Inhibition of intestinal degradation of somatostatin by rat milk

In vitro degradation of ¹²⁵I-labeled somatostatin-14 (Tyr¹¹) [¹²⁵I-SS-14(Tyr¹¹)] by luminal flushings of rat gastrointestinal segments was studied to characterize the fate of somatostatin in the gastrointestinal lumen. In addition, we evaluated the effect of rat milk as a potential inhibitor of luminal degradation of ¹²⁵I-SS-14(Tyr¹¹). Degradation of ¹²⁵I-SS-14(Tyr¹¹) was not detected in stomach flushings from either suckling or weanling rats. Luminal flushings from the small intestine degraded ¹²⁵I-SS-14(Tyr¹¹), with a gradient increase of activity from duodenum to midjejunum (degradation in suckling rat midjejunum and ileum was about five times lower than that in weanling rat). Degradation of ¹²⁵I-SS-14(Tyr¹¹) by luminal flushings of suckling rat midjejunum was dose dependently inhibited by rat milk casein and soluble fractions. Inhibitory activity of rat milk soluble fraction was heat labile and several times more potent than that of casein fraction. Casein fraction appeared to be stable at 100°C for up to 30 min of exposure. These studies suggest that somatostatin is stable in the gastric lumen and that milk protects somatostatin from intestinal luminal proteolysis, indicating a possible physiological significance of milkborne SS-14 for the suckling rat gastrointestinal tract.