LOW-AMILORIDE-AFFINITY NA-CELLS ISOLATED FROM THE ENDOLYMPHATIC SAC OF GUINEA-PIGS( CHANNEL IN THE EPITHELIAL)

By using the whole-cell parch-clamp technique, an amiloride-sensitive Na+-selective conductance was found in epithelial cells from the endol ymphatic sac (ES) epithelia of guinea-pi,os. In the current-clamp conf iguration, the average resting membrane potential was -41.7+/-8.4 mV ( n = 22). Application of amiloride at a concentration of 20 mu M elicit ed a decrease in cation conductance that was responsible for a membran e hyperpolarization by 17.9+/-6.0 mV (n = 22). Substitution of N-methy l D-glucamine chloride (NMDG-Cl) for external NaCl led to a more signi ficant membrane hyperpolarization by 28.4+/-8.3 mV (n = 22). At holdin g potential of -70 mV, amiloride and ethylisopropylamiloride (EIPA) bl ocked the inward current in a concentration-dependent manner over the range of concentrations of between 0.1 mu M and 50 mu M, with an inhib itory constant (K-i) of 1.3+/-0.4 mu M (n = 7) and 1.5+/-0.3 mu M (n = 5), respectively. In the voltage-clamp configuration, substitution of NMDG-Cl for external NaCl significantly reduced the inward current (n = 9), indicating that the whole-cell conductance has a high permeabil ity for Na+. Superfusion with 20 mu M amiloride induced a significant reduction of the inward current, shifted the reversal potential from - 39.4+/-8.8 mV to -60.4+/-10.5 mV (n = 12), and de creased the inward c onductance from 5.0+/-1.3 nS to 3.7+/-1.5 nS (n = 12). The permeabilit y ratio of Na+ over K+, calculated from the difference in reversal pot ential between the currents before and after application of amiloride, was approximately 5:1. Additionally, the conductance was not activate d by application of forskolin, 3-isobutyl-1-methylxanthine (IBMX) and 8-bromo-cAMP (8-Br-cAMP). These findings suggest that a low-amiloride- affinity Naf channel localized in the ES epithelial cells may be invol ved in uptake of Na+ in the ES.