Isoproterenol Widens the Source of Release Flux Underlying Ca Sparks

Abstract

Our previous work [Biophys. J. 98(10):2111-20; Biophys. J. 98(3):102a] suggested that the diastolic ryanodine receptor (RyR) mediated leak (Jleak) from the sarcoplasmic reticulum (SR) of intact ventricular myocytes occurs in spark and non-spark forms. We further showed that the fraction of spark-mediated Jleak increases upon isoproterenol treatment in intact rabbit ventricular myocytes, suggesting that the effective sensitivity to cytosolic Ca is increased by RyR phosphorylation [Biophys. J. 98(3):102a]. We now present an extension of this work, focused on aspects of individual sparks taken from cells at matched [Ca] and SR load. Events were wider in isoproterenol (8.5% greater FWHM of the F/F0 profile) but had similar amplitudes than control. A backward reconstruction of the release flux density, when applied to average sparks assumed to be spherically symmetric, rendered a source that was wider for the isoproterenol event, indicating the recruitment of peripheral RyRs. A forward release flux reconstruction which recapitulates the steps of spark formation could not simultaneously fit the amplitudes and sizes of any of the two average sparks when using realistic radii for the junctional SR. This result may be interpreted as implying the existence of RyRs, peripheral to and perhaps outside the couplon. Compounded with the increased CICR sensitivity upon isoproterenol treatment (see above), the greater spark width of isoproterenol events may increase the probability of Ca wave generation.