K201 (JTV-519) alters the spatiotemporal properties of diastolic Ca²⁺ release and the associated diastolic contraction during β-adrenergic stimulation in rat ventricular cardiomyocytes

K201 has previously been shown to reduce diastolic contractions in vivo during β-adrenergic stimulation and elevated extracellular calcium concentration ([Ca²⁺]_o). The present study characterised the effect of K201 on electrically stimulated and spontaneous diastolic sarcoplasmic reticulum (SR)-mediated Ca²⁺ release and contractile events in isolated rat cardiomyocytes during β-adrenergic stimulation and elevated [Ca²⁺]_o. Parallel experiments using confocal microscopy examined spontaneous diastolic Ca²⁺ release events at an enhanced spatiotemporal resolution. 1.0 μmol/L K201 in the presence of 150 nmol/L isoproterenol (ISO) and 4.75 mmol/L [Ca²⁺]_o significantly decreased the amplitude of diastolic contractions to ~16% of control levels. The stimulated free Ca²⁺ transient amplitude was significantly reduced, but stimulated cell shortening was not significantly altered. When intracellular buffering was taken into account, K201 led to an increase in action potential-induced SR Ca²⁺ release. Myofilament sensitivity to Ca²⁺ was not changed by K201. Confocal microscopy revealed diastolic events composed of multiple Ca²⁺ waves (2–3) originating at various points along the cardiomyocyte length during each diastolic period. 1.0 μmol/L K201 significantly reduced the (a) frequency of diastolic events and (b) initiation points/diastolic interval in the remaining diastolic events to 61% and 71% of control levels respectively. 1.0 μmol/L K201 can reduce the probability of spontaneous diastolic Ca²⁺ release and their associated contractions which may limit the propensity for the contractile dysfunction observed in vivo.