Ca2+ dependence of transverse tubule-mediated calcium release in skinned skeletal muscle fibers.
Isometric force and 45Ca efflux from the sarcoplasmic reticulum were measured at 19 degrees C in frog skeletal muscle fibers skinned by microdissection. After Ca2+ loading, application of the ionophores monensin, an Na+(K+)/H+ exchanger, or gramicidin D, an H+ greater than K+ greater than Na+ channel-former, evoked rapid force development and stimulated release of approximately 30% of the accumulated 45Ca within 1 min, whereas CCCP (carbonyl cyanide pyruvate p-trichloromethoxyphenylhydrazone), a protonophore, and valinomycin, a neutral, K+-specific ionophore, did not. When monensin was present in all bathing solutions, i.e., before and during Ca2+ loading, subsequent application failed to elicit force development and to stimulate 45Ca efflux. 5 min pretreatment of the skinned fibers with 50 microM digitoxin, a permeant glycoside that specifically inhibits the Na+,K+ pump, inhibited monensin and gramicidin D stimulation of 45Ca efflux; similar pretreatment with 100 microM ouabain, an impermeant glycoside, was ineffective. Monensin stimulation of 45Ca efflux was abolished by brief pretreatment with 5 mM EGTA, which chelates myofilament-space calcium. These results suggest that: monensin and gramicidin D stimulate Ca2+ release from the sarcoplasmic reticulum that is mediated by depolarization of the transverse tubules, which seal off after sarcolemma removal and form closed compartments; a transverse tubule membrane potential (myofilament space-negative) is maintained and/or established by the operation of the Na+,K+ pump in the transverse tubule membranes and is sensitive to the permeant inhibitor digitoxin; the transverse tubule-mediated stimulation of 45Ca efflux appears to be entirely Ca2+ dependent.
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D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D004074:Digitoxin / Q000494:pharmacology; D006096:Gramicidin / Q000494:pharmacology; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism; D008985:Monensin / Q000494:pharmacology; D009119:Muscle Contraction / Q000187:drug effects; D009132:Muscles / Q000187:drug effects / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism; D011898:Ranidae; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D012964:Sodium / Q000378:metabolism
D007473:Ion Channels; D006096:Gramicidin; D008985:Monensin; D012964:Sodium; D004074:Digitoxin; D011188:Potassium; D002118:Calcium