Fig. 1. In vitro microperfusion of Malpighian tubules of the yellow fever mosquito, Aedes aegypti. (A) Perfusion of a tubule segment containing a stellate cell. (B) Perfusion of a tubule segment without stellate cells. The lumen of Malpighian tubules was cannulated with a double-barreled glass pipette for perfusion of the tubule lumen and for measurements of the transepithelial voltage (V_t) and resistance (R_t). V_l is the voltage measured at the distal end of the perfused segment. A current (I=50 nA) was injected into the tubule lumen when measurements of R_t were desired. BB, brush border; PC, principal cell; SC, stellate cell; SJ, septate junction. (C) Simple electrical equivalent circuit of transepithelial electrolyte transport across Malpighian tubules. E_c, electromotive force of the active transport pathway; I, intraepithelial current; R_c, transcellular resistance; R_sh, shunt resistance. R_t is the total resistance of R_c and R_sh in parallel.