T-tubules, couplings and myofibrillar arrangements in rat atrial myocardium

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Abstract
Myocardial cells of rat atria have been studied using standard fixation and tannic acid mordanting. Cells were arranged in bundles or fascicles with little connective tissue. Fiber size varied, with complex cell junctions and in many fibers, particularly those of small diameter, no T-tubules or internal couplings were seen. Most cells, however, contained T-tubules, mainly coupled as triads at Z-lines but showing considerable variation in morphology. Direct continuity of T-tubules with the sarcolemma was demonstrated. Apparently unrelated to T-tubules and internal couplings, all cells showed the presence of peripheral couplings between sarcoplasmic reticulum and sarcolemma and, often, the sub-sarcolemmal element of reticulum was observed to pass internally in the sarcoplasm from a peripheral coupling. Also present were subsarcolemmal caveolae. The occasional presence within a single cell of myofibrils orientated in different planes was noted also. The findings are discussed in relation to the role of T-tubules and couplings in excitation-contraction.

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