| Abstrakt: |
The effects of short (1 sec) and long (1 min) transepithelial current clamps on membrane voltages and resistances of Necturus gallbladder were investigated. Transepithelial and cell membrane current-voltage relationships determined from 1-sec clamps revealed that: a) depolarization of the apical membrane voltage (Vmc) results in a marked decrease in apical membrane fractional resistance (fRa), whereas hyperpolarization of Vmc results in either no change in fRa or a small increase, and b) the voltage-dependent changes in fRa are essentially complete within 500 msec. Exposure of the tissue to 5 mM TEA+ on the mucosal side caused no significant change in baseline Vmc (-69 +/- 2 mV) and yet virtually abolished the voltage dependence of fRa. A possible interpretation of these results is that two types of K+ channels exist in the apical membrane, with different voltage dependencies and TEA+ sensitivities. Acidification or Ba2+ addition to the mucosal solution also reduced the voltage-dependent changes in fRa. The time courses of the changes in fRa and in the cable properties of the epithelium were assessed during 1-min transepithelial current clamps (+/- 200 microA/cm2). No secondary change in fRa was observed with mucosa-to-serosa currents, but a slow TEA+-sensitive decrease in fRa (half-time of seconds) was evident with serosa-to-mucosa currents. Cable analysis experiments demonstrated that the initial (less than 500 msec) voltage-dependent decrease in fRa is due to a fall in apical membrane resistance. The later decrease in fRa is due to changes in both cell membrane resistances attributable to the increase in transcellular current flow resulting from a fall in paracellular conductance. The voltage dependence of the apical membrane conductance is a more significant problem in estimating fRa than the current-induced effects on the lateral intercellular spaces. In principle, TEA+ can be used to prevent the nonlinear behavior of Ra during measurements of the voltage divider or membrane resistance ratio. [ABSTRACT FROM AUTHOR] |
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