Late Ca(2+) Sparks and Ripples During the Systolic Ca(2+) Transient in Heart Muscle Cells

RATIONALE: The development of a refractory period for Ca(2+) spark initiation after Ca(2+) release in cardiac myocytes should inhibit further Ca(2+) release during the action potential plateau. However, Ca(2+) release sites that did not initially activate or which have prematurely recovered from refractoriness might release Ca(2+) later during the action potential and alter the cell-wide Ca(2+) transient. OBJECTIVE: To investigate the possibility of late Ca(2+) 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 Ca(2+) 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 Ca(2+) current. After the upstroke of the Ca(2+) transient, LCSs were detected which had increased amplitude compared with diastolic Ca(2+) sparks. LCS are triggered by both L-type Ca(2+) channel activity during the action potential plateau, as well as by the increase of cytosolic Ca(2+) associated with the Ca(2+) transient itself. Importantly, a mismatch between sarcoplasmic reticulum load and L-type Ca(2+) 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 Ca(2+) transient and, if frequent, formation of microscopic propagating Ca(2+) release events (Ca(2+) ripples). Ca(2+) ripples resemble Ca(2+) waves in terms of local propagation velocity but spread for only a short distance because of limited regeneration. CONCLUSIONS: These new types of Ca(2+) signaling behavior extend our understanding of Ca(2+)-mediated signaling. LCS may provide an arrhythmogenic substrate by slowing the Ca(2+) transient decline, as well as by amplifying maintained Ca(2+) current effects on intracellular Ca(2+) and consequently Na(+)/Ca(2+) exchange current.