QUANTITATIVE RELATIONSHIP BETWEEN ALPHA(1)-ADRENERGIC RECEPTOR DENSITY AND THE RECEPTOR-MEDIATED CALCIUM RESPONSE IN INDIVIDUAL ASTROGLIAL CELLS

Alpha1-Adrenergic receptor (alpha1-AR) agonists elevate the intracellu lar calcium concentration ([Ca2+]i) in 60-80% of astroglia in vitro. L ikewise, 60-70% of astroglia exhibit specific binding sites for the al pha1-AR-selective antagonist beta-(4-hydroxyphenyl)-ethylaminomethyl]t etralone. The density of alpha1-AR binding sites varies markedly on in dividual cells, ranging from a few to >2000 binding sites/1000 mum2 of surface area. In the present study, we examined the relationship betw een the density of alpha1-AR binding sites on astroglia and their abil ity to respond to alpha1-AR agonists with a rise in [Ca2+]i. A video-b ased imaging system was used to monitor calcium responses in individua l astroglial cells, which were subsequently assessed for their express ion of alpha1-ARs using receptor binding autoradiography. The ability of a given concentration of phenylephrine (PE) to elicit a calcium res ponse correlated well with alpha1-AR density (r = 0.94), i.e., the hig her the receptor density the greater the probability that a given astr oglial cell would respond to alpha1-AR agonists. However, the amplitud e of the calcium response did not correlate with the alpha1-AR density . Cells with low al-AR density (<10 binding sites/1000 mum2) could gen erate a response with an amplitude comparable to that seen in cells wi th high alpha1-AR density (>1000 binding sites/1000 mum2). To evaluate the relationship between receptor occupancy and calcium response, PE concentrations and alpha1-AR density were varied while the calcium res ponse in individual cells was monitored. Interestingly, for a given ce ll the amplitude of calcium response reached its maximum with a small step increase in the concentration of PE (<5-fold), whereas the latenc y of the response decreased when PE concentrations were increased. Irr eversible inactivation of alpha1-ARs by phenoxybenzamine reduced the p otency of PE but not the maximal calcium response. Cells that responde d to 100 nm PE were able to generate a comparable response to 10 mum P E after inactivation of 90% of the total alpha1-AR binding sites with phenoxybenzamine treatment. In summary, our results indicate that most astroglial cells express a substantial level of ''spare'' alpha1-ARs that increase the sensitivity of these cells to alpha1-AR agonists. On ce activated, individual astroglial cells tend to generate a maximal [ Ca2+]i elevation that is independent of the total alpha1-AR density or the concentration of ligand.