Juxtaglomerular apparatus

Cyclooxygenase (COX) activity is responsible for the formation of prostaglandins from their arachidonic acid precursor. Two COX isoforms have been identified, COX-1 and COX-2. While COX-1 is constitutively expressed in most tissues, COX-2 is typically only found after induction by proinflammatory stimuli. However, a constitutively expressed and highly regulated COX-2 is found in the kidney, both in the renal medulla and in the renal cortex. Renal cortical COX-2 is located in the area ofthe juxtaglomerular apparatus, and prostaglandins formed by COX-2 regulate the expression and secretion of renin in response to a reduction in NaCl concentration at the macula densa. 

Tubuloglomerular feedback. Tubuloglomerular feedback involves the activity of the juxtaglomerular apparatus (see Figure 19.1). This structure is located where the distal tubule comes into contact with the afferent and efferent arterioles adjacent to the glomerulus. The juxtaglomerular apparatus is composed of the following ... 

Sympathetic stimulation of adrenergic receptors on the granular cells of the juxtaglomerular apparatus promotes secretion of renin and, consequently, formation of angiotensin II. Angiotensin II then causes ... 

A correlation between the activity of G-6-PDH and extractable renin in rat kidneys has been reported, suggesting that the macula densa cells and also the juxtaglomerular apparatus are parts of a system related to the formation of renin (H6). 

Specialized cells, called the juxtaglomerular apparatus (JGA), within the renal cortex are able to detect a fall in blood pressure and respond by secreting a proteolytic enzyme called renin (not to be confused with rennin). The substrate for renin, a liver-derived peptide called angiotensinogen, circulates in the plasma. Renin removes two amino acids from the N-terminal to produce angiotensin I, which is itself a substrate for angiotensin-converting enzyme (ACE). ACE removes two more amino acids to produce angiotensin II. 

The novel endocrine glands are the skin, gastrointestinal tract, adipose tissue, kidney (juxtaglomerular apparatus in the cortex which secretes renin that indirectly controls aldosterone secretion, via angiotensin-11), pineal gland, which secretes melatonin, and the heart (cardiac myocytes in the atria, which secrete atrial natriuretic peptide). 

L A. By blocking renal prostaglandin synthesis, COX-2 inhibitors, such as rofecoxib, decrease the blood flow to the juxtaglomerular apparatus, thus stimulating the release of renin and subsequent Na" retention and blood pressure elevation. Rofecoxib is neither metabolized nor induced by CYP2C9. It decreases rather than increases renal blood flow and does not increase the excretion of hydrochlorothiazide. Item D is incorrect because rofecoxib has very little effect on COX-1 and prostaglandins are not a major controlling factor of peripheral vascular tone. Rofecoxib does not decrease basal metabolic rate. 

Peti-Peterdi, J., Morishima, S., BeU, P. D., and Okada, Y. 2002. Two-photon excitation fluorescence imaging of the living juxtaglomerular apparatus. Am. J. Physiol. Renal Physiol. 283(1) F197-F201. 

Beta Postsynaptic effector cells, especially heart, lipocytes, brain presynaptic adrenergic and cholinergic nerve terminals, juxtaglomerular apparatus of renal tubules, ciliary body epithelium Stimulation of adenylyl cyclase, increased cAMP... 

The site of formation of renin is not known, although the indirect and circumstantial evidence favors slightly the juxtaglomerular apparatus rather than the tubules as a source (18). Crude renin, however, is extracted readily from renal cortex by saline extraction, acidification, and precipitation with ammonium sulfate and sodium chloride. Other active protein substances are likewise extracted, and their separation from renin is often a matter of considerable difficulty. The renin substrate, an arglobulin, is found in blood serum and is probably formed by the liver. It can be easily salted out of beef serum as a crude preparation. 

Figure 29.2. Schematic representation of nephron and vasculature. The glomerulus is positioned between afferent and efferent arterioles, and the juxtaglomerular apparatus is the point of contact between the vascular pole and distal tubule of the nephron. A capillary network surrounds tubular structures. (From Guyton, A. C., and Hall, J. E., Textbook of Medical Physiology, 11th ed. Elsevier Saunders Company, Philadelphia, 2006. Reproduced with permission.)...

Reduced renin secretion from the juxtaglomerular apparatus in the renal cortex. 

The renin angiotensin response to trauma has been investigated in some detail in reference to its controlling action on aldosterone (C9). Experimental studies in animals suggested that nerve impulses act directly on the juxtaglomerular apparatus to release renin. An intramuscular injection of sterile saline in unanesthetized rats produced an increase in plasma renin (B9). After extensive burns in man very high levels of renin were found. A detailed study on the effects of anesthesia,... 

Ito 5 Characteristics of isolated perfused juxtaglomerular apparatus. Kidney Int SuppI 67 546-548,1998... 

Yanagisawa H, Nodera M, Kurihara N, Wada O Altered expression of endothelin-1 and endothelial nitric oxide synthase in the juxtaglomerular apparatus of rats with HgCl2-induced acute renal failure.Toxicol Lett 98 181-8,1998... 

Yamada K, SugIsakI Y, Akimoto M, Yamanaka N. FK506-induced juxtaglomerular apparatus hyperplasia and tubular damage In rat kidney-morphologic and biologic analysis. Transplant Proc 1992 24 1396-1398. 

---