Galanin binding sites in rat gastric and jejunal smooth muscle membrane preparations

Receptors for galanin in membranes from the rat gastric and jejunal smooth muscle were studied using [¹²⁵I] radioiodinated synthetic porcine galanin. Specific binding was time and temperature dependent. At 32°C radioligand was degraded in the presence of smooth muscle membranes in a time-dependent manner. At optimal experimental conditions, the equilibrium binding analyses showed the presence of a single population of high affinity binding sites in both the rat stomach and jejunum (K_d value of 2.77±0.78 nM and 4.93±1.74 nM for stomach and jejunal smooth muscle membranes, respectively). The concentration of the high affinity binding sites was 58.19±11.04 and 32.36±5.68 fmol/mg protein, for gastric and jejunal preparations, respectively. Specific binding was completely inhibited by 10^-6 M of nonradioactive galanin; was 75% blocked by 1 μM of galanin(9-29); it was 10% blocked by 1 μM of galanin(15-29). Galanin(1-15) at a concentration of 1 μM was ineffective for inhibiting [¹²⁵I]galanin binding. Deletion of four C-terminal amino acid residues from galanin(9-29) to give galanin(9-25) also resulted in almost complete loss of affinity. Radioiodinated galanin and N-terminally deleted fragments had receptor binding potency in the following order: galanin(1-29) > galanin(9-29) > galanin(15-29). We conclude that the C-terminal part of the galanin chain is important for the rat gastric and jejunal smooth muscle membrane receptor recognition and binding and that N-terminal amino acid sequences are probably not so important, since galanin(1-15) was not active but galanin(9-29) retained most of the receptor binding activity.