Question

Define and explain the Paracrine and hormone signaling involving the juxtaglomerular apparatus.

Answer 1

Ans:

Paracrine signaling is a form of cell-to-cell communication in which a cell produces a signal to induce changes in nearby cells, altering the behaviour of those cells. Signaling molecules known as paracrine factors diffuse over a relatively short distance, as opposed to cell signaling by endocrine factors, hormones which travel considerably longer distances via the circulatory system; juxtacrine interactions; and autocrine signaling. Cells that produce paracrine factors secrete them into the immediate extracellular environment. Factors then travel to nearby cells in which the gradient of factor received determines the outcome. However, the exact distance that paracrine factors can travel is not certain.

Macula densa cells combines the approach of renal haemodynamics, glomerular filtration and renin release. These macula densa cells detect changes in tubular fluid composition through a complex of intracellular signalling events that are mediated by membrane transport pathways. Macula densa signalling then involves the production and release of specific paracrine signalling molecules at their basolateral membrane. Paracrine signalling by the macula densa cells therefore controls juxtaglomerular function, renal vascular resistance and participates in the regulation of renin release.

Hormone signaling primarily involves the role of hormones as member of a class of signaling molecules that are produced by glands in multicellular organisms and are transported by the circulatory system to target distant organs to regulate physiology and behavior.

Renin (a peptide hormone) is an enzyme involved in the renin angiotensin aldosterone system (RAAS), which regulates the fluid and electrolyte balance and blood pressure level. The system regulates the extracellular volume in the blood plasma, lymph and interstitial fluid, as well as controlling constriction of blood vessels. This peptide hormone is secreted from specialized cells called granular cells found in the juxtaglomerular apparatus of the kidney. There are two cellular pathways involved in the secretion of renin: (i) Prorenin is secreted through a constitutive pathway; and (ii) Renin is secreted through a regulated pathway.

The secretion of renin is stimulated by the following three factors:

• Decrease in arterial blood pressure detected by pressure sensitive receptors (baroreceptors) in the arterial vessels.
• Decrease in sodium chloride detected in the kidney by the macula densa in the juxtaglomerular apparatus.
• Sympathetic nervous system activity detected through beta1 adrenergic receptors.