Subcontracture Depolarization Alters Doublet Potentiation in Isolated Skeletal Muscle

The addition of two closely spaced stimulating impulses (doublets) at the beginning of a low‐frequency train of stimuli has been shown to increase isometrictension and maximize tension‐time integral (TTI). The purpose of this investigation was to determine the combined effects of subcontracture K + ‐induced depolarization, itself a powerful potentiating influence, and doublet potentiation on peak muscle tension development. Sartorii muscles (60–70 mg) from small male R. pipiens were dissected free and mounted vertically in water‐jacketed (20°C) chambers containing oxygenated (100%) amphibian Ringer's solution (pH 7.2) containing 2.5 mM KCl (R+KCl 2.5 ). After equilibrium was achieved, muscles were exposed serially to single and paired (ISI = 20ms) supramaximal square wave pulses (1 ms) via field stimulation. Peak tension (PT) in response to paired stimulation (PT D , g/g) was greater when compared to single pulse stimulation (PT S , g/g): 202.2 ± 18.4 v. 104.8 ± 6.7, respectively. Following complete recovery, the stimulation protocol was repeated after substituting Ringer's containing 15 mM KCl (R+KCl 15 ) for R+KCl 2.5 . PT S increased from 101.0 ± 4.9 (R+KCl 2.5 ) to 152.7 ± 14.9 (R+KCl 15 ). PT D increased from 101.0 ± 4.9 (R+KCl 2.5 ) to 184.5 ± 18.61 (R+KCl 15 ). Corresponding values for the TTI (g·s) mimicked results for PT. Subcontracture K + ‐induced depolarization lessened the effect of subsequent paired stimulation: (+ 92.9% (R+KCl 2.5 ) v. + 82.7% (R+KCl 15 )). These results suggest that alterations in extracellular environment may mitigate the potentiating effects of paired stimulation and alter it's efficacy as a motor control strategy.