Late Ca2+ Sparks and Ripples During the Systolic Ca2+ Transient in Heart Muscle Cells

Abstract

Rationale: The development of a refractory period for Ca²⁺ spark initiation after Ca²⁺ release in cardiac myocytes should inhibit further Ca²⁺ release during the action potential plateau. However, Ca²⁺ release sites that did not initially activate or which have prematurely recovered from refractoriness might release Ca²⁺ later during the action potential and alter the cell-wide Ca²⁺ transient.

Objective: To investigate the possibility of late Ca²⁺ spark (LCS) activity in intact isolated cardiac myocytes using fast confocal line scanning with improved confocality and signal to noise.

Methods and results: We recorded Ca²⁺ transients from cardiac ventricular myocytes isolated from rabbit hearts. Action potentials were produced by electric stimulation, and rapid solution changes were used to modify the L-type Ca²⁺ current. After the upstroke of the Ca²⁺ transient, LCSs were detected which had increased amplitude compared with diastolic Ca²⁺ sparks. LCS are triggered by both L-type Ca²⁺ channel activity during the action potential plateau, as well as by the increase of cytosolic Ca²⁺ associated with the Ca²⁺ transient itself. Importantly, a mismatch between sarcoplasmic reticulum load and L-type Ca²⁺ trigger can increase the number of LCS. The likelihood of triggering an LCS also depends on recovery from refractoriness that appears after prior activation. Consequences of LCS include a reduced rate of decline of the Ca²⁺ transient and, if frequent, formation of microscopic propagating Ca²⁺ release events (Ca²⁺ ripples). Ca²⁺ ripples resemble Ca²⁺ waves in terms of local propagation velocity but spread for only a short distance because of limited regeneration.

Conclusions: These new types of Ca²⁺ signaling behavior extend our understanding of Ca²⁺-mediated signaling. LCS may provide an arrhythmogenic substrate by slowing the Ca²⁺ transient decline, as well as by amplifying maintained Ca²⁺ current effects on intracellular Ca²⁺ and consequently Na⁺/Ca²⁺ exchange current.

Keywords: action potential; cardiac myocytes; heart; heart muscle cell; sodium–calcium exchanger.