We used immunochemical quantification and indirect immunofluorescence to investigate the cell content, distribution, and organization of microtubules in adult rat slowtwitch soleus and fast-twitch vastus lateralis muscles. An immunoblotting assay demonstrated that the soleus muscle (primarily Type I fibers) was found to have a 1.7-fold higher relative content of a-tubulin compared with the superficial portion of the vastus lateralis muscle (primarily Type IIb fibers). Both physiological muscle types revealed a complex arrangement of microtubules which displayed oblique, longitudinal, and transverse orientations within the sarcoplasmic space. The predominance of any one particular orientation varied significantly from one musde tissue section to another. Nuclei were completely surrounded by a dense netlike structure of microtubules. Both m d e fiber types were found to possess a higher density of microtubules in the subsarcolemmal region. These microtubules followed the contour of the sarcolemma in slightly contracted fibers and showed a fine punctate appearance indicative of a restricted distribution. The immunofluorescence results indicate that microtubules are associated with the sarcolemma and therefore may form a part of the membrane cytoskeletal domain of the muscle fiber. We conclude that the microtubule network of the adult mammalian skeletal musde fibet constitutes a bone fide component of the srosarcomeric cytoskeletal lattice domain along with the intermediate filaments, and as such could therefore partici@e in the mechanical integration of the various organelles of the myofibers during the contraction-relaxation cycle. (J Histochem C y t d e m 41:KEY WORDS: Microtubules; Muscle fiber types; Cytoskeleton; Nucleus; Membrane skeleton; Immunofluorescence; Tubulin. 1013-1021, 1993) IntroductionThe basic contractile unit of the mature skeletal muscle fiber, the sarcomere, represents a highly organized and elaborate forceproducing structure which consists of contractile proteins and associated regulatory proteins as well as an increasingly complex array of cytoskeletal proteins. The cytoskeletal system of the sarcomere has been subdivided into two distinct domains: the endosarcomeric lattice composed of the elastic titin filaments, the inextensible nebulin system, and an exosarcomeric domain consisting of desmincontaining intermediate filaments (Wang, 1985) and possibly other protein species (Tokuyasu et al., 1984). Signifcant progress has been realized with regard to our understanding of the titin and nebulin filament systems. Evidence has been obtained suggesting that both elasticity and compliance of the skeletal muscle cell are directly related to the expression of specific titin isoforms (Wang et al., 1991), and a strong correlation between the size of the nebulin molecule Supported by grants from the Natural Science and Engineering Council of Canada and the Muscular Dystrophy Association of Canada.Correspondence to: Dr. Peter A. Rogers. Laval University Hospital, Research Center S-750, 2705 Blvd. Lauri...

In the heart, brief repeated episodes of ischemia prior to a sustained occlusion (ischemic preconditioning; PC) significantly delay the onset of necrosis and arrhythmogenesis. Ischemia has been reported to influence gap junction organization and connexin43 (Cx43) content, but whether PC affects these structures is not known. We investigated the effect of PC (2 cycles of 5-min ischemia plus 10-min reperfusion) followed by prolonged reperfusion without concomitant regional coronary occlusion on the myocardial Cx43 content and its spatial distribution in rabbit hearts. We also compared the effect of sustained ischemia with or without PC on Cx43 spatial distribution. In experiments with PC only, there was an initial decrease in Cx43 levels within the ischemic zone followed by a progressive increase after 48 h reperfusion. End-to-end immunolabeling of Cx43 was augmented in the ischemic region between 24 and 48 h reperfusion; labeling was not uniquely confined to myocyte abutments, but was also dispersed along the sarcolemma. Cx43 immunolabelling was more intense and diffuse in hearts subjected to PC before sustained coronary occlusion (compared to non-PC). These data indicate that gap junctions are significantly altered during brief episodes of ischemia. Reorganization of the gap junction complex could contribute to PC-mediated reductions in cardiac arrhythmias.

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