Modulation of caffeine contractures in mammalian skeletal muscles by variation of extracellular potassium

Abstract

Caffeine contractures were induced after K(+)-conditioning of skeletal muscles from pigs and mice. K(+)-conditioning is defined as the partial depolarization caused by increasing external potassium (K+0) with [K+]x[Cl-] constant. Conditioning depolarizations that rendered muscles refractory to brief electrical stimulation still enhanced the contracture tension elicited by subsequent direct caffeine stimulation of sarcoplasmic reticulum (SR) calcium release. The effects of K(+)-conditioning on caffeine-induced contractures of intact cell bundles reached a maximum at 15-30 mM K+0 and then progressively declined at higher [K+]0. Conditioning with 30 mM K+ for 5 min, which inactivates excitation-contraction (EC) coupling in response to action potentials, both increased the magnitude of caffeine contractures 2-10-fold and shifted the contracture threshold toward lower caffeine concentrations. Enhanced sensitivity to caffeine was inhibited by dantrolene (20 microM) and its watersoluble analogue azumolene (150 microM). These drugs decreased caffeine-induced contractures following depolarization with 4-15 mM K+ to 25-50% of control tension. The inorganic anion perchlorate (CIO-4), which like caffeine potentiates twitches, increased caffeine-induced contractures approximately twofold after K(+)-conditioning (> 4 mM). The results suggest that CIO-4 and dantrolene, in addition to caffeine, also influence SR calcium release either directly or by mechanism(s) subsequent to depolarization of the sarcolemma. Moreover, since CIO-4 is known to shift the voltage-dependence of intramembrane charge movement, CIO-4 may exert effects on the transverse-tubule voltage sensors as well as the SR.

Dantrolene Inhibition of Sarcoplasmic Reticulum Ca2+Release by Direct and Specific Action at Skeletal Muscle Ryanodine Receptors

“…Dantrolene also inhibited [ 3 H]ryanodine binding to purified skeletal muscle ryanodine receptor protein reconstituted into liposomes. In contrast, cardiac sarcoplasmic reticulum vesicle 45 …”

“…Here we provide evidence of a high-affinity, monophasic inhibition by dantrolene of ryanodine receptor Ca 2؉ channel function in isolated sarcoplasmic reticulum vesicles prepared from malignant hyperthermia-susceptible and normal pig skeletal muscle. In media simulating resting myoplasm, dantrolene increased the half-time for 45 Ca 2؉ release from both malignant hyperthermia and normal vesicles approximately 3.5-fold and inhibited sarcoplasmic reticulum vesicle [ 3 …”

“…The effects of dantrolene on cardiac SR vesicle 45 Ca 2ϩ release and [ 3 H]ryanodine binding were also examined. Our results demonstrate specific inhibitory effects of dantrolene on the functional activity of skeletal muscle RYR1 channels that are consistent with the effects of dantrolene on E-C coupling in intact muscle and suggest a mechanism of action in which a direct, high-affinity interaction of dantrolene with the RYR1 channel complex may limit the activation of this channel by calmodulin (CaM) and Ca Isolation of SR Vesicles-Skeletal muscle SR vesicles were isolated from the longissimus dorsi muscle of MHS and normal pigs as described previously (17).…”

“…Finally, whether the effects of dantrolene may be restricted to the RYR1 channel isoform or may extend to other intracellular Ca 2ϩ release channel isoforms is also in question (16). The answers to these questions may be important not only in defining the mechanism of action of dantrolene but also in clarifying those aspects of RYR channel regulation that may be specific to skeletal muscle E-C coupling and that may be altered in MH.In this study, we have examined the effects of dantrolene on the 45 Ca 2ϩ release and [ 3 H]ryanodine binding activity of isolated SR vesicles prepared from MH-susceptible (MHS) and normal pig skeletal muscle. The effects of dantrolene on cardiac SR vesicle 45 Ca 2ϩ release and [ 3 H]ryanodine binding were also examined.…”

“…In this study, we have examined the effects of dantrolene on the 45 Ca 2ϩ release and [ 3 H]ryanodine binding activity of isolated SR vesicles prepared from MH-susceptible (MHS) and normal pig skeletal muscle. The effects of dantrolene on cardiac SR vesicle 45 Ca 2ϩ release and [ 3 H]ryanodine binding were also examined.…”