Mucosal gastrin receptor. I. Assay standardization and fulfillment of receptor criteria

Using iodinated gastrin with demonstrable biological activity, this study has shown that optimal specific gastrin binding occurs in rat gastric mucosal 270--30,000 g membrane preparations after an incubation period of 30 min at 30 degrees C (pH 7.4) with a protein concentration of 150--200 micrograms per assay tube. The gastrin binding was shown to be saturable with an equilibrium Ka of approximately 0.25 X 10(10) M-1 and an equilibrium Kd of approximately 4 X 10(10) M. The binding capacity was approximately 4 fmol/mg protein. Specific gastrin binding was shown to be present in the oxyntic gland and duodenal mucosa and to be absent from the antral mucosa, liver, spleen, and kidney. In order to decrease the specific binding of gastrin by 50% the competitors in order of potency are 15-Leu G-17 greater than cholecystokinin greater than caerulein greater than pentagastrin; secretin did not display a response similar to the other four competitors tested, indicating that its inhibition may be non-competitive. Fasting decreased the binding capacity of the gastrin receptor and refeeding brought the receptor levels back to control range; this result parallels the decrease seen in serum gastrin after fasting and the return to normal levels with refeeding. This suggests that rat gastric mucosal gastrin receptors may exhibit autoregulation. This study is the first to meet all the criteria for establishing the existence of a mucosal gastrin receptor.