Methyl Jasmonate-Induced Stimulation of Sarcoplasmic Reticulum Ca -ATPase Affects Contractile Responses in Rat Slow-Twitch Skeletal Muscle

The purpose of this study was to determine whether methyl jasmonate, a stimulator of Ca -adenosine triphosphatase (ATPase) activity of the purified ATPase from fast-twitch skeletal muscle, could affect contractile responses in small bundles of rat isolated slow-twitch (soleus) fibers. In saponin-skinned fibers, sarcoplasmic reticulum (SR) Ca loading was performed in pCa 7.0 solution. The amount of Ca taken up was monitored by use of the amplitude of contraction following application of 10 mM caffeine. Results indicate that the increased loading rate in the presence of methyl jasmonate is likely due to stimulation of the SR Ca -ATPase. In Triton-skinned fibers, the myofibrillar Ca sensitivity was not changed by methyl jasmonate (50–200 M). In intact fibers, the amplitude and the time constant of relaxation of twitch and potassium contracture were reversibly reduced after 2 min of application of methyl jasmonate at a concentration of up to 125 M. At higher concentrations ( 150 M), effects were not reversible. In the presence of methyl jasmonate (100 M), the relationship between the amplitude of potassium contractures and the membrane potential shifted to more positive potentials, whereas the steady-state inactivation curve was unchanged. These observations suggest that methyl jasmonate has no effect on voltage sensors. Taken together, our results show that methyl jasmonate is a potent, reversible, and specific stimulator of the SR Ca pump in slow-twitch skeletal muscle and is an extremely valuable pharmacological tool for improving relaxation and studying calcium-signaling questions. In mammalian skeletal muscle, triggering of the action potential along the membrane of the T-transverse tubule system increases intracellular Ca concentration and produces contraction through the interaction of actin and myosin. Contraction is then terminated through Ca uptake by the sarcoplasmic reticulum (SR) via the Ca pump. This series of events is referred to as excitation-contraction coupling. During contraction, two key proteins play a major role in the calcium release mechanism: the sarcolemmal dihydropyridine receptor (voltage sensor) and the SR Ca release channel (ryanodine receptor). To induce relaxation, Ca is removed from the cytosol by extrusion through the sarcolemmal Na /Ca exchanger and Ca adenosine triphosphatase (Ca -ATPase), mitochondria, and by sequestration in the SR mediated by the Ca -ATPase. However, in slowtwitch muscle, the SR Ca pump is the major process responsible for reducing cytosolic Ca from a high level to a low resting level during relaxation (Leong and Maclennan, 1998; Lamb, 2000). Pharmacological tools predominantly inhibiting SR Ca uptake, i.e., cyclopiazonic acid, thapsigargin, and 2,5-di-(tert-butyl)-1,4-hydroquinone, have long been used to separate the various cellular mechanisms that regulate the contraction-relaxation cycle in muscle. Other tools that act as specific stimulators of the SR Ca -ATPase activity of muscle cells could be useful for investigating SR Ca