Pertussis toxin-sensitive inhibition of glucagon-like peptide 1-stimulated acid production by epidermal growth factor and transforming growth factor α in rat parietal cells

We have recently shown that the intestinal hormone glucagon-like peptide-1 (GLP-1)-7-36) amideis a cAMP-dependent stimulant of rat parietal cell H⁺ production. Epidermal growth factor (EGF) and transforming growth factor-α (TGFα) are known to inhibit histamine-stimulated parietal cell function by reducing cAMP productions in a pertussis toxin-sensitive manner. Pertussis toxin blocks G_iα, the inhibitory subunit of adenylate cyclase, thereby preventing inhibitors from acting via G_iα. Therefore, we used pertussis toxin as a tool to determine whether EGF and TGFα inhibit GLP-1-stimulated parietal cell function via G_iα. In enriched (76 ± 4%) rat parietal cells [¹⁴C]aminopyrine accumulation and cAMP production were maximally stimulated by GLP-1-(7-36) amide (10^-8 and 10^-7 M, respectively) or by histamine (10^-4 and 10^-3 M, respectively). EGF and TGFα (10^-13-10^-7 M) caused concentration-dependent inhibition of GLP-1-stimulated parietal cell function. Maximal inhibition (33% and 37% of the response to GLP-1-(7-36) amide was observed at 10^-8 M EGF and 10^-9 M TGFα, respectively. There was a close correlation (r = 0.83; P < 0.05; n = 7) between the inhibition by EGF and TGFα of [¹⁴C]aminopyrine accumulation and the fall in cAMP production in GLP-1-stimulated parietal cells. The identical concentrations of both growth factors which maximally reduced GLP-1-stimulated parietal cell function inhibited [¹⁴C]aminopyrine accumulation in response to histamine by approximately 30%. Thus, the effect of EGF and TGFα on the response to GLP-1 closely resembles that on histamine-induced parietal cell function. Pretreatment with pertussis toxin (300 ng/ml; 37°C; 4 h) completely reversed inhibition by EGF and TGFα of aminopyrine accumulation and cAMP production following stimulation by either GLP-1 or histamine. In crude membranes prepared from enriched parietal cells, GLP-1-induced adenylate cyclase activity was inhibited by EGF (10^-8 M) and TGFα (10^-9 M) in a pertussis toxin-sensitive manner. We conclude that EGF and TGFα inhibit GLP-1-(7-36)-stimulated parietal cell function via a pertussis toxin-sensitive substrate, presumably G_iα, the inhibitory subunit of adenylate cyclase.