Hydrochlorothiazide action on the apical Cl-, Ca2+ and K+ conductances in rabbit gallbladder epithelium. Presence of an apamin-sensitive, Ca(2+)-activated K+ conductance.
In the rabbit gallbladder epithelium, hydrochlorothiazide (HCTZ) was shown to inhibit the transepithelial NaCl transport and the apical Na(+)-Cl- symport, to depolarize the apical membrane potential and to enhance the cell-to-lumen Cl- backflux (radiochemically measured), this increase being SITS-sensitive. To better investigate the causes of the depolarization and the Cl- backflux increase, cells were punctured with conventional microelectrodes on the luminal side (incubation in bicarbonate-free saline at 27 degrees C) and the apical membrane potential (Vm) was studied either with prolonged single impalements or with a set of short multiple impalements. The maximal depolarization was of 3-4 mV and was reached with 2.5 x 10(-4) M HCTZ. It was significantly enhanced by reducing luminal Cl- concentration to 30 mM; it was abolished by SCN-, furosemide, SITS; it was insensitive to DPC. SITS converted the depolarization into a hyperpolarization of about 4 mV; this latter was apamin, nifedipine and verapamil sensitive. It was concluded that HCTZ concomitantly opens apical Cl- and (probably) Ca2+ conductances and, indirectly, a Ca(2+)-sensitive, apamin inhibitable K+ conductance: since the intracellular Cl- activity is maintained above the value predicted at the electrochemical equilibrium, the opening of the apical Cl- conductance depolarizes Vm and enhances Cl- backflux. In the presence of apamin or verapamil, to avoid the hyperpolarizing effects due to HCTZ, the depolarization elicited by this drug was fully developed (7-10 mV) and proved to be Ca2+ insensitive. On this basis and measuring the transepithelial resistance and the apical/basolateral resistance ratio, the Cl- conductance opened by HCTZ has been estimated and the Cl- backflux increase calculate: it proved to be in the order of that observed radiochemically. The importance of this Cl- leak to the lumen in the overall inhibition of the transepithelial NaCl transport by HCTZ has been evaluated.
Cremaschi
D
D
Dipartimento di Fisiologia e Biochimeca Generali, Università degli Studi di Milano, Italy.
Vallin
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P
Porta
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C
D000818:Animals; D001030:Apamin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002712:Chlorides / Q000378:metabolism*; D004553:Electric Conductivity; D004848:Epithelium / Q000187:drug effects / Q000378:metabolism; D005704:Gallbladder / Q000187:drug effects* / Q000378:metabolism*; D006852:Hydrochlorothiazide / Q000494:pharmacology*; D066298:In Vitro Techniques; D017136:Ion Transport / Q000187:drug effects; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D011188:Potassium / Q000378:metabolism*; D011817:Rabbits
D002712:Chlorides; D006852:Hydrochlorothiazide; D001030:Apamin; D011188:Potassium; D002118:Calcium