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The Division of Cardiology and the Nora Eccles Harrison Cardiovascular Research and Training Institute, University of Utah Health Sciences Center, Salt Lake City, Utah 84132
The relative importance of the
Na+/Ca2+
exchanger in the initial and terminal phases of relaxation and the
decline in the
[Ca2+]i
transient was investigated in adult rabbit ventricular myocytes loaded
with the Ca2+ indicator fluo 3. For electrically stimulated contractions, the peak intracellular
Ca2+ concentration
([Ca2+]i)
was 700 ± 87 nM and end-diastolic
[Ca2+]i
was 239 ± 30 nM (0.25 Hz, 37°C, 1.08 mM
extracellular Ca2+ concentration;
n = 14). Abrupt inhibition of
Na+/Ca2+
exchange was produced by removal of extracellular
Na+ (KCl substitution) and
Ca2+ [2 mM
Ca2+-free ethylene
glycol-bis(
-aminoethyl
ether)-N,N,N',N'-tetraacetic acid] by means of a rapid switcher device (SW). Abrupt exposure to
high K+ induced an action
potential, although sufficient
Ca2+ remained adjacent to the
sarcolemma to induce a contraction (SW beat) and
[Ca2+]i
transient that were identical in amplitude to those induced by
electrical stimulation (ES beat). The initial relaxation and decline in
the
[Ca2+]i
transient was not significantly prolonged by abrupt elimination of the
Na+/Ca2+
exchanger, but the rate and extent of the terminal phase of the decline
in the
[Ca2+]i
transient were significantly reduced. The first derivative of
[Ca2+]i
with respect to time versus
[Ca2+]i
during the decline of the
[Ca2+]i
transient attributable to sarcoplasmic reticulum (SR) function was
estimated from the average SW transients, and that attributable to
Na+/Ca2+
exchange was estimated from the difference between SW and ES transients. By this analysis, the
Na+/Ca2+
exchanger produces 13% of the first half of the decline in
[Ca2+]i
and 45% of the second half of the decline. We conclude that abrupt
inhibition of forward
Na+/Ca2+
exchange does not significantly affect the amplitude or the initial rate of decline of the
[Ca2+]i
transient and relaxation. However, its contribution to the reduction of
[Ca2+]i
becomes apparent late during the
[Ca2+]i
transient, when cytosolic
[Ca2+]i
has been reduced.
sodium/calcium exchange; fluo 3; relaxation; calcium ion transient
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