Do metformin and phenformin potentiate differently B-cell response to high glucose? An in vitro study on isolated rat pancreas.

The study investigated the effects of metformin and phenformin, at "therapeutic" concentrations, on the pancreatic A-, B- and D- cell response to glucose using the isolated perfused rat pancreas model. Changes in the rate of pancreatic lactate output after these biguanides were also evaluated. Metformin--at 1.5 micrograms/ml--and phenformin--at 100 ng/ml--were separately infused both at 160 mg/dl and 300 mg/dl glucose levels. Neither metformin nor phenformin affected glucagon or somatostatin secretion during these two metabolic stimuli with glucose, nor did they significantly influence insulin response to the lower glucose stimulus. Both metformin and phenformin enhanced insulin response to 300 mg/dl glucose infusion and increased the second phase of the B-cell secretory profile but only phenformin significantly enhanced the pancreatic lactate output rate during the 300 mg/dl glucose infusion. Infusion with dichloroacetate (a stimulator of the mitochondrial pyruvate oxidation) or with verapamil (a calcium antagonist) alone did not modify the insulin response to high glucose concentrations. During metformin infusion dichloroacetate neither modified metformin's effects on B-cell response to high glucose nor did it affect the pancreatic lactate output rate. On the other hand dichloroacetate opposed phenformin's effects on the B-cell response to high glucose and reversed the rise in the pancreatic lactate output rate. Verapamil inhibited the effect of metformin on the B-cell response to high glucose but failed to affect phenformin's influence on high-glucose induced insulin release. These data suggest both metformin and phenformin potentiate--at least in rats--the late phase of insulin secretory response to high glucose. However metformin seems to influence pancreatic B-cell activity mainly by facilitating the trans-membrane calcium ion influx responsible for the second phase of insulin release. Phenformin's influence seems indirect since it increases pancreatic lactate production which mediates the enhanced B-cell response to glucose.