Nitric oxide and guanosine 3 ',5 '-cyclic monophosphate stimulate bile secretion in isolated rat hepatocyte couplets, but not in isolated bile duct units

Nitric oxide (NO) and guanosine 3',5'-cyclic monophosphate (cGMP) have rece ntly been shown to stimulate bile acid-independent bile flow in the isolate d perfused rat liver (IPRL), However, the cellular origin and mechanisms of this choleresis have not yet been determined, To address these questions, we examined the effects of NO and cGMP on bile secretion in isolated rat he patocyte couplets (IRHC) and in isolated bile duct units (IBDU), both of wh ich are isolated cell systems in which cell polarity is maintained and secr etion can be measured directly. Changes in the area of the canalicular and ductular lumens were determined in IRHC and IBDU, respectively, as indicato rs of the rate of fluid secretion using video microscopy. In addition, Cl-/ HCO3- exchanger activity in IBDU was evaluated by measuring changes in intr acellular pH (pH(i)) after Cl- removal/readmission by microfluorometric met hods. In the presence of HCO3-, both the NO donor, S-nitroso-acetyl-penicil lamine (SNAP), and the cell-permeant cGMP analogue, dibutyryl cGMP (DBcGMP) , stimulated canalicular bile secretion (P <.05), as did the cell-permeant cAMP. analogue, dibutyryl cAMP (DBcGMP) (P <.05). Removal of HCO3- from the buffer completely abolished the choleretic effects of DBcGMP, but had no e ffect on NO-induced choleresis. In contrast, secretion in IBDU was not stim ulated following incubations with SNAP or DBcGMP over 30 minutes, whereas D BcAMP and secretin, a cholangiocyte secretagogue and cAMP agonist, both had a marked effect on ductular secretion over this same time interval (P <.05 ), SNAP also had no effect on Cl-/HCO3- exchanger activity in IBDU, and inh ibition of endogenous NO synthesis by N-G-monomethyl-L-arginine (L-NMMA) di d not alter secretin-induced stimulation of ductular bile secretion and Cl- /HCO3- exchanger activity. in summary, NO and cGMP stimulate bile secretion exclusively at the the level of hepatocytes, whereas cAMP mediates cholere sis at both hepatocyte and bile duct levels. These findings may have import ant implications for the regulation of ductular bile secretion by hormones and neuropeptides, as well as under pathological conditions with increased hepatic NO synthesis.