Andrade Francisco H, Hatala Denise A, McMullen Colleen A
Department of Neurology, Case Western Reserve University, 11100 Euclid Avenue, Cleveland, OH 44106, USA.
Pflugers Arch. 2004 Aug;448(5):547-51. doi: 10.1007/s00424-004-1284-3. Epub 2004 Apr 27.
Carbonic anhydrase (CA) accelerates contractile function, particularly in fast-twitch skeletal muscles. Since the extraocular muscles are considered to be amongst the fastest skeletal muscles in mammals, this study tested two hypotheses: (1) CA is expressed at higher levels in rat extraocular muscles than in extensor digitorum longus (EDL, a fast limb muscle), and (2) inhibition of CA activity increases twitch duration and force in the extraocular muscles to a greater extent than in EDL. By real-time quantitative PCR we determined that the expression of CA3 isoform, typically high in skeletal muscles, is significantly depressed in extraocular muscles. Message levels for the CA2 and CA4 isoforms were higher in the extraocular muscles, while CA5 expression was equivalent in both muscles. Strong CA activity was demonstrated by histochemistry in frozen EDL muscle sections, in particular along the sarcolemma and in capillaries. By contrast, extraocular muscle had very low sarcolemmal or cytosolic CA activity. CA inhibition with 6-ethoxyzolamide (ETZ) reversibly increased twitch duration and force in EDL muscle bundles. In the extraocular muscles, ETZ did not alter twitch kinetics. Based on these results, we reject our initial hypotheses and conclude that CA does not influence the fast contractile kinetics characteristic of the extraocular muscles.
碳酸酐酶（CA）可加速收缩功能，尤其是在快肌纤维的骨骼肌中。由于眼外肌被认为是哺乳动物中收缩速度最快的骨骼肌之一，本研究检验了两个假设：（1）大鼠眼外肌中CA的表达水平高于趾长伸肌（EDL，一种快速的肢体肌肉）；（2）抑制CA活性对眼外肌收缩持续时间和力量的增加幅度大于对EDL的影响。通过实时定量PCR，我们确定在骨骼肌中通常含量较高的CA3亚型在眼外肌中的表达显著降低。CA2和CA4亚型在眼外肌中的信使核糖核酸水平较高，而CA5在两种肌肉中的表达相当。通过组织化学方法在冷冻的EDL肌肉切片中，特别是沿着肌膜和毛细血管处，证实了较强的CA活性。相比之下，眼外肌的肌膜或胞质CA活性非常低。用6-乙氧基唑胺（ETZ）抑制CA可使EDL肌束的收缩持续时间和力量可逆性增加。在眼外肌中，ETZ并未改变收缩动力学。基于这些结果，我们否定了最初的假设，并得出结论：CA并不影响眼外肌快速收缩动力学的特征。
