Caffeine contractures were induced after Kkonditioning of skeletal muscles from pigs and mice. K+-conditioning is defined as the partial depolarization caused by increasing external potassium (K;) with [K+lx[CI-l constant. Conditioning depolarizations that rendered muscles refractory to brief electrical stimulation still enhanced the contracture tension elicited by subsequent direct caffeine stimulation of sarcoplasmic reticulum (SR) calcium release. The effects of K'-conditioning on caffeine-induced contractures of intact cell bundles reached a maximum at 15-30 mM K i and then progressively declined at higher [K+],,. Conditioning with 30 mM K ' for 5 min, which inactivates excitation-contraction (EC) coupling in response to action potentials, both increased the magnitude of caffeine contractures 2-1 0-fold and shifted the contracture threshold toward lower caffeine concentrations. Enhanced sensitivity to caffeine was inhibited by dantrolene (20 KM) and its watersoluble analogue azumolene (1 50 pM). These drugs decreased caffeine-induced contractures following depolarization with 4-1 5 mM K' to 25-50% of control tension. The inorganic anion perchlorate (Cloy), which like caffeine potentiates twitches, increased caffeine-induced contractures -twofold after Ki-conditioning (>4 mM). The results suggest that CIO, and dantrolene, in addition to caffeine, also influence SR calcium release either directly or by mechanism(s) subsequent to depolarization of the sarcolemma. Moreover, since CIO, is known to shift the voltage-dependence of intramembrane charge movement, CIO, may exert effects on the transverse-tubule voltage sensors as well as the SR. 0 1995 ~i l e y -~i s s , Inc