EFFECT OF INSULIN ON AREA AND NA+ CHANNEL DENSITY OF APICAL MEMBRANE OF CULTURED TOAD KIDNEY-CELLS

1. The stimulation of transepithelial Na+ transport caused by insulin in A6 cultured toad kidney cells was investigated by determination of membrane capacitance (C-m), short circuit current (I-sc) and current fluctuation analysis. Values of C-m are proportional to membrane area while blocker-induced current fluctuation analysis provides an estimate of the number of active amiloride-sensitive Na+ channels in the apical membrane. 2. Insulin simultaneously increased C-m, I-sc and C-t (transepithelial conductance) in epithelia incubated with Na+-containing solutions on both sides. 3. Analysis of 6-chloro-3,5-diaminopyrazine-2-carboxamide (CDPC)-induced noise showed that insulin increased the number of active Na+ channels in the apical membrane, without altering the single channel current. 4. When nystatin was used to permeabilize the apical membrane the impedance data revealed the presence of a second time constant. Analysis of these data indicated that the basolateral membrane capacitance (C-b) is much larger than the apical membrane capacitance (C-a). Insulin administered to nystatin-treated epithelia increased the values for both capacitances. 5. We suggest that the stimulation of transepithelial Na+ transport caused by insulin may be associated with the exocytotic delivery of transporters to the apical membrane.