Partial inhibition of Ca2+ current by methoxyverapamil (D600) reveals spatial nonuniformities in [Ca2+]i during excitation-contraction coupling in cardiac myocytes.
The laser scanning confocal microscope was used in conjunction with the Ca2+ indicator fluo 3 to examine the spatiotemporal properties of free Ca2+ ([Ca2+]i) transients in isolated rat cardiac myocytes. We show that localized increases in [Ca2+]i (Ca2+ sparks) can be triggered by membrane depolarization in cardiac myocytes when the sarcolemmal Ca2+ current amplitude is reduced by methoxyverapamil (D600). These depolarization-evoked Ca2+ sparks are similar in amplitude and spatiotemporal properties to spontaneous Ca2+ sparks previously observed at rest. These observations support the idea that Ca2+ sparks are the result of the activation of functional elementary units of sarcoplasmic reticulum (SR) Ca2+ release. The synchronous activation of a large number of Ca2+ sparks can explain the increased amplitude and slower time course of the electrically evoked [Ca2+]i transient as well as the presence of spatial nonuniformities in [Ca2+]i during its rise. The data shown here suggest a model for excitation-contraction coupling in which the amplitude of the [Ca2+]i transient is regulated by variations in the probability of recruitment of elementary SR Ca2+ release units as well as the amount of Ca2+ released by each unit. Since the activation of each release unit will depend on the local amplitude of the Ca2+ current, this model can explain the regulation of the amplitude of the [Ca2+]i transient by the Ca2+ current. In addition, these data indicate that caution should be applied to the interpretation of signals obtained with nonlinear Ca2+ indicators during the rising phase of the [Ca2+]i transient, when the nonuniformities in [Ca2+]i are largest.
Department of Physiology, University of Maryland, Baltimore School of Medicine 21201.
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H
Cheng
M B
MB
Cannell
W J
WJ
Lederer
D000818:Animals; D002118:Calcium / Q000502:physiology*; D004553:Electric Conductivity; D004594:Electrophysiology; D005711:Gallopamil / Q000494:pharmacology*; D006321:Heart / Q000502:physiology*; D007425:Intracellular Membranes / Q000378:metabolism; D009200:Myocardial Contraction; D009206:Myocardium / Q000166:cytology; D009994:Osmolar Concentration; D051381:Rats
D005711:Gallopamil; D002118:Calcium