In ventricular myocytes of large mammals with low T-tubule density, a significant number of ryanodine receptors (RyRs) are not coupled to the sarcolemma; cardiac remodeling increases noncoupled RyRs.

Our aim was to test the hypothesis that coupled and noncoupled RyRs have distinct microdomain-dependent modulation.

We studied single myocytes from pig left ventricle. The T-tubule network was analyzed in 3-dimension (3D) to measure distance to membrane of release sites. The rising phase of the Ca²⁺ transient was correlated with proximity to the membrane (confocal imaging, whole-cell voltage-clamp, K₅fluo-4 as Ca²⁺ indicator). Ca²⁺ sparks after stimulation were thus identified as resulting from coupled or noncoupled RyRs. We used high-frequency stimulation as a known activator of Ca²⁺/calmodulin-dependent kinase II. Spark frequency increased significantly more in coupled than in noncoupled RyRs. This specific modulation of coupled RyRs was abolished by the Ca²⁺/calmodulin-dependent kinase II blockers autocamtide-2–related inhibitory peptide and KN-93, but not by KN-92. Colocalization of Ca²⁺/calmodulin-dependent kinase II and RyR was not detectably different for coupled and noncoupled sites, but the F-actin disruptor cytochalasin D prevented the specific modulation of coupled RyRs. NADPH oxidase 2 inhibition by diphenyleneiodonium or apocynin, or global reactive oxygen species scavenging, also prevented coupled RyR modulation. During stimulated Ca²⁺ transients, frequency-dependent increase of the rate of Ca²⁺ rise was seen in coupled RyR regions only and abolished by autocamtide-2–related inhibitory peptide. After myocardial infarction, selective modulation of coupled RyR was lost.

Coupled RyRs have a distinct modulation by Ca²⁺/calmodulin-dependent kinase II and reactive oxygen species, dependent on an intact cytoskeleton and consistent with a local Ca²⁺/reactive oxygen species microdomain, and subject to modification with disease.