Zusammenfassung
The relationship between changing driving force of the Na+/Ca2+-exchanger
(deltaG(exch)) and associated cytosolic calcium fluxes was studied
in rat ventricular myocytes. DeltaG(exch) was abruptly reversed by
the reduction of extracellular sodium (Na+o) with or without sustained
depolarization by the elevation of potassium (K+o). Cytosolic sodium
(Na+i) and calcium (Ca2+i) were measured with SBFI and indo-1
respectively and the time course of recovery of deltaG(exch) was
calculated. Following abrupt reversal of deltaG(exch) from +4.1 to
-9.2 kJ/mol Na+i exponentially decreased from 9.6-2.5 mmol/l (t(1/2)
about 30 s) and Ca2+i transiently increased to a peak value after
about 30 s. Negative values of deltaG(exch) were associated with
an increase and positive values with a decrease of Ca2+i. Equilibrium
(deltaG(exch) = 0) was reached after about 30 s coinciding with the
time to peak Ca2+i. After 180 s deltaG(exch) reached a new steady
state at +3.5 kJ/mol. Inhibition of SR with ryanodine or thapsigargin
reduced the amplitude of the Ca2+i transient and shifted its peak
to 80 s, but did not affect the time course of Na+i changes. In
the presence of ryanodine or thapsigargin the time required for deltaG(exch)
to recover to equilibrium was also shifted to 80 s. When we changed
the deltaG(exch) to the same extent by the reduction of Na+o in
combination with a sustained depolarization, Na+i decreased less
and the amplitude of Ca2+i transient was much enhanced. This increase
of Ca2+i was completely abolished by verapamil. DeltaG(exch) only
recovered to a little above equilibrium (+1 kJ/mol). Inhibition of
the Na+/K+-ATPase with ouabain entirely prevented the decrease of
Na+i and caused a much larger increase of Ca2+i, which remained
elevated; deltaG(exch) recovered to equilibrium and never returned
to positive values. The rate of change of total cytosolic calcium
was related to deltaG(exch), despite the fact that the calcium flux
associated with the exchanger itself contributed only about 10\%;
SR related flux contributed by about 90\% to the rate of change of
total cytosolic calcium. In summary, reduction of Na+o causes reversal
of the Na+/Ca2+-exchanger and its driving force deltaG(exch), a transient
increase of Ca2+i and a decrease of Na+i. The influx of calcium
associated with reversed deltaG(exch) triggers the release of calcium
from SR. Both the decrease of Na+i and the increase of Ca2+i
contribute to the recovery of deltaG(exch) to equilibrium. The time
at which deltaG(exch) reaches equilibrium always coincides with the
time to peak of Ca2+i transient. Activation of the Na+/K+-ATPase
is required to reduce Na+i and recover deltaG(exch) to positive
values in order to reduce Ca2+i. We conclude that deltaG(exch)
is a major regulator of cytosolic calcium by interaction with SR.
Nutzer