Fluid secretion in interlobular ducts isolated from guinea-pig pancreas

摘要

class="enumerated" style="list-style-type:decimal">Pancreatic HCO3⁻ and fluid secretion were studied by monitoring luminal pH (pHL) and luminal volume simultaneously in interlobular duct segments isolated from guinea-pig pancreas. The secretory rate and HCO3⁻ flux were estimated from fluorescence images obtained following microinjection of BCECF-dextran (70 kDa, 20 μM) into the duct lumen.Ducts filled initially with a Cl⁻-rich solution swelled steadily (2.0 nl min⁻¹ mm⁻²) when HCO3⁻²/CO was introduced, and the luminal pH increased to 8.08. When Cl⁻ was replaced by glucuronate, spontaneous fluid secretion was reduced by 75 %, and pHL did not rise above 7.3.Cl⁻-dependent spontaneous secretion was largely blocked by luminal H2DIDS (500 μM). We conclude that, in unstimulated ducts, HCO3⁻ transport across the luminal membrane is probably mediated by Cl⁻-HCO3⁻ exchange.Secretin (10 nM) and forskolin (1 μM) both stimulated HCO3⁻ and fluid secretion. The final value of pHL (8.4) and the increase in secretory rate (1.5 nl min⁻¹ mm⁻²) after secretin stimulation were unaffected by substitution of Cl⁻.The Cl⁻-independent component of secretin-evoked secretion was not affected by luminal H2DIDS. This suggests that a Cl⁻-independent mechanism provides the main pathway for luminal HCO3⁻ transport in secretin-stimulated ducts.Ducts filled initially with a HCO3⁻-rich fluid (125 mM HCO3⁻, 23 mM Cl⁻) secreted a Cl⁻-rich fluid while unstimulated. This became HCO3⁻-rich when secretin was applied.Addition of H2DIDS and MIA (10 μM) to the bath reduced the secretory rate by 56 and 18 %, respectively. Applied together they completely blocked fluid secretion. We conclude that basolateral HCO₃⁻ transport is mediated mainly by Na⁺-HCO₃⁻ cotransport rather than by Na⁺-H⁺ exchange.