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

Abstract

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 ug/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 ,ug/ml) but not by phentolamine (1-5 gg/ml).3. In EDL, isoprenaline prolonged the time to peak tension and the halfrelaxation 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 gg/ml) and by reduction of the temperature from 330 to 180 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 69 1 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.

The effect of catecholamines on Na—K transport and membrane potential in rat soleus muscle

“…The results obtained with specific a-and fl-adrenoceptor antagonists also confirm those of Raper & Bowman (1968) and Tashiro (1973), and it seems reasonable to assume that the hyperpolarization is mediated via fiadrenoceptors. Furthermore, as already pointed out by Bray et al (1976), the combination of dbcAMP and theophylline mimics the effect of ADR on EM, and, like other fl-adrenoceptor effects, the hyperpolarization of the sarcolemma seems to be mediated by cAMP.…”

“…Table 9), the calculated potential (E') will only change from -63-7 to -67-6 mV. The fourth possibility considered here, that the hyperpolarization is mainly due to stimulation of an electrogenic Na-K pump has already received some support in the studies of Tashiro (1973) and Smith & Thesleff (1976) Clausen, 1973;Clausen & Hansen, 1974;Clausen & Kohn, 1977). Since ouabain was found to produce a rapid depolarization (Fig.…”

“…c is the concentration of the transported ion. We have furthermore assumed that chloride ions are passively distributed across the muscle plasma membrane in accordance with a Donnan equilibrium (Tashiro, 1973;Akaike, 1975). Alternative views have been presented by Renner & Westphal (1975).…”

“…In addition, it is evident from studies with avian slow muscle fibres that catecholamines induce hyperpolarization by an ouabain-suppressible mechanism (Somlyo & Somlyo, 1969). In soleus and extensor digitorum longus muscles of the guinea-pig, it was recently demonstrated that stimulation of f8-adrenoceptors with isoprenaline induces an increase in the resting membrane potential (Tashiro 1973).…”

Transient hyperpolarization of non‐contracting muscle fibres in anaesthetized rats.

“…In the opposite leg, the soleus was selected for study; this muscle had an advantage over tibialis anterior in that any hyperpolarizing action of circulating adrenaline would be expected to be larger (Tashiro, 1973). In addition the sodium pump stimulation appears to be greater in a slow-twitch muscle, such as soleus, than in a fast-twitch muscle (Everts et al 1988).…”