Effects of substance P and neurotensin on somatostatin levels in rat portal plasma.

A sensitive and specific RIA system has been developed for measuring somatostatin (SRIF) in plasma. On chromatographic analysis of the plasma of portal blood of rats, two peaks of SRIF-like immunoreactivity were found, one (large form) near the position of albumin and the other (small form) in the same position as synthetic SRIF. The nature of the large form is unknown, but the small form was immunologically and physicochemically indistinguishable from synthetic cyclized SRIF. After iv injection of substance P (10 micrograms/100 g BW) into rats, there was a significant increase in the portal plasma level of SRIF from the basal values of 277 +/- 75 to 1130 +/- 111 pg/ml at 15 min. On the other hand, the pancreatic SRIF concentration decreased from 0.27 +/- 0.01 to 0.16 +/- 0.04 ng/mg wet wt at 15 min and was maintained at a low level up to 30 min after the injection. Injection of substance P did not affect the SRIF concentration in the stomach. Similar results were obtained after iv administration of neurotensin (3 micrograms/100 g BW). These findings suggest that increases in the portal plasma levels of SRIF in rats treated with substance P or neurotensin may reflect, at least in part, the release of SRIF from the pancreas. The physiological significance of SRIF secretion remains to be clarified, but this study suggests that SRIF can be released from the digestive organs, including the pancreas, into the portal vein and may act on the tissues in the peripheral area.