ONTOGENY OF GASTRIN, SOMATOSTATIN, AND THE H+/K+-ATPASE IN THE OVINE FETUS

In the term human and ovine fetus, plasma gastrin is elevated, but gastric acid secretion is below adult levels, suggesting a developmentally related immaturity in gastrin and gastric acid regulation. This study investigated a number of elements of the gastric acid regulatory system: gastrin and its glycine-extended precursor, somatostatin, and the H+/K+-ATPase. Measurements were made in blood, antrum, and fundus of the ovine fetus during the last half of gestation, of 15-day-old lambs, and of adult sheep at the level of mRNA synthesis, tissue storage, and secretion. Plasma amidated gastrin (gastrin-amide) was elevated at or above adult values from 125 days (term is 145 days) and steadily increased with development, peaking in the lamb. Similar changes occurred with plasma glycine-extended gastrin (gastrin-gly). The peak concentration of antral gastrinamide was present in the lamb, while the maximum antral gastrin-gly level occurred 1 week before birth. Gastrin mRNA paralleled the changes in antral gastrin-gly. The proportion of higher mol wt species of gastrin decreased during gestation in both plasma and antrum. Low amounts of mRNA for the H+/K+-ATPase was present from at least 120 days of gestation and antedated gastric acid secretion. However, there was a 3-fold increase in H+/K+-ATPase mRNA from the 140-day-old fetus to the lamb, the period when the greatest reduction in gastric pH occurred (pH 5 to 2). Antral and fundic somatostatin increased rapidly in the fetus at 120 days gestation and were above adult values at term and in the lamb. Somatostatin mRNA changed in parallel to somatostatin peptide. Somatostatin-14 was the major species in antrum and fundus throughout development. The increase in circulating and antral gastrin-amide after birth may be the result of increased amidation of gastrin-gly as well as increased expression of gastrin mRNA. Amidation of gastrin may be a regulatory step in the production of biologically active gastrin during development. The major increase in gastrin and the H+/K+-ATPase that occurs in the week before and after gestation correlated with the onset of increased gastric acidity.