A comparison between the effects of a tetanus and the effects of sympathomimetic amines on fast- and slow-contracting mammalian muscles.

The effects of adrenaline and isoprenaline on the tension and time-course of the contractions of the tibialis anterior and soleus muscles of cats and rabbits have been compared with the effects of previous high-frequency stimulation. Like a tetanus, adrenaline possessed a facilitating action on neuromuscular transmission and an action exerted directly on the muscle fibres. Isoprenaline possessed only the second of these two actions. The effect of adrenaline and isoprenaline on the muscle fibres was blocked by dichloroisoprenaline and by 1-(2 naphthyl)-2-isopropylaminoethanol, whereas the effect of adrenaline on neuromuscular transmission was blocked by phentolamine and by phenoxybenzamine. In the soleus muscle, both the catechol amines and a previous tetanus caused similar decreases in maximal twitch tension and in the times to peak tension and to half-relaxation. The muscle action potentials were unaltered or slightly increased in amplitude. In the tibialis anterior muscle, a previous tetanus and the catechol amines caused an increase in twitch tension and an increase in the overall duration of the twitch. The muscle action potentials were either unchanged or were slightly decreased in amplitude. In this muscle the effect of a tetanus differed from that of the catechol amines in that the large post-tetanic change was associated with a marked increase in the rate of rise of twitch tension.