FUNCTIONAL-CHARACTERIZATION OF A MUSCARINIC RECEPTOR STIMULATING GASTRIN-RELEASE FROM RABBIT ANTRAL G-CELLS IN PRIMARY CULTURE

In previous studies carbachol-induced stimulation of gastrin release from antral G-cells in primary culture suggested the presence of muscarinic acetylcholine receptors on this cell type. Therefore, we attempted to pharmacologically characterize the muscarinic acetylcholine receptor subtype involved. Enzymatically isolated rabbit antral mucosal cells (0.8% G-cells) were separated by counterflow elutriation yielding a fraction (1.7% G-cells) that was placed in culture on collagen-coated well plates. After 24-36 h of culture 13.0 +/- 2.4% of total adherent cells were immunoreactive for gastrin as shown by immunocytochemical staining using the avidin-biotin complex method. In this preparation basal gastrin release ranged from 3.3 +/- 0.3 to 4.1 +/- 0.3% of total cellular content. Maximal gastrin release in response to the acetylcholine receptor agonist carbachol (10(-4) M) or the selective muscarinic receptor agonist arecaidine propargyl ester (10(-4) M) was 8.5 +/- 0.4% and 7.6 +/- 0.4% of total cellular content, respectively. The EC(50) values were 3.7 +/- 0.5 X 10(-6) M carbachol and 1.8 +/- 0.4 X 10(-6) M arecaidine propargyl ester. At a concentration of 10(-6) M the non-selective muscarinic receptor antagonist atropine and the muscarinic M(3) receptor preferring antagonist hexahydro-sila-difenidol (HHSiD; M(3) greater than or equal to M(1) > M(2)) completely inhibited gastrin release in response to carbachol (K-i values: 52 X 10(-9) M atropine and 29 X 10(-9) M HHSiD) and arecaidine propargyl ester (K-i values: 11 X 10(-9) M atropine and 13 X 10(-9) M HHSiD). In contrast the muscarinic M(1) receptor preferring antagonist pirenzepine (M(1) > M31 M(2)) even at high concentrations (10(-6)-10(-5) M) inhibited the carbachol- and arecaidine propargyl ester-induced gastrin release by only about 40%, while the muscarinic M(2)/M(4) receptor antagonist 11-[[4-[4-(diethylamino)butyl]-1-piperidnyl]aceyl]-5,1-dihydro-6H-pyrido[2,3-b][1,4]benzodiazepin-6-one hydrochloride (AQ-RA 741; M(2) = M(4) > M(1) > M(3)) was ineffective at all concentrations tested (10(-9)-10(-5) M). We conclude that muscarinic receptor agents may directly stimulate gastrin release from antral G-cells via activation of muscarinic M(3) receptors most probably residing on the G-cells themselves.