Aa. Altamirano et al., EFFECTS OF OKADAIC ACID AND INTRACELLULAR CL- ON NA-K+-CL- COTRANSPORT(), American journal of physiology. Cell physiology, 38(4), 1995, pp. 878-883
The Na+-K+-Cl- cotransporter of the squid giant axon requires ATP and
is inhibited by intracellular Cl- (Cl-i(-))in a concentration-dependen
t manner ([Cl-](i) greater than or equal to 200 mM completely inhibits
the cotransporter). In the present study we address the question of w
hether inhibition of cotransport by Cl-i(-) is due to a Cl-i(-)-induce
d increase of protein phosphatase activity. Intracellular dialysis was
used to apply the phosphatase inhibitor okadaic acid (OKA) under cond
itions of [Cl-](i) at 0, 150, or 300 mM during measurement of cotransp
orter-mediated unidirectional Cl- influx into axone. At 0 mM [Cl-](i),
the application of 250 nM OKA had no effect on the cotransport-mediat
ed Cl- influx when axone were dialyzed with the normal intracellular A
TP concentration ([ATP](i) = 4 mM). Reduction of [ATP] to 50 mu M resu
lted in a significant decrease of the bumetanide-sensitive Cl- influx,
which could be partially reversed by OKA treatment. Similarly, in ATP
-limited axons with [Cl-](i) at 150 mM, cotransporter influx was parti
ally stimulated by treatment with OKA. However, axons dialyzed with 30
0 mM [Cl-](i) ([ATP](i) = 50 mu M) had no measurable cotransport influ
x, nor was subsequent treatment with OKA able to induce a cotransport-
mediated Cl- influx. We conclude that the inhibition of cotransport ca
used by Cl-i(-) is not the result of an increase in the OKA-sensitive
protein phosphatase activity.