Prolonged force increase following a high-frequency burst is not due to a sustained elevation of [Ca2+]i

1 Institute for Fundamental and Clinical Human Movement Sciences, Vrije Universiteit, 1081 BT Amsterdam, The Netherlands; 2 Department of Physiology and Pharmacology, Karolinska Institutet, SE 171 77 Stockholm, Sweden; and 3 Neuromuscular Biology Research Group, Manchester Metropolitan University, Alsager, ST 7 2 HL, United Kingdom A brief high-frequency burst of action potentials results in a sustained force increase in skeletal muscle. The present study investigates whether this force potentiation is the result of a sustained increase of the free myoplasmic [Ca 2+ ] ([Ca 2+ ] i ). Single fibers from mouse flexor brevis muscles were stimulated with three impulses at 150 Hz (triplet) at the start of a 350-ms tetanus or in the middle of a 700-ms tetanus; the stimulation frequency of the rest of the tetanus ranged from 20 to 60 Hz. After the triplet, force was significantly ( P