The contractile responses of the inferior rectus, one of the extraocular muscles of the rat, to a depolarization induced by an elevation of the potassium concentration in the external medium ([K]O) have been studied 'in vitro'. 2. 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. 3. The difference in the repriming of the phasic and tonic responses when tensions were induced with salines containing low or normal [Cl] suggests that the muscle fibres responsible for the tonic tension are poorly permeable to Cl-. 4. The amplitude of the tonic tension was reduced by Ca deprivation and by an elevation of [Ca] in the saline to 10 mM. 5. It is concluded that in rat extraocular muscles, an increase in [K]O activates two types of muscle fibres: singly and multiply innervated. These appear to be functionally equivalent to the twitch and slow fibres of amphibian and avian muscle and would give rise to the phasic and tonic components of the contracture, respectively.
摘要
已在体外研究了大鼠眼外肌之一的下直肌对细胞外液中钾离子浓度升高（[K]O）所诱导的去极化的收缩反应。2. 将[K]O升高至20和30 mM - K会产生挛缩，其表现为持续或强直性张力。当[K]O增加至50 mM或更高时，在强直性反应之前会出现明确的短暂或相位性张力。[K]O高于50 mM的增加增强了相位性成分并降低了强直性张力。通常由50 mM - K诱发的最大强直性张力约为强直收缩张力的50%，随时间逐渐下降并持续数小时。在膈肌上进行的对照实验表明，该肌肉仅产生相位性张力反应。3. 当用含低或正常[Cl]的盐水诱导张力时，相位性和强直性反应的再激发存在差异，这表明负责强直性张力的肌纤维对Cl-的通透性较差。4. 钙缺乏以及盐水中[Ca]升高至10 mM会降低强直性张力的幅度。5. 得出结论，在大鼠眼外肌中，[K]O的增加激活了两种类型的肌纤维：单突触和多突触支配的。它们在功能上似乎分别等同于两栖动物和鸟类肌肉的快肌纤维和慢肌纤维，并分别产生挛缩的相位性和强直性成分。
