ACTIVATION OF 2 TYPES OF FIBERS IN RAT EXTRAOCULAR-MUSCLES

The contractile responses of the inferior rectus, one of the extraocular muscles of the rat, to a depolarization induced by an elevation of the K concentration in the external medium ([K]O) were studied in vitro. The elevation of [K]O to 20 and 30 mM-K produced contractures that consisted of a sustained or tonic tension. When [K]O was increased to 50 mM or more a well-defined transient or phasic tension appeared before the tonic response. The increment of [K]O above 50 mM enhanced the phasic component and depressed the tonic tension. The maximal tonic tension, usually evoked by 50 mM-K, is about 50% of the tetanic tension, shows a gradual decline with time and lasts for hours. Control experiments performed in diaphragm showed that this muscle only responds with phasic tensions. The difference in the repriming of the phasic and tonic responses when tensions were induced with salines containing low or normal Cl- concentration [Cl] suggests that the muscle fibers responsible for the tonic tension are poorly permeable to Cl-. The amplitude of the tonic tension was reduced by Ca deprivation and by an elevation of [Ca] in the saline to 10 mM. In rat extraocular muscles, an increase in [K]O activates 2 types of muscle fibers: singly and multiply innervated. These appear to be functionally equivalent to the twitch and slow fibers of amphibian and avian muscle and would give rise to the phasic and tonic components of the contracture, respectively.