Gastrin receptors in normal and malignant gastrointestinal mucosa: Age-associated changes

Synthetic human gastrin 17-I(MG) and an analogue, [Leu¹⁵] gastrin-17-I (LG), were radiolabeled with Na¹²⁵I by Iodo-Gen, EnzymoBead, and chloramine-T methods, and the characteristics of the radiolabeled peptides were determined. When ¹²⁵I-MG was iodinated by chloramine-T, its biological activity and its binding activity were almost abolished, whereas the biological activity of ¹²⁵I-LG, iodinated by either of the methods, and of ¹²⁵I-MG, iodinated by Iodo-Gen and EnzymoBeads, was not significantly affected. The kinetic,s affinity, and specificity of binding of ¹²⁵I-MG, iodinated by Iodo-Gen, to crude and purified membranes from rat fundic mucosa were examined and found to be similar to that for ¹²⁵I-LG. Age-associated changes in the number and affinity of gastrin receptors (GR) on the crude membranes of gastrointestinal mucosa of rats was also examined. Significantly fewer GR were observed on the crude membranes of fundic mucosa of aged (24 mo old) compared with young (3- and 6-mo-old) rats. In addition, specific gastrin-binding sites (4.7 ± 0.9 fmol/mg prot) with low affinity (K(d) = 3.7 ± 1.2 nM) were observed in the antrum of aged rats, the significance of which is not understood. There were, however, no differences in the number and characteristics of GR in other regions of the intestine of old and young rats. The presence of GR was additionally assessed in cell lines of gastrointestinal cancers from humans, mice, and hamsters. Cells from cultured cell lines of a human stomach cancer (AgS), a human colon cancer (LoVo), and a mouse colon cancer (MC-26) were found to be highly positive for GR, whereas others were either slightly positive or negative. The binding affinity of gastrin to GR on MC-26 and LoVo cells (K(d) = 0.2-0.6 nM) was found to be similar to that on normal rat fundic mucosal membranes (K(d) = 0.4 nM).