Biphasic Insulin Secretion After Intravenous but Not After Intraportal CCK-8 Infusion in Rats

The effects of physiological doses of sulfated cholecystokinin-8 (CCK-8) on insulin secretion were investigated in unrestrained unanesthetized rats. The routes of administration were intravenous or intraportal infusion. Intravenous infusion (0.33–5.0 μg CCK-8 · kg⁻¹ · 20 min⁻¹) resulted in a biphasic response pattern consisting of a fast 1st-min rise in plasma insulin concentration and a slower second phase that lasted throughout the infusion. The first phase showed the same amplitude with all amounts of CCK-8 administered in this study, whereas the second phase exhibited dose dependency. Blood glucose levels were lowered during all infusions of CCK-8, although the second phase of insulin release was absent with the lowest dose. These results suggest a strong stimulatory effect of CCK-8 on the pancreatic β-cells, probably by changing the set point for glucose. The described effects of intravenous administration of CCK-8 cannot be produced when the infusion is given into the portal vein. Only very high concentrations of CCK-8 (15 μg · kg⁻¹ · 20 min⁻¹) produced a small increase in plasma insulin levels, indicating a strong CCK-8-eliminating mechanism in the liver. These results indicate that 1) CCK-8 evokes biphasic insulin release and a concomitant drop in glucose levels, and 2) CCK-8 acting on the β-cell in vivo is not of intestinal origin but is probably released by the pancreatic vagai branch.