Electrogenic properties of the cloned Na+/glucose cotransporter: I. Voltage-clamp studies |
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Authors: | Lucie Parent Stéphane Supplisson Donald D. F. Loo Ernest M. Wright |
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Affiliation: | (1) Division of Biomedical Sciences, University of California, 92521-0121 Riverside, California |
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Abstract: | Summary Cystic fibrosis (CF) is characterized by abnormal epithelial Cl– conductance (GCl). In vitro studies that have shown that cAMP regulation is an intrinsic property of the CF-affected GCl(CF-GCl) have been carried out previously on cultured secretory cells and on nonepithelial cells. Even though GCl in absorption is defective in CF, a clear demonstration of cAMP regulation of CF-GCl in a purely absorptive tissue is lacking. We studied the cAMP regulation of CF-GCl in the microperfused intact human reabsorptive sweat duct. About 40% of the ducts responded to cAMP (responsive) while the remainder of the ducts did not. In responsive ducts, cAMP-elevating agents: -adrenergic agonist isoproterenol (IPR), CPT-cAMP, forskolin, theophylline or IBMX increased Gtby about 2.3-fold (n = no. of ducts = 8). Removal of media Cl–, but not amiloride pretreatment (in the lumen), abolished the cAMP response, indicating exclusive activation of GCl. cAMP activated both apical and basolateral GCl. cAMP hyperpolarized gluconate: Cl– (lumen: bath) transepithelial bionic potentials ( Vt=–20.3±5.2 mV, mean ±se, n=9) and transepithelial 3 1 luminal NaCl dilution diffusion potentials ( Vt=–8.8±2.9 mV, n=5). cAMP activated basolateral GCl as indicated by increased bi-ionic (gluconate: Cl–, bath: lumen) diffusion potentials (by about 12 mV). The voltage divider ratio in symmetric NaCl solutions increased by 60%. Compared to responsive ducts, nonresponsive ducts were characterized by smaller spontaneous transepithelial potentials in symmetrical Ringer's solution (Vt=–6.9±0.8 mV, n=24, nonresponsive vs. –19.4±1.8 mV, n=22, responsive ducts) but larger bi-ionic potentials (–94±6 mV, n=35, nonresponsive vs. –65±5 mV, n=17, responsive ducts) and dilution diffusion potentials (–40±5 mV, n=11, nonresponsive vs. –29±3 mV, n=7, responsive ducts). These results are consistent with an inherently (prestimulus) maximal activation of GCl in nonresponsive ducts and submaximal activation of GCl in responsive ducts. We conclude that cAMP activates CF-GCl which is expressed and abnormal in both apical and basal membranes of this absorptive epithelium in CF.Abbreviations CF cystic fibrosis - Gt transepithelial conductance - Vb electrical potential across the basolateral membrane - Va electrical potential across the apical membrane - Vt transepithelial potential - Vb transepithelial currentinduced voltage deflections across the basolateral membrane - Va transepithelial current-induced voltage deflections across the apical membrane - Vt transepithelial current-induced voltage deflection across the epithelium - VDR voltage divider ratio - GCl transepithelial Cl– conductance - CF-GCl cystic fibrosis-affected Cl– conductance - EMF electromotive force - IPR isoproterenol - IBMX 3-isobutyl-1-methylxanthine - CPT-cAMP chlorophenylthio-adenosine 3 -5 cyclic monophosphate - PGE2 prostaglandin E2 |
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Keywords: | CFTR regulation sweat duct cystic fibrosis |
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