Abstract
Glycoside-induced cardiac inotropy has traditionally been attributed
to direct Na(+)-K(+)-ATPase inhibition, causing increased intracellular
Na(+) and consequent Ca(2+) gain via the Na(+)-Ca(2+) exchanger
(NCX). However, recent studies suggested alternative mechanisms of
glycoside-induced inotropy: (1) direct activation of sarcoplasmic
reticulum Ca(2+) release channels (ryanodine receptors; RyRs); (2)
increased Ca(2+) selectivity of Na(+) channels (slip-mode conductance);
and (3) other signal transduction pathways. None of these proposed
mechanisms requires NCX or an altered Na(+) gradient. Here we tested
the ability of ouabain (OUA, 3 microm), digoxin (DIG, 20 microm)
or acetylstrophanthidin (ACS, 4 microm) to alter Ca(2+) transients
in completely Na(+)-free conditions in intact ferret and cat ventricular
myocytes. We also tested whether OUA directly activates RyRs in permeabilized
cat myocytes (measuring Ca(2+) sparks by confocal microscopy). In
intact ferret myocytes (stimulated at 0.2 Hz), DIG and ACS enhanced
Ca(2+) transients and cell shortening during twitches, as expected.
However, prior depletion of Na(+)(i) (in Na(+)-free, Ca(2+)-free
solution) and in Na(+)-free solution (replaced by Li(+)) the inotropic
effects of DIG and ACS were completely prevented. In voltage-clamped
cat myocytes, OUA increased Ca(2+) transients by 48 +/- 4\% but OUA
had no effect in Na(+)-depleted cells (replaced by N-methyl-d-glucamine).
In permeabilized cat myocytes, OUA did not change Ca(2+) spark frequency,
amplitude or spatial spread (although spark duration was slightly
prolonged). We conclude that the acute inotropic effects of DIG,
ACS and OUA (and the effects on RyRs) depend on the presence of Na(+)
and a functional NCX in ferret and cat myocytes (rather than alternate
Na(+)-independent mechanisms).
- /&/
- agents,
- analogs
- animals;
- bacterial
- calcium
- cardiac
- cardiac,
- cardiotonic
- cats;
- channel,
- contraction;
- cytology/drug
- derivatives/pharmacology
- digoxin,
- drug
- effects/metabolism;
- exchanger,
- ferrets;
- glycosides,
- heart
- membrane
- metabolism;
- myocardial
- myocytes,
- ouabain,
- patch-clamp
- pharmacology;
- potentials;
- proteins,
- receptor
- release
- ryanodine
- signaling;
- sodium,
- sodium-calcium
- streptolysins,
- strophanthidin,
- techniques;
- ventricles,
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