Evidence for KCNQ1 [K.sup.+] channel expression in rat zymogen granule membranes and involvement in cholecystokinin-induced pancreatic acinar secretion

Secretion of enzymes and fluid induced by [Ca.sup.2+] in pancreatic acini is not completely understood and may involve activation of ion conductive pathways in zymogen granule (ZG) membranes. We hypothesized that a chromanol 293B-sensitive [K.sup.+] conductance carried by a KCNQ1 protein is expressed in ZG membranes (ZGM). In suspensions of rat pancreatic ZG, ion flux was determined by ionophore-induced osmotic lysis of ZG suspended in isotonic salts. The KCNQ1 blocker 293B selectively blocked [K.sup.+] permeability (I[C.sub.50] of ~10 [micro]M). After incorporation of ZGM into planar bilayer membranes, cation channels were detected in 645/150 mM potassium gluconate cis/trans solutions. Channels had linear current-voltage relationships, a reversal potential ([E.sub.rev]) of -20.9 [+ or -] 0.9 mV, and a single-channel [K.sup.+] conductance (gK) of 265.8 [+ or -] 44.0 pS (n = 39). Replacement of cis 500 mM [K.sup.+] by 500 mM [Na.sup.+] shifted [E.sub.rev] to -2.4 [+ or -] 3.6 mV (n = 3), indicating [K.sup.+] selectivity. Single-channel analysis identified several [K.sup.+] channel groups with distinct channel behaviors. [K.sup.+] channels with a gK of 651.8 [+ or -] 88.0 pS, [E.sub.rev] of -22.9 [+ or -] 2.2 mV, and open probability ([P.sub.open]) of 0.43 [+ or -] 0.06 at 0 mV (n = 6) and channels with a gK of 155.0 [+ or -] 11.4 pS, [E.sub.rev] of -18.3 [+ or -] 1.8 mV, and [P.sub.open] of 0.80 [+ or -] 0.03 at 0 mV (n = 3) were inhibited by 100 [micro]M 293B or by the more selective inhibitor HMR-1556 but not by the maxi-[Ca.sup.2+]-activated [K.sup.+] channel (BK channel) inhibitor charybdotoxin (5 nM). KCNQ1 protein was demonstrated by immunoperoxidase labeling of pancreatic tissue, immunogold labeling of ZG, and immunoblotting of ZGM. 293B also inhibited cholecystokinin-induced amylase secretion of permeabilized acini (I[C.sub.50] of ~10 [micro]M). Thus KCNQ1 may account for ZG [K.sup.+] conductance and contribute to pancreatic hormone-stimulated enzyme and fluid secretion. exocytosis; acinar cell; secretory granule; planar bilayer