Role of the neuropeptide, bombesin, in bile secretion

Since ancient times, bile secretion has been considered vital for maintaining health. One of the main functions of bile secretion is gastric acid neutralization with biliary bicarbonate during a meal or Pavlovian response. Although the liver has many extrinsic and intrinsic nerve innervations, the functional role of these nerves in biliary physiology is poorly understood. To understand the role of neural regulation in bile secretion, our recent studies on the effect of bombesin, a neuropeptide, on bile secretion and its underlying mechanisms will be reviewed. Using isolated perfused rat livers (IPRL) from both normal and 2 week bile duct ligated rats, as well as hepatocyte couplets and isolated bile duct units (IBDU) from normal rat livers, bombesin was shown to stimulate biliary bicarbonate and fluid secretion from bile ducts. Detailed pH studies indicated that bombesin stimulated the activity of Cl-/HCO3- exchanger, which was counterbalanced by a secondary activation of electrogenic Na+/HCO3- symport. Quantitative videomicroscopic studies showed that bombesin-stimulated fluid secretion in IBDU was dependent on Cl- and HCO3- in the media, anion exchanger(s), Cl- and K+ channels, and carbonic anhydrase, but not on the microtubular system. Furthermore, this bombesin response is inhibited by somatostatin but not substance P. Finally, studies of secondary messengers in isolated cholangiocytes and IBDU indicated that bombesin had no effect on intracellular cAMP, cGMP, or Ca++ levels in cholangiocytes. These results provide evidence that neuropeptides such as bombesin can directly stimulate fluid and bicarbonate secretion from cholangiocytes by activating luminal Cl-/HCO3- exchange, but by different mechanisms from those established for secretin. These findings, in turn, suggest that neuropeptides may play an important regulatory role in biliary transport and secretion. Thus, this neuropeptidergic regulation of bile secretion may provide a plausible mechanism for the bicarbonate-rich choleresis seen with meals or Pavlovian response.