Abstract
Ca ions were ionophoretically injected through an intracellular microelectrode into the single muscle fiber of a crayfish, and the resulting contraction sphere was observed under a microscope and photographed with a movie camera. The minimum contraction produced by the threshold current involved usually three or four, sometimes two, sarcomeres on both sides of the injecting pipette but contraction involving only one sarcomere was not observed. The rheobase of the Ca-injectingcurrent was 3.2×10-9 A. The strength-duration curves were determined for Ca-, Sr-, and Ba-injecting currents; all fitted a similar hyperbolic equation. The threshold amount of Ca above rheobasic injection was 2.1×10-15 mol, and the ratios between threshold amounts were Ca:Sr:Ba=1:1.9:3.0. The effects of Ca and Sr were additive for the contraction. More current was required for the Ca-injection to produce the contraction in the K-depolarized- or 15mm-procaine-treated muscle, although less current was sufficient for the muscle treated with 0.5-1.0mM of caffeine.The participation of the Ca-induced Ca release mechanism in the contraction produced by Ca injection and the role of Sr or Ba as a substitute for Ca were discussed.
