The late component of L-type calcium current during guinea-pig cardiac action potentials and its contribution to contraction |
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Authors: | Klaus W. Linz R. Meyer |
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Affiliation: | (1) Physiological Institute, University of Bonn, Wilhelmstrasse 31, D-53111 Bonn, Germany e-mail: meyer@physio.uni-bonn.de Tel.: +49-228-2872311, Fax: +49-228-2872313, DE |
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Abstract: | L-Type Ca2+ current (I Ca,L) elicited during the action potential (AP) of guinea-pig ventricular myocytes exhibits an early and a late component. The whole-cell patch-clamp technique was used to characterize the process regulating the late I Ca,L component and to assess its contribution to excitation-contraction coupling. A stepwise decrease in repolarization rate of AP-like voltage-clamp pulses led to an exponential increase in Ca2+ charge carried by I Ca,L. This saturation behaviour was significantly reduced or absent when Ba2+ or monovalent cations were used as charge carriers, which suggests that the late component of I Ca,L is controlled mainly by Ca2+-dependent processes. Simultaneously recording I Ca,L and zero-load shortening or the internal Ca2+ concentration (fura-2) revealed that Ca2+ carried by the late component of I Ca,L markedly contributes to the Ca2+ content of the sarcoplasmic reticulum (SR). Reducing the charge transfer by late I Ca,L during a series of AP-like conditioning clamp pulses by 48% reduced the shortening amplitude during a subsequent test stimulation by 56%. This relationship was absent during long rectangular depolarizing conditioning clamps, during which Na+/Ca2+ exchange increased its influence on SR Ca2+ loading. The late component of I Ca,L developed only a minor direct influence on the simultaneous cell shortening. Thus, the main contribution of the late I Ca,L component is to supply Ca2+ for SR loading. Received: 5 November 1997 / Received after revision: 12 June 1998 / Accepted: 15 June 1998 |
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Keywords: | Action potential clamp Ca2+-dependent inactivation Cell shortening Excitation-contraction coupling Fura-2 Guinea-pig ventricular myocytes L-Type current SR Ca2+ content |
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