Pflfigers Arch. 361, 1-7 (1975) Pfli.igers Archiv European Journal of Physiology 9 by Springer-Verlag 1975 Influence of Intermittent Long-Term Stimulation on Contractile, Histochemical and Metabolic Properties of Fibre Populations in Fast and Slow Rabbit Muscles* Dirk Pette, Beatriz U. Ramirez, Werner Mfiller, Reinhard Simon, Gerhard U. Exner, and Rosemarie Hildebrand Fachbereich Biologie der Universit/it, D-7750 Konstanz, Postfach 7733, Federal Republic of Germany Summary. Slow (m.soleus) and fast (m.tibialis anterior) muscles of the rabbit were subjected to indirect long- term intermittent stimulation (3 weeks, 8 hrs daily) with a frequency pattern of 10 imp/sec. Whereas no changes were observed in case of the slow muscle, stimulation induced profound changes in the fast tibialis anterior muscle. These consisted in a re- arrangement of the enzyme activity pattern of energy- supplying metabolism, e.g. decrease in glycogenolytic and glycolytic enzyme activities and severalfold in- crease in key enzymes of aerobic endoxidation of substrates in fi,oxidation and the citric acid cycle. Concomitant with the increase in aerobic oxidative capacity, there was an increased resistance to fatigue, Histochemical studies revealed a strong increase in mitochondria of all fibres. The bimodal distribution of fibre cross-sectional area in the normal tibialis anterior muscle was changed by stimulation into a more homogeneous population of fibres with a smaller cross-sectional area. Despite a 50~ increase in time to peak of isometric twitch contraction no changes were observed in the fibre population with regard to myofibrillar ATPase reaction in quantitative evalu- ation of whole cross-sections of the muscles. The percentage of fibres histochemically classified as slow amounted to 2.8 ~ and 3.1 ~ in control and stimulated tibialis anterior muscle. Nevertheless the data suggest * This study was supported by Deutsche Forsctiungsgemein: schaft, Sonderforschungsbereich 138 ,,Biologische Grenzfl/ichen und Spezifit/it" and by grants from Deutscher Sportbund. B. U. R. obtained a fellowship from Deutscher Akademischer Austausch- dienst. G.U.E. and R.S. were fellows of Deutsche Forschungs- gemeinschaft. Present address of B. U. Ramirez: Departamento de Neuro- biologia, Universidad Cat61ica de Chile, Casilla 114-D, Santiago, Chile. Present address of R. Simon : Medizinische Hochschule Han- nover, Medizinische Klinik im Krankenhaus Oststadt, D-3000 Han- nover, Podbielskistr. 380, Federal Republic of Germany. Present address of G. U. Exner: Kinderspital der Universitfit Ziirich, Steinwiesstr., CH-8032 Ztirich, Switzerland. a transformation of the fibre population under the influence of long-term intermittent stimulation. Key words: Nerve-muscle interaction - Contractile parameters - Energy supplying metabolism- Enzyme activite pattern - Transformation of fibre types - Histochemical fibre typing. INTRODUCTION The tibialis anterior of the rabbit is a fast contracting muscle, composed of a mixed population of "red", "white" and "intermediate" fibres (Pette et al., 1973). Intermittent long-term stimulation of its nerve with a frequency pattern resembling that of a slow moto- neuron has been shown to induce in this muscle some physiological and biochemical features of a slow muscle, e,g. increase of the isometric twitch contraction time, changes in the enzyane activity pattern which signify a decrease in the glycolytic capacity, changes in the lactate dehydrogenase isozyme pattern, and increased activity levels of enzymes involved in sub- strate activation and aerobi c Substrate end0xidation (Salmons and Vrbovfi, 1969; Pette et al., 1972, 1973). Also the capillary density was increased in the stimu- lated muscle (Cotter et al., 1972), and according:to qualitative and semiquantitative histochemicaI deter- mination of succinate dehydrogenase it was suggested i a change in fibre population with a predominance of "red" fibres (Pette et al., 1972; 1973). Red fib~;es have been shown to be capable of sustained performance (Kugelberg and EdstrSm, 1968; Edstrfm and Kugel, berg, .1968; Burke et al., 1971, 1973). It appeared interesting, therefore, to undertake a detailed study on the induced changes in fibre population. In this connection we also examined the :quesiion ~i~ether the induced change !6ads to an increased ~capacity of sustained perf0rmance. ~TO examine the p0sslbility that the changes indUCed by.itl~e stimulation m~ght