In vivo absorption of pancreatic ribonuclease from the small intestine of young and mature rats.

It has been postulated that exocrine pancreatic enzymes may undergo an enteropancreatic recirculation as a mechanism for the conservation of digestive enzymes. Exocrine pancreatic secretions contain considerable amounts of ribonuclease (RNase). Therefore we examined the absorption of RNase from the small bowel of 4-week and 10-week-old rats by measuring concentration changes of the substrate in portal blood after instilling RNase into in vivo, in situ intestinal loops. Unphysiologically high intestinal luminal RNase concentrations were required for sufficient RNase to cross the mucosal barrier and raise portal vein blood RNase levels. A gradient of 1,000:1 existed between intestinal lumen and portal vein blood RNase concentrations in the 4-week-old rats. The gradient was 10 times greater in the mature animals. No significant differences in luminal absorption of RNase from duodenum or ileum were detected. The data indicate that RNase penetrates the intestinal mucosa in vivo but that only small amounts are absorbed. Therefore if an enteropancreatic recirculation of exocrine pancreatic RNase exists in the rat, only negligible quantities of RNase are included in such a recirculation.