Surface membrane receptors

In addition to platelet activation by a variety of G protein (guanine nucleotide binding proteins)-coupled surface membrane receptors, including receptors for... 

Surface Membrane Receptors. These receptors are located on the outer surface of the plasma membrane (see Fig. 28—2).6 Surface receptors tend to recognize the peptide hormones and some amino acid derivatives (e.g., pituitary hormones, catecholamines). They are typically linked to specific intracellular enzymes. When stimulated by a peptidelike hormone,... 

FIGURE 28-2 Primary cellular locations of hormone receptors. Peptide hormones tend to bind to surface membrane receptors (site 0 steroid hormones bind to cytosolic receptors [site 10 and thyroid hormones bind to receptors in the cell nucleus [site III). 

The hypothalamic and pituitary hormones are all peptides that exert their effects by binding to target cell surface membrane receptors with high specificity and affinity. 

Extracellular or cell surface membrane receptors for D-galactose are widespread among higher eukaryotes. Isothermal titration calorimetry of the C-type lectin receptors from the tunicate Polyandrocatpa misakiensis revealed the presence of a single calcium atom per monomer with a dissociation constant of 2.6 pM, and confirmed the monosaccharide speci-... 

The peptide hormone somatostatin inhibits secretion from a wide variety of both endocrine and exocrine cells. It functions as a neurotransmitter and plays an important role in the regulation of cell proliferation and differentiation. Somatostatin exerts its effects through binding to specific surface membrane receptors. Somatostatin receptors are membrane glycoproteins with five different subtypes distributed in a variety of tissues throughout the body. 

This suggests that the disruption of phar-macomechanical coupling in saponin-treated smooth muscle is due to disruption of the coupling between surface-membrane receptors and their G proteins while the G protein effector cascade appears to be functional. 

The first demonstration of intracellular steroid-hormone receptors was by Jensen and Jacobson (1962) for estradiol. Levinson et al. (1972) showed that hormone binding to the glucocorticoid receptor in cultured rat hepatoma cells takes place inside the cell membrane rather than at the cell surface, but did not exclude the existence of other biological actions of steroids mediated by surface membrane receptors. Indeed, Ozegovic et al. (1977) have shown that aldosterone binds to isolated kidney plasma membranes and have proposed that this steroid-membrane interaction may reflect an early event in the transmembrance movement of aldosterone. 

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