Analysis of the ?-receptor mediated effect on fast-contracting skeletal muscle in vitro

Abstract

Subtetanic contractions of the isolated extensor digitorum longus (EDL) of the guinea-pig, a fast-contracting muscle, were evoked by transmural field stimulation. Isoprenaline, adrenaline, terbutaline, and noradrenaline each caused a dose-dependent increase in the force of contraction, their potencies decreasing in that order. Tyramine was without effect in this respect. Curare depressed the contractions of EDL by about 20% but did not appreciably change the response to the β-adrenoceptor agonists. The effects of isoprenaline and noradrenaline were blocked by propranolol (unselective) and H 35/25 (1-(p-tolyl)-2-isopropylamino-1-propanol, β₂-selective) but not by practolol (β₁-selective). Moreover, the increase in the force of subtetanic contractions of EDL produced by noradrenaline was unaffected by phentolamine. It is concluded that the adrenoceptor mediating the increase in the force of contraction of the isolated EDL is of the β₂-type and that the site of action is direct on the muscle. Its similarity to the receptor mediating the inverse effect on the slow-contracting soleus-muscle is pointed out.