Parenchymal cells receptor

The evidence supporting the existence of a specific category of dopamine receptor on the parenchymal cells of the bovine parathyroid gland and the possible biochemical mechanisms by which dopamine stimulates the release of parathyroid hormone are reviewed. The dopamine receptor on the bovine parathyroid cell is compared to other dopamine receptors. 

Dopaminergic neurons synapse upon the parenchymal cells of the intermediate lobe (IL) of the rat pituitary gland. Dopamine decreases the capacity of the IL cells to synthesize cyclic AMP and inhibits the release of otMSH and other peptides from this tissue. The presence of a D-2 receptor accounts for both of these phenomena. This D-2 dopamine receptor can be studied in a binding assay using [3H]-spiroperidol, a dopamine... 

There is abundant evidence that glucagon elevates cAMP levels in isolated liver parenchymal cells, in perfused liver and in the liver in vivo [58,59], As illustrated in Fig. 2, this occurs rapidly and with concentrations of the hormone [59] within the range found in portal venous blood in vivo i.e., 0.2-2 x 10-10 M. When sufficiently sensitive and accurate methods are employed to measure cAMP, an increase in the nucleotide is consistently observed in situations where the hormone induces metabolic responses [58,59]. However, an increase of only 2- to 3-fold is capable of inducing full stimulation of some major hepatic responses, e.g., phos-phorylase activation (Fig. 2) and gluconeogenesis [58,59]. Since higher concentrations of the hormone can elevate cAMP 10-fold or more [59] it appears that there is considerable receptor reserve for these responses. 

AAG is synthesized primarily by the hepatic parenchymal cells, but granulocytes and monocytes may also contribute to plasma levels in sepsis. Catabolism is believed to be primarily by removal of desialylated AAG by hepatic asialoglycoprotein receptors. The plasma half-life of intact AAG is 3 days, whereas that of the desialylated protein is only a few minutes. 

AMG is synthesized primarily by hepatic parenchymal cells. Catabolism is via two primary routes once the thiol ester bond is spht, AMG—regardless of whether complexed to a protease— is rapidly removed by a hepatic receptor that also acts to remove low-density lipoprotein. Desialylated... 

Sialic acid is slowly removed from circulating Cp by tissue and plasma neuraminidases, resulting in exposure of the penterminai galactose residue on the carbohydrate side chains. Once a critical number of galactose residues are exposed, the protein is rapidly removed by galactose receptors of the hepatic parenchymal cells and catabofized. Markedly increased catabolism by this route may result in deposition of excessive copper in the liver. The normal plasma half-life of intact, holoCp (copper replete) is 4 to 5 days, whereas that of apoCp is a few hours and that of desia-lylated Cp a few minutes. 

The chemistry and biochemistry of Hpx has been reviewed and a crystal structure is available. Hemopexin is present in serum at about 10 pM and its primary function is to transport released heme to its degradation site in the parenchymal cells of the liver via receptor-mediated endocytosis. Encapsulation of a single heme by Hpx occurs via bis-histidyl protein side-chain coordination of the Fe. Spectroelectrochemical investigation of the heme-Hpx assembly gives insight into the role of Hpx in controlling the reduction potential of the heme Fe, the efficiency of electron transfer at the metal centre, the influence of bis-histidyl coordination at the Fe centre, and the possible role of Fe redox in the Hpx-mediated transport and recycling of heme. 

TNF-a is released from virtually all brain parenchymal cells after trauma, hypoxia, epilepsy, neuro-AIDS, and inflammation [73]. Interestingly, TNF-a is not only specifically transported across the BBB but also modulates the functions of the specialized endothelial cells lining the BBB [74—76]. TNF-a transport across the BBB follows a circadian rhythm, and strikingly ABCBl expression at the BBB also displays a circadian rhythm [71, 77]. Upon receptor binding, TNF-a probably affects ABCBl expression by activating NFkB, which binds to the proximal promoter of ABCBl and activates its transcription [78]. 

Catabolism of chylomicron remnants may be viewed as the second step in the processing of chylomicrons. After the loss of apo C-II and other C and A apoproteins, LPL no longer acts upon the remnants, and they leave the capillary surface. Chylomicron remnants are rapidly removed by uptake into liver parenchymal cells via receptor-mediated endocytosis. Apo E is important in this uptake process. The chylomicron receptors in liver are distinct from the B-E receptor that mediates uptake of LDL. The hepatic receptor for chylomicrons binds with apo E, but not apo B-48. Another receptor, known as the LDL receptor-related protein (LRP), may also function in chylomicron uptake. Chylomicron remnants are transported into the lysosomal compartment where acid lipases and proteases complete their degradation. In the liver, fatty acids so released are oxidized or are reconverted to triacylglycerol, which is stored or secreted as VLDL. The cholesterol may be used in membrane synthesis, stored as cholesteryl ester, or excreted in the bile unchanged or as bile acids. 

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