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

摘要

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.