Prolonged force increase following a high-frequency burst is not due to a sustained elevation of [Ca2+]i.
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 [Ca2+] ([Ca2+]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 < 0.05) increased between 17 and 20% when the triplet was given at the start of the tetanus and between 5 and 18% when the triplet was given in the middle (n = 7). However, during this potentiation, [Ca2+]i was not consistently increased. Hence, the increased force following a high-frequency burst is likely due to changes in the myofibrillar properties.
Abbate
F
F
Institute for Fundamental and Clinical Human Movement Sciences, Vrije Universiteit, 1081 BT Amsterdam, The Netherlands. Fabio.Abbate@fyfa.ki.se
Bruton
J D
JD
De Haan
A
A
Westerblad
H
H
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D004558:Electric Stimulation / Q000379:methods; D006614:Hindlimb; D066298:In Vitro Techniques; D007425:Intracellular Membranes / Q000378:metabolism*; D008297:Male; D051379:Mice; D009119:Muscle Contraction / Q000502:physiology; D018485:Muscle Fibers, Skeletal / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology*; D009994:Osmolar Concentration; D013997:Time Factors
D002118:Calcium