Zusammenfassung
Chinese hamster ovary cells expressing the bovine cardiac Na/Ca exchanger
were treated with ouabain to increase Na$^+$i and stimulate
Ca$^2+$ influx by Na/Ca exchange. Depletion of cellular ATP inhibited
45Ca uptake by 40\% or more and reduced the half-maximal Na$^+$
concentration for inhibition of 45Ca uptake from 90 to 55 mM. ATP
depletion also reduced the rate of rise in Ca$^2+$i when Na$^+$o
was reduced and inhibited the decline in Ca$^2+$i when high
Na$^+$o was restored. The effects of ATP depletion were either
absent or reduced in cells expressing a mutant exchanger missing
most of the cytosolic hydrophilic domain. We were unable to detect
a phosphorylated form of the exchanger in immunoprecipitates from
32P-labeled cells. ATP depletion caused a breakdown in the actin
cytoskeleton of the cells. Treatment of the cells with cytochalasin
D mimicked the effects of ATP depletion on the Na$^+$ inhibition
profile for 45Ca uptake. Thus, ATP depletion inhibits both the Ca$^2+$
influx and Ca$^2+$ efflux modes of Na/Ca exchange, and may alter
the competitive interactions of extracellular Na$^+$ and Ca$^2+$
with the transporter. The latter effect appears to be related to
changes in the actin cytoskeleton.
Nutzer