Abstract:The coupling mechanism of the γ‐Aminobutyric acid (GABA)Breceptor, one of the subtypes of GABA receptors, with calcium ion channel and GTP‐binding protein was examined using a crude synaptic membrane (P2) fraction from the bovine cerebral cortex and a fraction solubilized with sodium deoxycholate. In the P2fraction, [3H]GABA binding to the GABABreceptor was increased significantly by the addition of calcium ion, and this enhancement was accentuated further by calcium ion channel blockers such as nicardipine and diltiazem. In contrast,N‐(6‐aminohexyl)‐5‐chloro‐1‐naphthalenesulfonamide (W‐7), a calmodulin antagonist, did not affect on the calcium ion‐induced enhancement of GABABreceptor binding. These results suggest that the GABABreceptor may be functionally coupled with the calcium ion channel, which exhibits an inhibitory modulation against the receptor. On the other hand, GABABreceptor binding, which was noncompetitively inhibited by guanine nucleotides such as GTP, guanosine 5′‐(3‐O‐thio)triphosphate (GTPγS), guanosine 5′‐(β,γ‐imido)triphosphate [Gpp(NH)p], and GDP, was competitively inhibited by (‐)‐baclofen. Although the affinity of (‐)‐baclo‐fen for the GABABreceptor was decreased in the presence of GTP, pretreatment of the P2fraction with islet‐activating protein (IAP) eliminated the effect of GTP. In addition, GABA and (‐)‐baclofen induced an increase of GTPase activity in the P2fraction, and this increase was also eliminated by treatment with IAP. These results suggest that the GABABreceptor may also be functionally coupled with IAP‐sensitive GTP‐binding protein. Treatment of the P2fraction with sodium deoxycholate resulted in the highest solubilization of GABABreceptor among various detergents examined. The solubilization, however, completely eliminated the stimulating effects of calcium ion and calcium ion channel blockers as well as the inhibitory effects of GTP and GTP analogues on GABABreceptor binding. Furthermore, the increase of GTPase activity induced by GABA and (‐)‐baclofen was also eliminated following the solubilization. These results suggest that functional coupling of the GABABreceptor with the calcium ion channel and GTP‐binding protein such