Free somatostatin in the circulation: amounts and molecular sizes of somatostatin-like immunoreactivity in portal, aortic, and vena caval plasma of fasting and meal-stimulated dogs.

Somatostatin-like immunoreactivity (SLI) of plasma from the portal vein, aorta, and inferior vena cava and of lymph from the thoracic duct of both fasted and meal-stimulated dogs was measured and characterized with respect to molecular size. Significant portal vein-arterial and arteriovenous SLI gradients were present in fasting dogs, and they increased sharply after the intragastric infusion of liver extract and HCl. Chromatography of fasting plasma at pH 7.4 revealed all measurable SLI to be confined to the void volume fractions of a Bio-Gel P-6 column, although, after a 7-fold concentration of fractions coeluting with somatostatin, approximately 1600-daltion SLI was detected in the portal venous plasma. The rise in SLI after a meal was due primarily to an increase of approximately 1600-dalton SLI; approximately 1600-dalton SLI was detectable in unconcentrated portal venous and aortic plasma and in the peripheral venous plasma concentrated 7-fold. SLI levels in lymph were similar to those of basal venous plasma and did not increase with a meal. This first demonstration at a physiological pH of a approximately 1600-dalton SLI in the arterial circulation suggests that a free, readily available form of endogenous somatostatin is present in the canine circulation and could be playing a hormonal role.