The effects of membrane potential, SR Ca2+ content and RyR responsiveness on systolic Ca2+ alternans in rat ventricular myocytes.

Previous work has shown that small depolarizing pulses produce a beat to beat alternation in the amplitude of the systolic Ca(2+) transient in ventricular myocytes. The aim of the present work was to investigate the role of changes of SR Ca(2+) content and L-type Ca(2+) current in this alternans. As the amplitude of the depolarizing pulse was increased from 10 to 30 mV the magnitude of alternans decreased. Confocal linescan studies showed that this was accompanied by an increase in the number of sites from which Ca(2+) waves propagated. A sudden decrease in the depolarisation amplitude resulted in three classes of behaviour: (1) a gradual decrease in Ca(2+) transient amplitude before alternans developed accompanied by a loss of SR Ca(2+), (2) a gradual increase in Ca(2+) transient amplitude before alternans accompanied by a gain of SR Ca(2+), and (3) immediate development of alternans with no change of SR content. We conclude that alternans develops if the combination of decreased opening of L-type channels and change of SR Ca(2+) content results in spatially fragmented release from the SR as long as there is sufficient Ca(2+) in the SR to sustain wave propagation. Potentiation of the opening of the ryanodine receptor (RyR) by low concentrations of caffeine (100 microm) abolished alternans for a few pulses but the alternans then redeveloped once SR Ca(2+) content fell to the new threshold for wave propagation. Finally we show evidence that inhibiting L-type Ca(2+) current with 200 mum Cd(2+) produces alternans by means of a similar fragmentation of the Ca(2+) release profile and propagation of mini-waves of Ca(2+) release.