Galanin receptor in plasma membrane of canine small intestinal circular muscle

High-affinity binding sites for galanin were identified and characterized in plasma membrane of circular muscle from canine small intestine using 125I-radioiodinated synthetic porcine galanin. Scatchard analysis indicated a high-affinity binding site on plasma membrane with a dissociation constant (Kd) of 0.58 nM and a binding capacity of 389 fmol/mg. Unlabeled galanin or NH2-terminal galanin fragments competitively inhibited the binding of 125I-galanin in a concentration-dependent manner, whereas the COOH-terminal fragment was inactive. Computer analysis of competitive binding data suggested a two-site model with a high-affinity (inhibitor constant, Ki = 0.01 nM) and a low-affinity (Ki = 2.8 nM) binding site. Guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) enhanced the dissociation of bound 125I-galanin. Cholera toxin (CTX) and GTP gamma S abolished the activity of the high-affinity binding site, leaving the low-affinity binding site. We conclude that galanin may act as an neurotransmitter to inhibit canine small intestinal smooth muscle contraction by interaction with a CTX-sensitive G protein-coupled specific receptor on muscle membrane. This receptor showed different G protein coupling from a synaptosomal receptor previously described in the same tissue preparations.