Electrophysiology of posterior, NaCl-absorbing gills of Chasmagnathus granulatus: rapid responses to osmotic variations.
2003
SUMMARY In the present study, the influence of short-term osmotic variations on
some electrophysiological properties related to NaCl absorption across
posterior gills of Chasmagnathus granulatus was investigated. The
transepithelial potential difference ( V te ) of isolated and
perfused gills increased significantly when hyposmotic saline (699 mosmol
l -1 ) was used instead of isosmotic solution (1045 mosmol
l -1 ). A reduction of the concentration of Na + or
Cl - at constant osmolarity did not produce any change in
V te . Transepithelial short-circuit current
( I sc ) and conductance ( G te ), measured
with split gill lamellae mounted in a modified Ussing chamber, also increased
after changing to hyposmotic salines ( I sc : from
-89.0±40.8 μA cm -2 to -179.3±37.0 μA
cm -2 ; G te : from 40.5±16.9 mS
cm -2 to 47.3±15.8 mS cm -2 ). The observed effects
of reduced osmolarity were fast, reversible and gradually dependent on the
magnitude of the osmotic variation. The activity of the
Na + /K + -ATPase increased significantly after perfusion
with hyposmotic saline, from 18.73±6.35 μmol P i
h -1 mg -1 to 41.84±14.54 μmol P i
h -1 mg -1 . Theophylline maintained part of the elevated
V te induced by hyposmotic saline, suggesting that an
increased cellular cyclic AMP level is involved in the response to reduced
osmolarity. In summary, the results indicate that the hemolymph osmolarity
regulates active transbranchial NaCl absorption by modulating the activity of
the basolateral Na + /K + -ATPase and by changing a
conductive pathway, probably at the apical membrane.
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