Fig. 1. The study of isolated Malpighian tubules. (A) The method of Ramsay (1953) for measurements of fluid secretion and for the compositional analysis of secreted fluid under well-defined experimental conditions in vitro. (B) The methods of Burg and Helman (Helman, 1972) for measurement of the transepithelial voltage (V_t) and resistance (R_t) in isolated perfused Malpighian tubules. V_bl is the voltage measured across the basolateral membrane of a principal cell impaled with a microelectrode. I is the current injected for the measurement of R_t. Voltage measurements yield electrochemical potentials of the major electrolytes, Na⁺, K⁺ and Cl^-, secreted into the tubule lumen. Resistance measurements give insights into conductive and non-conductive transport mechanisms. (C) The isolated Malpighian tubule of Aedes aegypti under control conditions. To move K⁺ from 3.4 mmol l^-1 in the peritubular bath to 91 mmol l^-1 in the tubule lumen requires a driving force (chemical potential) of 87.1 mV, calculated as E_K=61 mV log(91/3.4). Add to this the lumen-positive voltage of 52.6 mV (electrical potential), against which K⁺ is moved, to yield the total electrochemical potential (139.7 mV) needed to transport K⁺ into the tubule lumen. Similar calculations for Na⁺ yield an electrochemical potential of 40.2 mV against which this cation is secreted. To move Cl^- from 158 mmol l^-1 in the peritubular bath to 161 mmol l^-1 in the tubule lumen requires the small driving force of -0.5 mV [E_Cl=-61 mV log(161/158)]. However, the transepithelial voltage is lumen-positive (52.6 mV), `pulling' Cl^- into the tubule lumen. Thus, Cl^- moves into the tubule lumen down (passive) an electrochemical potential of 52.1 mV.

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