Figure 5.
Effects of voltage and Ca coupling on alternans dynamics. A–D, Action potential duration (APD)-to-Ca coupling is assumed to be positive and labels refer to Ca-to-APD coupling sign. A, Electromechanically concordant alternans (EMCA) in a rabbit ventricular myocyte with positive Ca-to-APD coupling.109 B, Electromechanically discordant alternans (EMDA) in a rabbit myocyte after blocking Na-Ca exchange current by SEA0400 to promote negative Ca-to-APD coupling.109 C, Theoretical stability boundary of alternans for positive Ca-to-APD coupling.123 Arrows along both axes indicate the directions of increasing instability and dashed lines are the stability boundaries (from stable to unstable) when Ca and APD are not coupled. Solid line is the stability boundary when Ca and APD are coupled, below which the system is stable without alternans and above which alternans occurs in the form of EMCA. D, Same as C but for negative Ca-to-APD coupling. The regions of EMCA, EMDA, and electromechanically quasiperiodic alternans (EMQA) are marked. E, Quasiperiodic APD alternans caused by infusion of erythromycin in guinea pig hearts.111