Current-voltage relations of the apical and basolateral membranes of the frog skin.

Abstract

We determined the current-voltage (I-V) relations of the apical and basolateral barriers of frog skins by impaling the cells with an intracellular microelectrode and assuming that the current across the cellular pathway was equal to the amiloride-inhibitable current. We found that : (a) The responses in transepithelial current and intracellular potential to square pulses of transepithelial potential (VT) varied markedly with time . (b) As a consequence of these transient responses, the basolateral I-V relation was markedly dependent on the time of sampling after the beginning of each pulse. The apical 1-V plot was much less sensitive to the time of sampling within the pulse. (c) The I-V data for the apical barrier approximated the I-V relations calculated from the Goldman constant field equation over a relatively wide range of membrane potentials (±100 mV). (d) A sudden reduction in apical bath [Na'] resulted in an increase in apical permeability and a shift in the apical barrier zero-current potential (Ea) toward less positive values . The shift in Ea was equivalent to a change of 45 mV for a 10-fold change in apical [Na'] . (e) The transient responses of the skin to square VT pulses were described by the sum of two exponentials with time constants of 114 and 1,563 ms, which are compatible with the time constants that would be produced by an RC circuit with capacitances of 65 and 1,718 AF . The larger capacitance is too large to identify it comfortably with a true dielectric membrane capacitance .

Whole-cell and single channel K+ and Cl- currents in epithelial cells of frog skin.

“…With this hypothesis it is possible to estimate the number of channels in a cell from the relation: (Schoen and Erlij, 1985;Nagel et al, 1988). Such behavior is comparable to that of the whole-cell currents in Na medium reported here in the voltage range between reversal and 20 mV.…”

“…The polarity is maintained by the tight junctions between cells of the outer S. granulosum, which also restricts diffusion between the outer solution and the intercellular spaces (Martlnez-Palomo, Erlij, and Bracho, 1971). These structural observations, together with the demonstration of the syncytial behavior of the epithelium (Rick et al, 1978) and the large capacitance of the basolateral membrane of the intact epithelium (Smith, 1971 ;Garcia-Dfaz and Essig, 1985;Schoen and Erlij, 1985) indicates that, functionally, all innermost cell membranes behave as basolateral membrane. This implies that the majority of isolated cells will exhibit macroscopic properties that correspond to the basolateral membrane of the intact epithelium and there would be a much higher probability of recording single channels of basolateral origin/ One can also make the argument that, since the apical membrane of R. pipiens skin does not exhibit any significant conductive permeability to ions other than Na (Helman and Fisher, 1977;Nagel, 1977;Garcia-Diaz et al, 1985;Stoddard, Jakobsson, and Helman, 1985;D6rge, Beck, Wienecke, and Rick, 1989) the K and CI selective channels found in this study are necessarily of basolateral origin.…”

“…Noise analysis of the basolateral membrane has been hampered by the attenuating effect of the high resistance apical membrane (Van Driessche and Hillyard, 1985). Different approaches have been used to elucidate the I-V relation and conductance of the basolateral membrane, with results that do not generally agree (Nagel, 1985;Schoen and Erlij, 1985;Nagel, Garcia-Diaz, and Essig, 1988). Such problems complicate the study of regulation of transport at this membrane and its possible relation with alterations in Na absorption.…”

Cell sodium activity and sodium pump function in frog skin

“…Thomas et al (1983) found that a~a in Necturus urinary bladder, calculated by fitting the current-voltage (I-V) relation of the apical membrane to the Goldman-Hodgkin-Katz equa-tion, did not change significantly when apical [Na] was lowered from 45 to 5 raM. However, using the same approach in frog skins, Schoen and Erlij (1985) calculated that a~a decreased from 25.1 to 17.2 mM in less than 1 min after [Na]o was reduced from 120 to 24 mM. We do not know the reasons for these discrepancies, but we would like to point out the indirect nature of the calculation of a~a from the I-V relations and the assumptions involved, in particular that the paracellular I-V relation is unchanged by amiloride.…”

Fast and slow voltage modulation of apical Cl- permeability in toad skin at high [K+]

“…Leaving out the first 600 ms (where capacitance artifacts might be involved (22)) it is clear from Figure 4B that the skin regains, in a fraction of a second, the conductance prior to the +100 mV maneuver. On the other hand, when the voltage is brought slowly (in sinusoidal form) from +100 to -100 mV the current follows a return pathway clearly distinct from the ongoing -100 to +100 mV one, indicating a more persistent memory of the inhibiting positive voltage ( Figure 5A).…”