D. Erlij et al., Forskolin increases apical sodium conductance in cultured toad kidney cells (A6) by stimulating membrane insertion, PFLUG ARCH, 438(2), 1999, pp. 195-204
The role of membrane traffic in the stimulation of apical Na+ permeability
caused by increases in cytoplasmic cyclic AMP was assessed by measuring the
effects of forskolin on transepithelial capacitance (C-T), transepithelial
conductance (G(T)), and short-circuit current (I-sc) in A6 cultured toad k
idney cells. Apical water per meability was probed by recording cell volume
changes after reducing the osmolality of the apical bath. We found that fo
rskolin does not increase the osmotic water permeability of the apical memb
rane of A6 cells, and thus does not stimulate the insertion of water channe
ls. Comparison of the effects of forskolin and insulin on Na+ transport dem
onstrated that both agents produce reversible increases in C-T, G(T) and I-
sc. G(T) and C-T increased proportionally during the rising phase of the in
sulin response. However, a non-linear relationship between both parameters
was recorded when forskolin was given in NaCl Ringer's solution. The relati
onship between C-T and G(T) became linear after the effects of forskolin on
Cl- conductances were eliminated by substituting Cl- by an impermeant anio
n. In contrast, in Cl--containing Na+-free solutions, the nonlinearity beca
me more pronounced. Successive additions of insulin and forskolin caused ad
ditive increases in C-T. Because increases in C-T and Na+ transport occurre
d in the absence of stimulation of water permeability and increases of C-T
and G(T) were directly proportional when Na+ was the major permeating ion a
cross the apical membrane, we suggest that the increase in apical Na+ perme
ability in the presence of either forskolin or insulin is due to the insert
ion of channels residing in intracellular pools. In contrast, the increased
Cl- permeability caused by forskolin may be related to the activation of c
hannels already present in the membrane.