Excitation-contraction coupling in voltage-clamped cardiac myocytes

Summary

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 (I_Ca)-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 I_Ca. In these measurements Ca²⁺ preload was likely to increase with increasing potential. It is concluded that I_Ca 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.