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 (I(Na))- and L-type Ca2+ current (I(Ca,L))-induced Ca2+ transients in voltage-clamped ventricular myocytes isolated from guinea-pigs. Both currents, I(Na) and I(Ca,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 microM verapamil to partially block L-type Ca2+ current reduced the corresponding Ca2+ transients down to individual Ca2+ sparks. In contrast, I(Na)-induced Ca2+ responses were still spatially homogeneous and lacked Ca2+ sparks even for small current amplitudes. By using high resistance patch pipettes (> 4 MOmega) to exaggerate the loss of voltage control during I(Na), Ca2+ sparks appeared superimposed on a homogeneous Ca2+ release component and were exclusively triggered during the flow of I(Na). In the presence of 10 microM ryanodine both I(Ca,L) and I(Na) elicited small, residual Ca2+ transients that were spatially homogeneous but displayed distinctively different temporal profiles. We conclude that I(Na) is indeed able to cause Ca2+ release in guinea-pig ventricular myocytes. In contrast to I(Ca,L)-induced Ca2+ transients, which are built up from the recruitment of individual Ca2+ sparks, the I(Na)-evoked cellular responses were always homogeneous, indicating that their underlying elementary Ca2+ release event is distinct from the Ca2+ spark. Thus, I(Na)-induced Ca2+ transients are composed of smaller Ca2+ signals, most likely Ca2+ quarks.
Lipp
Peter
P
Department of Physiology, University of Bern, Bühlplatz 5, 3012 Bern, Switzerland. peter.lipp@uniklinik-saarland.de
Egger
Marcel
M
Niggli
Ernst
E
D000818:Animals; D002118:Calcium / Q000502:physiology*; D020746:Calcium Channels, L-Type / Q000502:physiology*; D020013:Calcium Signaling / Q000502:physiology*; D006168:Guinea Pigs; D006321:Heart / Q000502:physiology*; D006352:Heart Ventricles; D066298:In Vitro Techniques; D032342:Muscle Cells / Q000502:physiology*; D018408:Patch-Clamp Techniques; D012519:Sarcoplasmic Reticulum / Q000502:physiology*; D015222:Sodium Channels / Q000502:physiology*
D020746:Calcium Channels, L-Type; D015222:Sodium Channels; D002118:Calcium