EFFECTS OF DILTIAZEM UPON A RAPIDLY EXCHANGING CALCIUM COMPARTMENT RELATED TO REPRIMING IN FROG SKELETAL-MUSCLE

Following spontaneous relaxation, fast skeletal muscle must first repo larize and then undergo a first-order repriming reaction before depola rization will result in maximal tension production. Ca-45 exposure dur ing repriming defined two Ca compartments during subsequent efflux, na med Ca-fast and Ca-slow. Ca-slow had an average time constant of 112 /- 17 min. On the basis of slow turnover and content determined by a v ariety of methods, I suggest Ca-slow represent Ca within the sarcoplas mic reticulum. Ca-fast contaomed 12 pmol Ca per fibre and resting exch ange had a time constant of 5.1 +/- 0.4 min. A total of 12 pmol Ca-45 within Ca-fast was released during a maximal contracture. Most of the Ca released from Ca-fast rapidly entered the extracellular space; howe ver, 0.39 +/- 0.15 pmol Ca per fibre transferred from Ca-fast into Ca- slow when the muscle bundle contracted. When 1-10 mu M diltiazem reduc ed contracture time-tension, release of Ca-fast was reduced proportion ally When 10 mu M diltiazem paralyzed excitation-contraction coupling, Ca-fast was not released. Refilling of Ca-fast was proportional to th e extent of repriming during Ca-45 exposure. Although release and refi lling of Ca-fast is related to contraction, its role in excitation-con traction coupling remains to be elucidated.