EFFECTS OF RYANODINE RECEPTOR AGONIST 4-CHLORO-M-CRESOL ON MYOPLASMICFREE CA2-MUSCLE( CONCENTRATION AND FORCE OF CONTRACTION IN MOUSE SKELETAL)

In single mouse skeletal muscle fibers injected with fluorescent Ca2indicator Indo-1, 4-chloro-m-cresol (chlorocresol, 4-CmC) and its lipo philic analogue 4-chloro-3-ethylphenol (4-CEP) increased resting myopl asmic free [Ca2+] ([Ca2+](i)) in a dose-dependent manner. In this rega rd, 4-CEP was more potent than 4-CmC and both were more potent than ca ffeine. High concentrations of 4-CmC (1 mM) or 4-CEP (500 mu M) caused large and irreversible increase in resting [Ca2+](i) leading to contr acture, 4-CmC potentiated the [Ca2+](i) increase and force of contract ion induced by tetanic stimulation. Unlike caffeine, 4-CmC did not aff ect the activity of sarcoplasmic reticulum Ca2+ pump or the myofibrill ar Ca2+ sensitivity. A low concentration of 4-CEP (20 mu M) had no eff ect on resting [Ca2+](i) on its own, but it enhanced the resting [Ca2](i) increase induced by caffeine and also potentiated the [Ca2+](i) i ncrease and contraction induced by tetanic stimulation. However, a rel atively high concentration of 4-CEP (200 mu M) inhibited tetanic stimu lation-induced [Ca2+](i) increase and contraction. Dantrolene, a muscl e relaxant, inhibited 4-CmC-induced [Ca2+](i) increase under resting c onditions. However, when 4-CEP was applied in the presence of dantrole ne, 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 sk eletal muscle fibers. These compounds, specially 4-CmC, may be useful for mechanistic and functional studies of ryanodine receptors and exci tation-contraction coupling in skeletal muscles.