Calcium/calmodulin-dependent kinase II and nitric oxide synthase 1 dependent modulation of ryanodine receptors during β-adrenergic stimulation is restricted to the dyadic cleft.

In cardiac myocytes, β‐adrenergic stimulation enhances Ca2+ cycling through an integrated signalling cascade modulating L‐type Ca2+ channels (LTCCs), phospholamban and ryanodine receptors (RyRs). Ca2+/calmodulin‐dependent kinase II (CaMKII) and nitric oxide synthase 1 (NOS1) are proposed as prime mediators for increasing RyR open probability. We investigate whether this pathway is confined to the high Ca2+ microdomain of the dyadic cleft and thus to coupled RyRs. Pig ventricular myocytes are studied under whole‐cell voltage‐clamp and confocal line‐scan imaging with Fluo‐4 as a [Ca2+]i indicator. Following conditioning depolarizing pulses, spontaneous RyR activity is recorded as Ca2+ sparks, which are assigned to coupled and non‐coupled RyR clusters. Isoproterenol (ISO) (10 nm) increases Ca2+ spark frequency in both populations of RyRs. However, CaMKII inhibition reduces spark frequency in coupled RyRs only; NOS1 inhibition mimics the effect of CaMKII inhibition. Moreover, ISO induces the repetitive activation of coupled RyR clusters through CaMKII activation. Immunostaining shows high levels of CaMKII phosphorylation at the dyadic cleft. CaMKII inhibition reduces ICaL and local Ca2+ transients during depolarizing steps but has only modest effects on amplitude or relaxation of the global Ca2+ transient. In contrast, protein kinase A (PKA) inhibition reduces spark frequency in all RyRs concurrently with a reduction of sarcoplasmic reticulum Ca2+ content, Ca2+ transient amplitude and relaxation. In conclusion, CaMKII activation during β‐adrenergic stimulation is restricted to the dyadic cleft microdomain, enhancing LTCC‐triggered local Ca2+ release as well as spontaneous diastolic Ca2+ release whilst PKA is the major pathway increasing global Ca2+ cycling. Selective CaMKII inhibition may reduce potentially arrhythmogenic release without negative inotropy.