@article {Shao247, author = {Y Shao and K D McCarthy}, title = {Quantitative relationship between alpha 1-adrenergic receptor density and the receptor-mediated calcium response in individual astroglial cells.}, volume = {44}, number = {2}, pages = {247--254}, year = {1993}, publisher = {American Society for Pharmacology and Experimental Therapeutics}, abstract = {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.}, issn = {0026-895X}, URL = {https://molpharm.aspetjournals.org/content/44/2/247}, eprint = {https://molpharm.aspetjournals.org/content/44/2/247.full.pdf}, journal = {Molecular Pharmacology} }