Effects of diltiazem upon a rapidly exchanging calcium compartment related to repriming in frog skeletal muscle.

Following spontaneous relaxation, fast skeletal muscle must first repolarize and then undergo a first-order repriming reaction before depolarization will result in maximal tension production. 45Ca exposure during repriming defined two Ca compartments during subsequent efflux, named Ca(fast) and Ca(fast). Ca(slow) had an average time constant of 112 +/- 17 min. On the basis of slow turnover and content determined by a variety of methods, I suggest Ca(slow) represents Ca within the sarcoplasmic reticulum. Ca(fast) contained 12 pmol Ca per fibre and resting exchange had a time constant of 5.1 +/- 0.4 min. A total of 12 pmol 45Ca within Ca(fast) was released during a maximal contracture. Most of the Ca released from Ca(fast) rapidly entered the extracellular space; however, 0.39 +/- 0.15 pmol Ca per fibre transferred from Ca(fast) into Ca(slow) when the muscle bundle contracted. When 1-10 microM diltiazem reduced contracture time-tension, release of Ca(fast) was reduced proportionally. When 10 microM diltiazem paralyzed excitation-contraction coupling, Ca(fast) was not released. Refilling of Ca(fast) was proportional to the extent of repriming during 45Ca exposure. Although release and refilling of Ca(fast) is related to contraction, its role in excitation-contraction coupling remains to be elucidated.