerular blood flow and glomerular filtration rate.
SECRETION OF HORMONES
Juxtaglomerular apparatus secretes two hormones:
1. Renin
2. Prostaglandin.
1. Renin
Juxtaglomerular cells secrete renin. Renin is a peptide with 340 amino acids. Along with angiotensins, renin forms the renin-angiotensin system, which is a hormone system that plays an important role in the maintenance of blood pressure.
Stimulants for renin secretion
Secretion of renin is stimulated by four factors:
i. Fall in arterial blood pressure
ii. Reduction in the ECF volume
iii. Increased sympathetic activity
iv. Decreased load of sodium and chloride in macula densa.
Renin-angiotensin system
When renin is released into the blood, it acts on a specific plasma protein called angiotensinogen or renin substrate. It is the α2-globulin. By the activity of renin, the angiotensinogen is converted into a decapeptide
ii. It increases water intake by stimulating the thirst center
iii. It increases the secretion of Corticotropin-releasing hormone (CRH) from hypothalamus. CRH in turn increases secretion of adrenocorticotropic hormone (ACTH) from pituitary
iv. It increases secretion of antidiuretic hormone (ADH) from hypothalamus.
Other actions:
Angiotensin II acts as a growth factor in heart and it is thought to cause muscular hypertrophy and cardiac enlargement.
Angiotensin III
Angiotensin III increases the blood pressure and stimulates aldosterone secretion from adrenal cortex. It has 100% adrenocortical stimulating activity and 40% vasopressor activity of angiotensin II.
Angiotensin IV
It also has adrenocortical stimulating and vasopressor activities.
Sembuling, K. and Sembuling, P. (2012) Essential of Medical Physiology. 6th Edition, New Jaypee Brothers Medical Publishers, Delhi, India.