Dynamic control of cystic fibrosis transmembrane conductance regulator Cl(-)/HCO3(-) selectivity by external Cl(-).
HCO(3)(-) secretion is a vital activity in cystic fibrosis transmembrane conductance regulator (CFTR)-expressing epithelia. However, the role of CFTR in this activity is not well understood. Simultaneous measurements of membrane potential and pH(i) and/or current in CFTRexpressing Xenopus oocytes revealed dynamic control of CFTR Cl(-)/HCO(3)(-) permeability ratio, which is regulated by external Cl(-) (Cl(-)(o)). Thus, reducing external Cl(-) from 110 to 0-10 mm resulted in the expected increase in membrane potential, but with no corresponding OH(-) or HCO(3)(-) influx. Approximately 3-4 min after reducing Cl(o)(-) to 0 mm, an abrupt switch in membrane potential occurs that coincided with an increased rates of OH(-) and HCO(3)(-) influx. The switch in membrane permeability to OH(-)/HCO(3)(-) can also be recorded as a leftward shift in the reversal potential. Furthermore, an increased rate of OH(-) influx in response to elevating pH(o) to 9.0 was observed only after the switch in membrane potential. The time to switch increased to 11 min at Cl(o)(-) of 5 mm. Conversely, re-addition of external Cl(-) after the switch in membrane potential did not stop HCO(3)(-) influx, which continued for about 3.9 min after Cl(-) addition. Importantly, addition of external Cl(-) to cells incubated in Cl(-)-free medium never resulted in HCO(3)(-) efflux. Voltage and current clamp experiments showed that the delayed HCO(3)(-) transport is electrogenic. These results indicate that CFTR exists in two conformations, a Cl(-) only and a Cl(-) and OH(-)/HCO(3)(-) permeable state. The switch between the states is controlled by external Cl(-). Accordingly, a different tryptic pattern of CFTR was found upon digestion in Cl(-)-containing and Cl(-)-free media. The physiological significance of these finding is discussed in the context of HCO(3)(-) secretion by tissues such as the pancreas and salivary glands.
Shcheynikov
Nikolay
N
Department of Physiology and Graduate Program in Molecular Biophysics, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75390-9040, USA.
Kim
Kil Hwan
KH
Kim
Kyung-Mi
KM
Dorwart
Michael R
MR
Ko
Shigeru B H
SB
Goto
Hidemi
H
Naruse
Satoru
S
Thomas
Philip J
PJ
Muallem
Shmuel
S
D000818:Animals; D027963:Chloride-Bicarbonate Antiporters / Q000096:biosynthesis* / Q000737:chemistry; D002713:Chlorine / Q000378:metabolism*; D019005:Cystic Fibrosis Transmembrane Conductance Regulator / Q000737:chemistry*; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D005260:Female; D006863:Hydrogen-Ion Concentration; D008564:Membrane Potentials; D051379:Mice; D009865:Oocytes / Q000378:metabolism; D010179:Pancreas / Q000378:metabolism; D018408:Patch-Clamp Techniques; D011487:Protein Conformation; D018075:RNA, Complementary / Q000378:metabolism; D012469:Salivary Glands / Q000378:metabolism; D013997:Time Factors; D014357:Trypsin / Q000494:pharmacology; D014981:Xenopus; D014982:Xenopus laevis
D027963:Chloride-Bicarbonate Antiporters; D018075:RNA, Complementary; D019005:Cystic Fibrosis Transmembrane Conductance Regulator; D002713:Chlorine; D014357:Trypsin