Temperature‐effects on motor unit doublet potentiation in isolated skeletal muscle in vitro

Abstract

The additionof an extra stimulating pulse, or doublet, at the beginning of a low-frequency train of stimuli has been shown to substantially increase isometricforce. This study examined the effects of temperature on doublet potentiation. Sartorii muscles (60–70 mg) from small male R. pipiens were dissected free and mounted vertically in water-jacketed chambers containing oxygenated (100%) Ringer solution (pH 7.2). After recovery, muscles were exposed serially to three different incubation temperatures: 10°, 20°, and 30° C. Twitch characteristics were examined via field stimulation with supramaximal square wave pulses (1 ms). At ambient temperature (20°), peak tension in response to paired stimulation (PTD, g/g) was greater when compared to single (PTS, g/g) stimulation (308.4 ± 23.4 v. 188.3 ± 16.8, respectively, mean ± SE). Peak tension in response to both single and paired stimulation increased with decreasing temperature (PTS = 159.0 ± 20.7 (30°) v. 188.3 ± 16.8 (20°) v. 248.7 ± 16.3 (10°); PTD =302.2 ± 32.7(30°) v. 308.4 ± 23.4 (20°) v. 322.9 ± 21.3 (10°)). Furthermore, the tension-time integral (TTI, g·s) was also found to be increased with paired stimulation and decreased temperature. However, the doublet-to-singlet tension ratio (PTD/ PTS) was observed to decrease with decreasing temperature (PTD /PTS = 1.95 ± 0.09 (30°) v. 1.66 ± 0.08 (20°) v. 1.30 ± 0.03 (10°). The results presented here indicate that doublet potentiation is temperature-sensitive and suggest that the efficacy of paired stimulation in vivo, as a potential neural control strategy to augment force production, may be somewhat mediated by changes in muscle temperature.