Fig. 1. Block diagram for investigating electrical coupling of principal cells through gap junctions in isolated Malpighian tubules. Cell 1 was voltage clamped at a desired command voltage and the voltage deflections in cells 1, 2 and 3 were recorded. The input resistance R_input of cell 1 was calculated from the values of (1) current injected into cell 1 to hold it at the desired command voltage, and (2) the change in the basolateral membrane voltage of cell 1 (V_bl1).