The direct effect of insulin on canalicular bile flow

Effect of insulin on hepatic bile secretion during normoglycaemia and hyperglycaemia

The roles of hyperosmolality, hyperglycaemia, and insulin in glucose-induced reduction of bile secretion were examined in anaesthetized pigs. Compared with normoglycaemia, intravenous infusion of isotonic glucose reduced bile acid-dependent bile secretion at a plasma glucose concentration of 18 mmol/l, with 34 +/- 4%. Lowering of plasma glucose concentration to normoglycaemia after administration of insulin (10 U/kg body wt. low dose) increased bicarbonate-dependent bile secretion by 23 +/- 3%. Induction of hyperglycaemia (plasma glucose concentration, 16 mmol/l) and the combined infusion of isotonic glucose and the low insulin dose decreased bile secretion by 22 +/- 3%. During hyperglycaemia (plasma glucose concentration, 16 mmol/l) the combined infusion of isotonic glucose and a high dose of insulin (60 U/kg body wt) increased bile acid-dependent bile secretion by 26 +/- 3%. Hyperglycaemia reduces bile secretion without altering plasma osmolality. Endogenous production (or too rapid degradation) of insulin may be too small during intravenous glucose infusion to cope with the metabolic demands of hepatocellular glucose conversion. This may be overcome by administration of insulin in a large dose, which stimulates bile acid secretion.

Effect of verapamil on insulin-stimulated choleresis

Insulin is one of several neurohumoral substances known to have a choleretic effect in vivo and in the isolated perfused rat liver. Infusion of insulin in the perfused rat liver preparation results in stimulation of bile acid-independent bile flow evidenced by increased bile flow, decreased bile acid concentration, and stable bile acid output. The mechanism of insulin-stimulated choleresis is unknown but may involve calcium as an intracellular second messenger. The present studies were performed to assess the role of membrane calcium channels in mediating choleresis and insulin-stimulated bile acid-independent bile flow in the in situ perfused rat liver. We have shown that verapamil, a specific calcium channel blocker, has no effect on bile flow, bile acid concentration, or bile acid output during bile acid-stimulated choleresis at a taurocholate infusion rate of 40 or 80 nmole/g liver/min. Insulin caused a significant increase in bile flow (18-30%) and a decrease in bile acid concentration (13-21%) without affecting bile acid output at a taurocholate infusion rate of 40 or 80 nmole/g liver/min. Verapamil failed to inhibit insulin-stimulated choleresis at a taurocholate infusion rate of 80 nmole/g liver/min. Although we observed an insulin-stimulated increase in bile flow and a decrease in bile acid concentration in the presence of verapamil at a taurocholate infusion rate of 40 nmole/g liver/min, these changes failed to reach statistical significance. We conclude that verapamil has no effect on choleresis or insulin-stimulated bile flow in the perfused rat liver and that the mechanism by which insulin promotes bile acid-independent bile flow is not mediated by verapamil-sensitive calcium channels.

Is somatostatin a directly acting cholestatic hormone?

Previous experiments have demonstrated the cholestatic effects of somatostatin administration in several animal species. These effects were confirmed in the rat. Nine pairs of intact awake rats received intravenous sodium taurocholate (23 mg hr-1) to stabilize bile flow, and half were later given somatostatin at doses of 185 micrograms hr-1. After 1 hr of somatostatin the experimental group showed a significant decrease in bile flow compared to the control group. Cholestasis reversed when somatostatin infusion was stopped. An in situ isolated perfused rat liver technique was used to assess the direct effects of somatostatin on biliary flow. In 10 pairs of rat livers, after achieving stable bile flow, half of those perfused (the "experimentals") received continuous (370 micrograms hr-1) somatostatin infusion, while the controls received saline. The percentage change in bile flow from baseline in the somatostatin group was not significantly different from that in controls for any test period. Bile analysis revealed no significant differences between groups for cholesterol, phospholipid, or bile salt concentrations or outputs. These data suggest that somatostatin inhibits bile secretion by some mechanism other than direct inhibition of bile secretory mechanisms.