Modulation by calcium of gamma-aminobutyric acid (GABA) binding to GABAA and GABAB recognition sites in rat brain. Involvement of different mechanisms.
Calcium modulates sodium-independent binding of gamma-aminobutyric acid (GABA) to GABAA and GABAB recognition sites located in synaptic membranes of rat brain. At 37 degrees the binding of [3H]GABA to the GABAB recognition site is dramatically stimulated by Ca2+ with an EC50 (half-saturation constant) of congruent to 10 microns, whereas the binding to the GABAA recognition site is only slightly, but significantly, potentiated by Ca2+ with an EC50 of congruent to 0.1-1.0 micron. The effect of calcium on GABAA recognition sites requires a temperature of 37 degrees and the presence of calmodulin. Only GABAA recognition sites are linked to benzodiazepine recognition sites, and the interaction between these sites is modulated by Ca2+ at physiological ion concentrations. When free Ca2+ in the assay medium is below 10 nM, only one population of low-affinity GABAA recognition sites can be measured; however, when free Ca2+ is at the micromolar level, or if diazepam is present, a high affinity-binding site appears in addition to the pre-existing low-affinity component. On the basis of affinity there is a single population of GABAB bindings sites, but the number of sites is about 90% greater at 37 degrees than at 4 degrees. This temperature-dependent increase in the number of GABAB recognition sites is calmodulin-independent, and data with leupeptin, hemin, and antipain suggest that this temperature-dependent increase in GABAB sites might involve the activity of Ca2+-dependent protease(s).
D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002540:Cerebral Cortex / Q000378:metabolism*; D003975:Diazepam / Q000494:pharmacology; D004533:Egtazic Acid / Q000494:pharmacology; D007700:Kinetics; D008297:Male; D011480:Protease Inhibitors / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D011956:Receptors, Cell Surface / Q000187:drug effects / Q000378:metabolism*; D011963:Receptors, GABA-A; D013570:Synaptic Membranes / Q000378:metabolism*; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
D011480:Protease Inhibitors; D011956:Receptors, Cell Surface; D011963:Receptors, GABA-A; D004533:Egtazic Acid; D005680:gamma-Aminobutyric Acid; D003975:Diazepam; D002118:Calcium