No abstract

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.

The passive-mechanical and dynamic properties of the rabbit inferior oblique muscle IO were studied in vitro at 35 degrees C. The influence of length on the resting tension and isometric contractions were determined. Maximum twitch tension and fusion tension were developed at optimum length (Lo) an extension of the muscle to about 1.15 times LR, the resting length of the IO in situ. A linear relation was found between length and tension in the activated muscle. An increase in stimulation frequency induced a parallel shift in the curves to higher tension but the slope of the curves remained unchanged. On an average the IO had in response to direct massive stimulation a twitch contraction time of 6.4 ms and a half-relaxation time of 7.0 ms. At stimulation with 300 Hz or above the tetanus fused. Stimulus frequencies above fusion frequency increased the rate of tension rise but not the maximum tetanic tension. The maximum tetanic tension was about 6.4 N/cm2, and the twitch:tetanus ratio was 0.1. To prolonged tetanic stimulations the IO exhibited a high fatigue resistance. Cooling the muscle to 25 degrees C was followed by an increase in the time parameters of single twitches and tetanic contractions, a decrease of the tension developed in a fused tetanus and a small potentiation of the twitch. Following a repetitive stimulation a small post-tetanic potentiation of the twitch was observed.

Six oculorotatory muscles and the levator palpebrae muscle of the rat were analysed by SDS-PAGE for their myosin heavy chain (MHC) isoform patterns. Oculorotatory muscles display a marked predominance of fast MHC isoforms. They contain, in addition to the slow (MHCI) and fast (MHCIIb, MHCIId, MHCIIa) skeletal MHCs, the neonatal MHC, and the extraocular MHC,,. The levator palpebrae, generally assumed to be a member of the extraocular muscle group because of its innervation by the ocohnnotor nerve, does not contain MHC, and MHC,,. It resembles a fast-twitch skeletal muscle with a predominance of MHCIId.

Constitutive expression of nitric oxide synthase (NOS) II was found in rat hindlimb muscles by immunohistochemistry and western blotting during development from embryonic day 21 to the adult stage of 75 days. The immunohistochemical NOS II expression pattern was related to the physiological metabolic fibre types SO (slow-oxidative), FOG I, II (fast-oxidative glycolytic; I more glycolytic, II more oxidative) and FG (fast-glycolytic) and to the myosin-based fibre types I and IIA, IIB (IIX not separated) identified in serial sections by enzyme histochemistry and immunohistochemistry. In adult muscles only the small population of FOG II fibres, which is a part of both IIA and IIB fibre population, showed NOS II immunoreactivity. This is the reason that only weak NOS II expression in adult hindlimb muscles has been detected by western blotting. Hindlimb muscles of embryonic, neonatal and young rats of 8 days expressed more NOS II as compared with adult rat hindlimb muscles. This can be explained by the findings that before the age of 21 days fast fibres were metabolically undifferentiated, all of them were NOS II positive and contribute to the NOS II expression of the muscle. In muscles of diabetic rats the NOS II expression was elevated indicating an inhibition of glucose uptake into the muscle fibres of diabetic muscles. Our findings suggest that the NOS II may be designated both as constitutive and inducible.