Abstract
The effects of caffeine on the process of excitation-contraction coupling in amphibian skeletal muscle fibers were investigated using the confocal spot detection technique. This method permits to carefully discriminate between caffeine effects on the primary sources of Ca2+ release at the Z-lines where the triads are located and secondary actions on other potential Ca Release sources. Our results demonstrate that 0.5 mM caffeine potentiates and prolongs localized action-potential evoked Ca2+ transients recorded at the level of the Z-lines, but that 1mM only prolongs them. The effects at both doses are reversible. At the level of the M-line, localized Ca2+ transients displayed more variability in the presence of 1 mM caffeine than in control conditions. At this dose of caffeine, extra-junctional sources of Ca2+ release also were observed occasionally.
Highlights
A critical step in the excitation-contraction (EC) coupling process in skeletal muscle is the release of Ca2+ ions, stored at a high concentration in the lumen of the terminal cisternae (TC) of the sarcoplasmic reticulum (SR), in response to electrical depolarization of the transverse tubular system (Peachey, 1965)
The localization of the sites of Ca2+ release at the Z-lines was first demonstrated experimentally by Escobar et al (1994) using a spot detection method and fluorescent Ca2+ indicators. These authors observed that, in amphibian muscle fibers, the action potentials (APs)-elicited [Ca2+] increase at the M-line was not delayed significantly relative to that at the Z-line (Z-M delay) and proposed that a broad band of the SR may participate in the release process (Escobar et al, 1994). This suggestion was confirmed in experiments that incorporated technical refinements to the spot detection method (DiFranco et al, 2002) and with quantitative modeling of the diffusion-reaction process involved in the Ca2+ redistribution within the sarcomere (Novo et al, 2003)
Since it has been well documented that caffeine is a strong modulator of CICR in several preparations, it seemed appropriate that its effects are studied in the context of localized detection of Ca2+ transients
Summary
A critical step in the excitation-contraction (EC) coupling process in skeletal muscle is the release of Ca2+ ions, stored at a high concentration in the lumen of the terminal cisternae (TC) of the sarcoplasmic reticulum (SR), in response to electrical depolarization of the transverse tubular system (Peachey, 1965). The localization of the sites of Ca2+ release at the Z-lines was first demonstrated experimentally by Escobar et al (1994) using a spot detection method and fluorescent Ca2+ indicators These authors observed that, in amphibian muscle fibers, the AP-elicited [Ca2+] increase at the M-line was not delayed significantly relative to that at the Z-line (Z-M delay) and proposed that a broad band of the SR may participate in the release process (Escobar et al, 1994). This suggestion was confirmed in experiments that incorporated technical refinements to the spot detection method (DiFranco et al, 2002) and with quantitative modeling of the diffusion-reaction process involved in the Ca2+ redistribution within the sarcomere (Novo et al, 2003). Since it has been well documented that caffeine is a strong modulator of CICR in several preparations, it seemed appropriate that its effects are studied in the context of localized detection of Ca2+ transients
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