Isolation and Partial Characterization of Gastrotropin from Canine Ileum: Further Studies of the Parietal and Chief Cell Response*
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Gastrotropin (GT), a protein previously isolated from porcine ileal mucosa, with a molecular mass of 14,054 daltons, was extracted from canine ileum and purified to homogeneity. The canine and porcine peptides had similar relative molecular mass, charge, hydrophobicity, and amino acid compositions. Direct Edman degradation yielded no free amino acids, indicating a blocked NH2-terminus, and a partial sequence determination of the CNBr fragments demonstrated a high degree of homology with porcine GT. Previous studies have indicated that GT is a potent enterooxyntin, and to further characterize these observations we have investigated the actions of both porcine and canine GT on isolated enriched preparations of guinea pig and dog parietal and chief cells. The results of these studies demonstrate that GT is present in more than one species and that the cellular response to porcine and canine GT is identical. The efficacies of canine and porcine GT preparations in stimulating pepsinogen secretion and [14C]aminopyrine uptake were identical and equal to those of cholecystokinin octapeptide (CCK8) and pentagastrin. GT was 100-fold more potent than either of these two major secretagogues. Maximal [14C]aminopyrine accumulation was observed with 10(-8) M GT, with an ED50 of 2 x 10(-9) M compared to pentagastrin, which caused maximal accumulation at 10(-6) M and had an ED50 of 5 x 10(-8) M. Maximal pepsinogen secretion was observed with 10(-7) M GT, with an ED50 of 10(-10) M, compared to 10(-6) M for CCK8, with an ED50 of 10(-8) M. The maximal chief cell response to GT was unaffected by the addition of CCK8 or carbachol, but responded additively with forskolin, indicating that GT uses the same transduction mechanism as CCK8 and carbachol and does not involve the activation of adenylate cyclase. The ED50 values observed with both parietal and chief cells in these studies were close to the basal circulating levels of GT (3.5 x 10(-9) M) in adult pigs. These results clearly demonstrate that GT is a potent component of the enterooxyntin factor identified in studies of the role of the small bowel in the regulation of gastric secretion.
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