In single mouse skeletal muscle fibers injected with fluorescent Ca2+ indicator Indo-1, 4-chloro-m-cresol (chlorocresol, 4-CmC) and its lipophilic analogue 4-chloro-3-ethylphenol (4-CEP) increased resting myoplasmic free [Ca2+] ([Ca2+](i)) in a dose-dependent manner. In this regard, 4-CEP was more potent than 4-CmC and both were more potent than caffeine. High concentrations of 4-CmC (1 mM) or 4-CEP (500 μM) caused large and irreversible increase in resting [Ca2+](i) leading to contracture. 4-CmC potentiated the [Ca2+](i) increase and force of contraction induced by tetanic stimulation. Unlike caffeine, 4-CmC did not affect the activity of sarcoplasmic reticulum Ca2+ pump or the myofibrillar Ca2+ sensitivity. A low concentration of 4-CEP (20 μM) had no effect on resting [Ca2+](i) on its own, but it enhanced the resting [Ca2+](i) increase induced by caffeine and also potentiated the [Ca2+](i) increase and contraction induced by tetanic stimulation. However, a relatively high concentration of 4-CEP (200 μM) inhibited tetanic stimulation-induced [Ca2+](i) increase and contraction. Dantrolene, a muscle relaxant, inhibited 4-CmC-induced [Ca2+](i) increase under resting conditions. However, when 4-CEP was applied in the presence of dantrolene, there was an exaggerated increase in [Ca2+](i). We conclude that 4-CmC and 4-CEP are potent agonists that can increase [Ca2+](i) rapidly and reversibly by activating ryanodine receptors in situ in intact skeletal muscle fibers. These compounds, specially 4-CmC, may be useful for mechanistic and functional studies of ryanodine receptors and excitation-contraction coupling in skeletal muscles.
|Number of pages||11|
|State||Published - Aug 1998|
Bibliographical noteFunding Information:
Financial support was obtained from the Swedish Medical Research Council (Project no 10842), The Swedish National Center for Sports Research, The Swedish Society of Medicine, National Board of Health and Welfare and Funds of the Karolinska Institute. FHA was supported by Baylor College of Medicine-Karolinska Institute Research Exchange Program. HW and MS1 have positions at the Swedish Medical Research Council.
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology