The contractile responses of single slow muscle fibres were investigated under isometric conditions. The fibres were isolated from normal and denervated iliofibularis muscle of Rana temporaria. Direct stimulation was achieved by external application of electrical pulses of depolarizing solutions (40 mM K and acetylcholine 10(-6) g/ml). 2. Upon electrical stimulation normal slow fibres developed contractions whose amplitude increased steadily with the strength of the pulses. These contractions did not exceed 0.22 kg/cm2, and their rates of rise and fall were approximately 1/6 of those observed in twitch fibres. During pulses of several 100 ms duration the tension continued to rise slowly until the end of the pulses. Application of 40 mM K or acetylcholine resulted in contractures which reached maximum values of 1.71 and 1.87 kg/cm2, respectively, after less than 1 min; little relaxation occurred during the following minutes. 3. The responses of slow fibres denervated for 9--76 days differed in several respects from those of normal slow fibres. a) The contractions elicited by electrical stimulation became faster but their amplitude decreased; the relationship between tension and pulse strength remained, however, essentially unchanged. b) Small and short contractions could be observed during and at the end of stimulating current pulses. Their amplitude was independent of the amplitude and duration of the stimuli; they were blocked by tetrodotoxin and must be attributed to action potentials. c) The ability to develop and maintain tension was reduced. Maximum values of K- and acetylcholine contractures were generally only 50% of those observed in normal slow fibres. Maintenance of tension was markedly reduced in some fibres, less so in others. 4. It is concluded that incorporation of Na channels into its membrane does not transform the slow fibre into a type of fibre which resembles a twitch fibre.
