Some observations on caffeine-induced contracture of barnacle muscle fibers

Isometric tension measurements carried out on uncannulated, intact barnacle muscle fibers show that the minimal concentration of caffeine which produces contracture is 2mM and that which produces maximal tension is 30mM. Pretreatment of barnacle fibers with 3mM-caffeine renders these fibers less sensitive to a second challege e.g. with 30mM-caffeine, the amplitude of the contractile response being time-dependent. Caffeine-treated fibers fail to relax completely. Isotonic tension measurements indicate that the residual tension caused by caffeine is dose-dependent. Significantly, the tension caused by caffeine is independent of the external Ca²⁺.