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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 (Vt) and
resistance (Rt) in isolated perfused Malpighian tubules.
Vbl 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 Rt. 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 EK=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
[ECl=-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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