Interaction of galanin fragments with galanin receptors on isolated smooth muscle cells from guinea pig stomach: identification of a novel galanin receptor subtype.

From structure-function studies it has been proposed that two subtypes of receptors may mediate galanin's actions in the gastrointestinal tract and other tissues and their effects can be either direct or neurally mediated. We have recently demonstrated that isolated gastric smooth muscle cells possess high-affinity galanin receptors, activation of which increases AMP and causes relaxation. Because this cell system contains no neural elements, contains a single class of galanin receptors and allows binding to be correlated with function, it is a good system to investigate peptide requirements for cell activation. Porcine galanin (p-Gal) and rat galanin (r-Gal) were equipotent to the NH2-terminal fragments r,p-Gal(1-10), r,p-Gal(1-15), r,p-Gal(1-20), p-Gal(2-29) and p-Gal(3-29) at inhibiting binding of 125I-galanin to gastric smooth muscle cells from guinea pig (Kd 5-8 nM). The midmolecule fragment p-Gal(9-25) and the long COOH-terminal fragment r-Gal(9-29) were equipotent and 25-fold less potent than p-Gal. Acetylation of r-Gal(9-29) increased potency to that of p-Gal. The short COOH-terminal fragment, p-Gal(21-29) and r-Gal(21-29), had very low affinity (Kd 3 microM). Each peptide alone (1 microM) caused no effect on cell length, but inhibited carbachol-induced contraction. The inhibition was 81% to 92% for r-Gal or p-Gal, r,p-Gal(1-10), r,p-Gal(1-15), r,p-Gal(1-20), p-Gal(2-29), p-Gal(3-29) and Ac-r-Gal(9-29); 47% for p-Gal(9-25) and r-Gal(9-29); and 16% to 17% for p-Gal(21-29).(ABSTRACT TRUNCATED AT 250 WORDS)