The inhibitory actions of eserine and ouabain on the K, Rb and Cs uptake in slow and fast twitch muscles of the rat.

Comparative, in vitro studies were carried out on the 42K, 86Rb, and 131Cs uptake in fast twitch extensor digitorum longus (EDL), slow twitch soleus (SOL) muscles of the rat, and in fast muscle (sartorius) of the frog. The inhibitory action of ouabain (10(-4) M) and eserine (10(-3) M) on the influxes of alkali cations was investigated. The rate of potassium influx in isolated EDL muscles was higher than that of SOL, while no difference could be found in 86Rb or 131Cs influx in the two types of mammalian muscles under in vitro condition. Ouabain inhibited to about the same extent the influx of K (25%) of both types of mammalian muscles and also in fast amphibian muscle. On the other hand, the eserine sensitive component of 42K influx in fast twitch mammalian muscle (EDL) was about one-fourth of the total influx and even less in slow twitch mammalian muscle (SOL), while in frog muscle it amounted to about two-third of the total. The "residual" potassium influx, which represents the influx remaining after simultaneous treatment of the muscles with cardiac glycoside and eserine was about half of the total in EDL and SOL, but it was only a fraction of it in the frog sartorius muscle. The results may be explained on the basis of the morphological differences of the transverse tubular and sarcoplasmic reticulum systems of fast and slow mammalian muscles.