Spatial and Functional Interactions between SK Channels and L-Type Calcium Channels in Cardiomyocytes

Xiao-Dong Zhang,Wei Chun Chen,Jeong-Han Lee,Zhong Jian,Gu Dai,Leighton Izu,Ye Chen-Izu,Nipavan Chiamvimonvat

doi:10.1016/j.bpj.2015.11.707

Xiao-Dong Zhang, Wei Chun Chen + Show 6 more

Open Access

https://doi.org/10.1016/j.bpj.2015.11.707

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Abstract

Small conductance Ca2+-activated K+ (SK) channels are gated solely by intracellular Ca2+ and hence, provide a critical link between changes in intracellular Ca2+ and membrane potentials. The identification and functional expression of SK channels in the heart provided additional insights into the regulation of cardiac excitability. Indeed, the crosstalk between Ca2+ signaling and SK channels is critical for shaping cardiac repolarization. Our prior studies documented the physical coupling between SK channels and L-type Ca2+ channels (LTCCs). In addition, functional dependence of SK channel on sarcoplasmic reticulum has also been demonstrated. However, the spatial and functional interactions are less well understood. We hypothesize that LTCCs provide the immediate Ca2+ domain for the activation of cardiac SK channels. Using stimulated emission depletion (STED) microscopy, we gained super-resolution images of SK channels, LTCCs and ryanodine receptors (RyR2) in rabbit ventricular myocytes, revealing the spatial coupling of the three molecules. Moreover, we designed a voltage-clamp protocol to test the functional coupling between SK and LTCCs. EGTA or BAPTA were applied to identify the intracellular Ca2+ domains for SK channel activation. SK currents were activated by Ca2+ influx via LTCCs after depletion of sarcoplasmic reticulum (SR) Ca2+ store. BAPTA removed the functional coupling between SKs and LTCCs. In addition, alterations in SK channel activation kinetics were observed after SR Ca2+ depletion. We conclude that the spatial proximity among SK, LTCCs and RyR2 provides the structural basis for the functional complex formation for SK channel activation. Ca2+ influx through LTCCs provides Ca2+ microdomain for SK channel activation as well as a joint contribution from LTCCs and SR for SK channel activation in cardiomyocytes.Funded by NIH (R01 HL085727 and R01 HL085844 to NC), VA (I01 BX000576 to NC) and AHA (14BGIA18870087 to XZ).

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