Effects of isoprenaline on contractions of directly stimulated fast and slow skeletal muscles of the guinea‐pig

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Abstract
1 The actions of isoprenaline on the contraction and the resting potential of the isolated extensor digitorum longus (EDL), a fast contracting muscle, and the soleus, a slow contracting muscle, of the guinea-pig were investigated. Twitch tension was elicited by direct supramaximal stimulation and recorded isometrically. 2 The twitch tension of EDL elicited by pulses of 0·5–10 ms duration was increased in the presence of isoprenaline (1 μg/ml). Isoprenaline increased the twitch tension of the soleus elicited by a pulse of more than 5 ms duration, but decreased it when elicited by a pulse of less than 1 millisecond. These effects were blocked by propranolol (1–3 μg/ml) but not by phentolamine (1–5 μg/ml). 3 In EDL, isoprenaline prolonged the time to peak tension and the half-relaxation time. The twitch of the soleus was shortened by isoprenaline due to an acceleration of relaxation. These findings were independent of stimulus duration. 4 The potentiating effects of isoprenaline on the twitch tension of EDL and the soleus were not observed in K+-free Krebs solution and were abolished by ouabain (1 μg/ml) and by reduction of the temperature from 33° to 18° C. The effects of isoprenaline on the relaxation process were not affected by these treatments. 5 In EDL, the resting potential increased from 77·3 mV to 78·5 mV after isoprenaline, whereas in the soleus it increased from 691 to 74·7 mV. These effects were blocked by propranolol, K+-deficiency, ouabain, and cooling to 18° C. Hyperpolarization by isoprenaline was increased by substitution of isethionate for the external chloride. 6 There was a good correlation between the potentiation of the mechanical response and the hyperpolarization of the membrane by isoprenaline. The hyperpolarization seems to be due to activation of the Na+-K+ pump.

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