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

Summary1 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.