Excitation-contraction coupling in voltage-clamped cardiac myocytes.
Myocytes isolated from guinea pig ventricles were voltage-clamped using patch pipettes in the whole-cell configuration. For proper voltage control fast Na+ current was blocked by TTX or inactivated by an appropriate prepulse. Zero-load cell shortening was monitored by a photoelectric device. The mechanical response to a short depolarizing clamp was mainly a phasic (transient) contraction. Long-lasting depolarizations caused a tonic (sustained) shortening of a cell. Different clamp patterns were used to study the mode of activation of phasic contraction. 1) With a constant Ca2+ preload established by a train of conditioning pulses, the shortening-voltage relation measured with test pulses of varying height was a bell-shaped curve reflecting the slow inward current (ICa)-voltage relation. The test pulse had a striking influence on the first contraction of the following conditioning series, resulting in an S-shaped relation between post-test contraction and test potential. 2) With series of identical clamps of varying height, steady-state contraction was maximal around 40 mV and not in proportion to ICa. In these measurements Ca2+ preload was likely to increase with increasing potential. It is concluded that ICa initiates phasic contraction by inducing a release of Ca2+ from internal stores while replenishment of the stores is largely determined by an electrogenic transsarcolemmal Na+-Ca2+ exchange. The data suggest that Na+-Ca2+ exchange is not only involved in long-term changes of cardiac contractility but also in beat-to-beat regulation.
Meyer
R
R
Department of Physiology II, University of Bonn, FRG.
Haas
H G
HG
Wiemer
J
J
D000200:Action Potentials; D000818:Animals; D002118:Calcium / Q000378:metabolism; D004560:Electricity; D006168:Guinea Pigs; D006321:Heart / Q000502:physiology*; D009200:Myocardial Contraction; D009206:Myocardium / Q000166:cytology*; D012519:Sarcoplasmic Reticulum / Q000502:physiology; D016276:Ventricular Function
D002118:Calcium