CALCIUM INFLUX IN CONTRACTING AND PARALYZED FROG TWITCH MUSCLE-FIBERS

Cal uptake produced by a K contracture in isolated frog twitch fibers was 6.7 .+-. 0.8 pmol in 0.7 cm of fiber (mean .+-. SEM, 21 observations) in the presence of 30 .mu.M D 600 [methoxyverafamil]. When K was applied to fibers paralyzed by the combination of 30 .mu.M D600, colds, and a prior contracture, the Ca uptake fell to 3.0 .+-. 0.7 pmol. The fibers were soaked in 45Ca in sodium Ringer for 3 min before 45Ca, in a K solution, was added for 2 min; each estimate of uptake was corrected for 5 min of resting influx, measured from the same fiber (average = 2.3 .+-. 0.3 pmol). The Ca influx into paralzyed fibers is unrelated to contraction. This voltage-sensitive, slowly inactivating influx, which can be blocked by 4 mM N, has properties similar to the Ca current described by several laboratories. The paired difference in Ca uptake between contracting and paralyzed fibers, 2.9 .+-. 0.8 pmol, is a component of influx related to contraction. Its size varies with contracture size and it occurs after tension production: 45Ca applied immediately after contracture is taken up in essentially the same amounts as 45Ca added before contraction. This delayed uptake is probably a reflux refilling a binding site on the cytoplasmic side of the T membrane, which was emptied during the prior contracture, perhaps to initiate it. No components of Ca uptake related to excitation-contraction coupling occurring before or during a contracture was detected.