Stimulation by bombesin of immunoreactive somatostatin release into rat hypophysial portal blood.
Bombesin was injected into the cerebral ventricle of male rats anesthetized with urethane to study its effect on plasma levels of immunoreactive somatostatin (IRS) in hypophysial portal and jugular blood. An intraventricular injection of bombesin (0.2 and 2 micrograms/rat) caused a significant and dose-related increase in plasma IRS in hypophysial portal blood but not in jugular blood. Although bombesin placed into the cerebral ventricle is known to stimulate glucagon and epinephrine release, an iv injection of glucagon (100 micrograms/100 g BW) or epinephrine (2.5 micrograms/100 g BW) did not cause any significant changes in plasma IRS levels in hypophysial portal and jugular blood, suggesting that these substances do not mediate bombesin stimulation of portal IRS release. Pretreatment with naloxone (75 micrograms/100 g BW, iv) failed to affect the portal IRS release induced by bombesin (2 micrograms/rat), indicating that the opiate receptor is not likely to be involved in this reaction. To ascertain whether IRS released by bombesin into hypophysial portal blood is biologically active, the effect of bombesin on the plasma GH level was then examined. Bombesin (2 micrograms/rat) injected intraventricularly completely suppressed the rise of plasma GH after the intraventricular injection of beta-endorphin (1 microgram/rat) or the iv injection of prostaglandin E1 (5 micrograms/100 g BW). Bombesin thus appears to stimulate the secretion of IRS, and probably biologically active somatostatin as well, from the hypothalamus into hypophysial portal blood, thereby inhibiting GH release from the anterior pituitary.