Agonist-induced alpha 1-adrenergic receptor changes. Evidence for receptor sequestration.

Short-term receptor regulation by agonists is a well-known phenomenon for a number of receptors but little is known about the regulation of alpha 1-adrenergic receptors. In the present study we provide evidence of alpha 1-adrenergic receptor changes induced by agonists on DDT1 MF-2 smooth muscle cells. The cells were preincubated with the agonist and receptor changes were investigated in the cells washed free of the agonist. On cells pretreated with norepinephrine the number of receptors recognized by [3H]prazosin at 4 degrees C was reduced by 38%. The receptors were not degraded as the number of sites was the same in control and norepinephrine-treated cells when binding was measured at 37 degrees C. When binding was measured on fragmented membranes (at 4 degrees C), the number of receptors was the same in control and norepinephrine-treated cells, suggesting that the disruption of cellular integrity might expose receptors which are probably sequestered after agonist treatment. We conclude that agonists induced rapid sequestration of receptors on intact DDT cells

Expression and Coupling to Polyphosphoinositide Hydrolysis of Group I Metabotropic Glutamate Receptors in Early Postnatal and Adult Rat Brain

We investigated the expression and coupling to the phospholipase C signal transduction pathway of metabotropic glutamate receptor (mGluR) subtypes by Western blot analysis and agonist-stimulated inositol monophosphate formation in several brain regions of postnatal day 9 (P9) and adult rats. In the cerebral cortex, hippocampus, corpus striatum, olfactory bulb, cerebellum and hypothalamus, the expression level of mGluR5 was greater at P9 than in adulthood. The mGluR5 signal was very low or absent in the adult cerebellum and hypothalamus. The expression of mGluR1a was slightly greater at P9 in the hypothalamus, hippocampus and olfactory bulb, whereas it substantially increased with age in the cerebellum, and did not change in the cerebral cortex and corpus striatum. mGluR1b and -1c were nearly undetectable by Western blot analysis. The expression level of mGluR5, but not that of mGluR1a, was significantly correlated with the extent of phosphoinositide hydrolysis stimulated by mGluR agonists in slices prepared from these brain regions. The mGluR antagonist cyclopropan[b]chromen-1a-carboxylic acid ethylester (CPCCOEt), potently antagonized responses mediated by mGluR1, but much less potently those mediated by mGluR5a in recombinant cells. CPCCOEt, at a concentration which efficently blocks mGluR1 responses, did not substantially affect the polyphosphoinositide response in hippocampal or cerebellar slices from newborn animals, and antagonized only a minor component of the polyphosphoinositide response in adult hippocampal slices. CPCCOEt, however, prevented the small stimulation of polyphosphoinositide hydrolysis by mGluR agonists in adult cerebellar slices. We conclude that (i) the efficient mGluR-mediated polyphosphoinositide hydrolysis in g-day-old rats is mediated by mGluR5; (ii) the increased expression of mGluR1 in the adult cerebellum does not substitute for the decline of mGluR5 expression in the ability to mediate polyphosphoinositide hydrolysis; and therefore (iii) mGluR1a might couple less efficiently than mGluR5 to polyphosphoinositide hydrolysis