Fig. 1. Electrophysiological study of a principal cell by the two-electrode voltage-clamp (TEVC) method in isolated Malpighian tubules of Aedes aegypti. (A) Transepithelial secretion of NaCl and KCl under control conditions. The question mark indicates uncertainty as to whether the exchanger transports both Na⁺ and K⁺. (B) Electrical equivalent circuit of transepithelial NaCl and KCl secretion consisting of the active transport pathway for Na⁺ and K⁺ ions through principal cells and the passive transport of Cl^- through the shunt pathway. E_bl and E_a are the electromotive forces at the basolateral and apical membrane, respectively. R_bl, R_a and R_sh are the resistances of the basolateral membrane, the apical membrane, and the shunt, respectively. V is voltage measured across basolateral (bl) and apical (a) membranes of the cell. V_t and R_t are the transepithelial voltage (lumen-positive) and resistance, respectively. Numerical data are from previous studies (Pannabecker et al., 1992). (C) TEVC study of a principal cell in an isolated Malpighian tubule. (D) The measurement circuit in TEVC studies yields the input resistance R_pc, which is the sum of the parallel resistances of the basolateral membrane and the apical membrane in series with the shunt.

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