Localization of K-NPPase and Li+ secretion in the exocrine pancreas of the pig.

To study the mode of transepithelial Na+ transport into pancreatic ducts during secretin-dependent NaHCO3 secretion, Na, K-ATPase was first localized within the exocrine pancreas of the pig using a cytochemical reaction for K-dependent p-nitrophenylphosphatase (K-NPPase). K-NPPase staining was confined to the lateral cell membrane bordering the intercellular spaces between ductal cells, negating the possibility of primary active, transcellular Na+ transport into pancreatic ducts. To assess how transepithelial Na+ transport may be coupled to HCO-3 secretion, net flux of Li+ into pancreatic juice was measured following intravenous systemic Li+ loading of 12 secretin infused, anaesthetized pigs. At plasma Li+ 32 (23-35) mmol l-1, Li+ displaced Na+ as accompanying cation to secreted HCO-3, and Li+/Na+ in pancreatic juice matched Li+/Na+ in arterial plasma. During superimposed inhibition of pancreatic water flux by hyperglycaemia, Li+ and Na+ were both transported against a transepithelial concentration gradient. Li+ reduced pancreatic HCO-3 secretion rate by 14 (-2 to -20)%, as well as Na,K-ATPase activity in a separate in vitro assay. The finding that Li+ substituted for Na+ in the secretion even during reduced osmotic water flow suggests that Na+ and Li+ are transported together with secreted HCO-3 into pancreatic juice by an electrogenic mechanism in addition to solvent drag and diffusion.