Evidence for an Indirect Effect of Somatostatin on Glucagon Secretion via Inhibition of Free Fatty Acids Release in the Duck

Summary

In ducks, the infusion of somatostatin (at a rate of 800 or 200 ng/kg/min for 30 min) elicited a prompt (within 5 min) fall in the plasma free fatty acids (FFA) levels. This effect is not mediated via modifications of pancreatic functions or growth hormone secretion as it occurs in pancreatectomized and hypophysectomized ducks infused with somatostatin, suggesting that it has a direct antilipolytic effect on adipose tissue. In intact ducks the infusion of somatostatin increased the level of plasma glucagon-like-immunoreactivity (GLI). As this effect was not observed in pancreatectomized birds glucagon solely of pancreatic origin is involved in the response to somatostatin.

To determine if somatostatin affected pancreatic A cells via lipid metabolism, ducks were infused with a mixture of somatostatin and oleic acid (in order to avoid the drop in plasma FFA levels induced by somatostatin). In controls, infused with oleic acid alone (at a rate of 12 mg/kg/min) plasma FFA levels rose and plateaued at the end of the infusion period. The levels of plasma immunoreactive glucagon (IRG) were reduced in control ducks during oleic acid infusion. When somatostatin (200 ng/kg/min) and oleic acid (12 mg/kg/min) were infused together the rise in FFA was similar during the first 20 min but by 30 min the levels had been reduced to the pretreatment concentration. In those birds IRG secretion was not reduced as in the controls and following infusion a rebound increase in IRG occurred. The infusions of oleic acid (6 mg/kg/min) with somatostatin (200 ng/kg/min) induced no change in FFA levels but blunted the IRG stimulation observed in birds infused with somatostatin alone.

These findings suggest that somatostatin is a potent antilipolytic hormone in ducks and that the stimulatory effect of somatostatin on glucagon secretion may be secondary to the inhibition of FFA release.

Key-Words:

Ducks - Glucagon Like Immunoreactivity - Immunoreactive Glucagon - Somatostatin - Free Fatty Acids - Hypophysectomy - Pancreatectomy