Spatial characteristics of sarcoplasmic reticulum Ca2+ release events triggered by L‐type Ca2+ current and Na+ current in guinea‐pig cardiac myocytes

Ca2+ signals in cardiac muscle cells are composed of spatially limited elementary events termed Ca2+ sparks. Several studies have also indicated that Ca2+ signals smaller than Ca2+ sparks can be elicited. These signals have been termed Ca2+ quarks and were proposed to result from the opening of a single Ca2+ release channel of the sarcoplasmic reticulum. We used laser‐scanning confocal microscopy to examine the subcellular properties of Na+ current (INa)‐ and L‐type Ca2+ current (ICa,L)‐induced Ca2+ transients in voltage‐clamped ventricular myocytes isolated from guinea‐pigs. Both currents, INa and ICa,L, evoked substantial, global Ca2+ transients. To examine the spatiotemporal properties of such Ca2+ signals, we performed power spectral analysis of these Ca2+ transients and found that both lacked spatial frequency components characteristic for Ca2+ sparks. The application of 10 μm verapamil to partially block L‐type Ca2+ current reduced the corresponding Ca2+ transients down to individual Ca2+ sparks. In contrast, INa‐induced Ca2+ responses were still spatially homogeneous and lacked Ca2+ sparks even for small current amplitudes. By using high resistance patch pipettes (> 4 MΩ) to exaggerate the loss of voltage control during INa, Ca2+ sparks appeared superimposed on a homogeneous Ca2+ release component and were exclusively triggered during the flow of INa. In the presence of 10 μm ryanodine both ICa,L and INa elicited small, residual Ca2+ transients that were spatially homogeneous but displayed distinctively different temporal profiles. We conclude that INa is indeed able to cause Ca2+ release in guinea‐pig ventricular myocytes. In contrast to ICa,L‐induced Ca2+ transients, which are built up from the recruitment of individual Ca2+ sparks, the INa‐evoked cellular responses were always homogeneous, indicating that their underlying elementary Ca2+ release event is distinct from the Ca2+ spark. Thus, INa‐induced Ca2+ transients are composed of smaller Ca2+ signals, most likely Ca2+ quarks.