Contribution of junctional conductance to the cellular voltage-divider ratio in frog skins

Abstract

It has been suggested that distribution of lateral interspace resistance in association with a highly conductive junction can significantly affect the measurement of outer membrane(o)/epithelial(t) voltage divider ratios (F _o=δV _o/δV _t), thereby leading to erroneous inferences regarding the outer membrane fractional resistance [fR _o=R _o/R _c=R _o/(R _o+R _i)], whereR _o andR _i are the outer and inner cell membrane resistance respectively andR _c is the total cell membrane resistance. We present here experimental evidence for this point of view. During seasons when frog skins were highly permeable to Cl, transepithelial conductanceg _t often exceeded 2 mS/cm². High concentrations of external amiloride rapidly blocked cellular transport, butg _t initially remained high andF _o remained appreciably less than 1.0. These values ofF _o were found here to result from low junctional resistanceR _j: increase ofR _j, either gradually following the administration of amiloride, or abruptly with external replacement of Cl by other anions, was associated with increase ofF _o to near unity, without effect on the membrane potential or significant change in the short-circuit current. Experimental results following amiloride validated a simple equivalent circuit model predicting near-linear increase inF _o with progressive decrease ing _t and led to plausible values ofR _j and lateral space resistanceR _l. The possible influence of the paracellular resistance pattern on the evaluation of cell membrane resistances from voltage divider ratios is discussed.