Plasma immunoreactive somatostatin levels in rat hypophysial portal blood: Effect of glucagon administration

Plasma somatostatin levels of the hypophysial portal blood was examined in urethane-anesthetized rats. Portal blood was withdrawn at a rate of 4.7 μl/min via a cannula placed over the stump of the pituitary stalk, collected into the ice-cold tube at 20min intervals before and after the intraventricular injection of test materials. Immunoreactive somatostatin extracted from the plasma with acid acetone was measured by a specific radioimmunoassay.Basal plasma somatostatin concentrations, 778±52pg/ml (means±SE), did not significantly change after the administration of control solution (0.1N acetic acid in physiological saline), while intraventricular injection of glucagon (2, 10 and 50 μg) caused a significant and dose-related increase in plasma somatostatin during the first 20min.These results suggested that somatostatin release from the median eminence into the hypophysial portal vessel was stimulated by glucagon, although the physiologycal significance remains to be clarified.