Quantitative relationship between alpha 1-adrenergic receptor density and the receptor-mediated calcium response in individual astroglial cells.
alpha 1-Adrenergic receptor (alpha 1-AR) agonists elevate the intracellular calcium concentration ([Ca2+]i) in 60-80% of astroglia in vitro. Likewise, 60-70% of astroglia exhibit specific binding sites for the alpha 1-AR-selective antagonist (+-)-125I-[beta-(4-hydroxyphenyl)-ethylaminomethyl]tetralone. The density of alpha 1-AR binding sites varies markedly on individual cells, ranging from a few to > 2000 binding sites/1000 microns 2 of surface area. In the present study, we examined the relationship between the density of alpha 1-AR binding sites on astroglia and their ability to respond to alpha 1-AR agonists with a rise in [Ca2+]i. A video-based imaging system was used to monitor calcium responses in individual astroglial cells, which were subsequently assessed for their expression of alpha 1-ARs using receptor binding autoradiography. The ability of a given concentration of phenylephrine (PE) to elicit a calcium response correlated well with alpha 1-AR density (r = 0.94), i.e., the higher the receptor density the greater the probability that a given astroglial cell would respond to alpha 1-AR agonists. However, the amplitude of the calcium response did not correlate with the alpha 1-AR density. Cells with low alpha 1-AR density (< 10 binding sites/1000 microns 2) could generate a response with an amplitude comparable to that seen in cells with high alpha 1-AR density (> 1000 binding sites/1000 microns 2). To evaluate the relationship between receptor occupancy and calcium response, PE concentrations and alpha 1-AR density were varied while the calcium response in individual cells was monitored. Interestingly, for a given cell the amplitude of calcium response reached its maximum with a small step increase in the concentration of PE (< 5-fold), whereas the latency of the response decreased when PE concentrations were increased. Irreversible inactivation of alpha 1-ARs by phenoxybenzamine reduced the potency of PE but not the maximal calcium response. Cells that responded to 100 nM PE were able to generate a comparable response to 10 microM PE after inactivation of 90% of the total alpha 1-AR binding sites with phenoxybenzamine treatment. In summary, our results indicate that most astroglial cells express a substantial level of "spare" alpha 1-ARs that increase the sensitivity of these cells to alpha 1-AR agonists. Once activated, individual astroglial cells tend to generate a maximal [Ca2+]i elevation that is independent of the total alpha 1-AR density or the concentration of ligand.
Shao
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Y
Department of Pharmacology, University of North Carolina, Chapel Hill 27599-7365.
McCarthy
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KD
D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D001253:Astrocytes / Q000187:drug effects / Q000378:metabolism*; D001345:Autoradiography; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D002540:Cerebral Cortex; D010627:Phenethylamines / Q000494:pharmacology; D010643:Phenoxybenzamine / Q000494:pharmacology; D010656:Phenylephrine / Q000494:pharmacology*; D051381:Rats; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000502:physiology*; D044024:Tetralones
D000317:Adrenergic alpha-Antagonists; D010627:Phenethylamines; D011942:Receptors, Adrenergic, alpha; D044024:Tetralones; D010643:Phenoxybenzamine; D010656:Phenylephrine; C010652:BE 2254; D002118:Calcium