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Maximum velocity of shortening, Vo, was measured by the method of Edman [J Physiol (Lond) 291:143-159, 1979] on extensor digitorum longus muscles of a mouse in vitro at 20 degreesC. Blockers of nitric oxide synthase, 10 mM nitro-l-arginine or 1 mM 7-nitroindazole, reduced Vo by 18% and 22%, respectively. On removal of the inhibitor, Vo returned to the control value. It was found that 10 mM nitro-d-arginine, an enantiomer of nitro-l-arginine inactive against nitric oxide synthase, did not affect Vo. A donor of nitric oxide, 0.1 mM nitroprusside, increased Vo by 15%. It removed the inhibition caused by nitro-l-arginine. Another donor of nitric oxide, 1 microM (+/-)-S-nitroso-N-acetylpenicillamine (SNAP), increased Vo by 8%. An inhibitor of cGMP synthase, 0.01 mM Ly-83583, decreased Vo by 18%. An analogue of cGMP, 0.1 mM 8-bromo-cGMP, increased Vo by 17%. A general inhibitor of phosphodiesterases, 0.02 mM 3-isobutyl-1-methylxanthine (IBMX), increased Vo by 17%. An inhibitor specific of cGMP phosphodiesterase, 0.01 mM dipyridamole, increased Vo by 8%. The maximal isometric force (F0) was not modified by the drugs, except by 7-nitroindazole and Ly-83583, which depressed F0 by 12%. The cGMP level in tetanized muscles decreased by 12-27% in the presence of blockers of nitric oxide synthase. We conclude that the level of intracellular nitric oxide modulates Vo through the cGMP pathway.

The contractile properties of the rabbit inferior oblique muscle (IO) were studied in vitro with direct stimulation at temperatures between 20 and 35 degrees C. Isovelocity releases were used to determine the force/velocity relation. Cooling the muscle from 35 degrees C to 20 degrees C increased contraction and half-relaxation times of single twitches with a temperature coefficient (Q10) of 0.4, but did not affect significantly the twitch tension. The tetanic tension increases with increasing temperature (Q10 = 1.32). Cooling decreased the maximum shortening velocity of the IO with a Q10 of 1.6 and the maximum mechanical power with a Q10 of 2.3. At 35 degrees C, the maximum speed of shortening of the muscle (19 +/- 2 muscle lengths/s, mean +/- SEM) corresponded to a maximum shortening velocity of the sarcomeres of 57 +/- 6 microns/s. This value is similar to data obtained for extraocular muscles (EOM) of smaller rodents (mice and rats). In comparison with mammalian limb muscles the isometric and force-velocity properties of mammalian EOM appear to be virtually independent of the size of the animal. Thus, IO is a fast-twitch muscle endowed with a maximum velocity of shortening higher than that of fast-twitch skeletal muscle, but using a tetanic mechanical power lower than that produced by slow-twitch muscle: the combination of these properties makes it ideally suited to move an ocular globe of low mass at high velocity.

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Native myosin isozymes of rat muscles have been isolated by electrophoreses in non-dissociating conditions. Their mobilities were measured, using taenia coli myosin as an internal standard and their relative concentrations were determined by computer planimetry of the electrophoretograms. Three isozymes were observed in extensor digitorum longus (EDL), two in soleus (SOL), four in neonatal muscles and four in muscles three days before birth. Regenerates of minced EDL or SOL muscles in adult animals had no native myosin the third day after surgery; they were similar to neonatal muscles 15 days after surgery and to adult muscles 60 days after surgery.Muscle myosin is an hexameric molecule in its native state. Various isozymes, differing by their heavy-chain primary structures or by the type of their alkaline light chains, can be separated in their native state by electrophoreses in nondissociating conditions [I -41. Slow muscle myosin can thus be separated from fast muscle myosin [I, 21. Neonatal [5, 61 and embryonic [6] isomyosins have also been separated by this technique, whilst their chemical individuality has been supported by immunochemical methods or peptide mapping after enzymatic cleavage of the myosin heavy chains. Using taenia coli myosin as an internal standard, we are able to measure accurately RF values for the various isoforms of myosin in adult, neonatal and embryonic muscles and, using an interactive computer-controlled scanning device, to measure the relative proportions of the various isoforms.Adult muscles regenerating from a local cold injury express transiently fetal myosin heavy chains [7]. We have therefore analysed the isomyosins of regenerating muscles, using a model described by Carlson [S] in which the muscle is minced and orthotopically autografted. After an initial phase of degeneration and fragmentation of the graft, regeneration proceeds, recapitulating ontogeny. 15 days after surgery, the regenerate is contractile [9] and partially innervated [lo]. 60 days later, the fibres are hypertrophied [S], their tension may be back to normal [Ill, their force-velocity is similar to that ofthe original muscle [I21 and the functional recovery is satisfactory [ll]. We show in this work that the isozymic transition characteristics of muscle growth from embryo to adult stages [6] also occur in adult rat during muscle regeneration. MATERIALS AND METHODSStriated muscles excised from rats (Wistar, outbred local stock, obtained from the Katholieke Universiteit te Leuven) were Abbreviations. SOL, adult soleus muscle; EDL, adult extensor digitorus longus muscle; FM,, FM, and FM,, myosins of adult fast muscle 1,2 or 3; SM;, SM, and SM,, myosins of adult slow muscle l', 1 or 2; fM,, fM,, fM,, fM, and fM,, myosins of neonatal muscle 1,2, 3,4 or 5 ; eM,, eM,, eM, and eM,, myosins of embryonic muscle, 1,2, 3 or 4; R,, ratio of the electrophoretic mobility of myosin to the mobility of an internal standard.Enzyme. Myosin ATPase (EC 3.6.1.3).used. Embryonic muscles (symbolized 'eM') were obtained by careful...