Abstract

Freeze fracture methods were used to examine the sarcoplasmic reticulum (SR) of dog cardiac muscle. The free SR consists of three regions: narrow tubules of the free SR, fenestrated membranes of the M rete, and flat cisternal SR. Cisternal SR is a new ultrastructural feature of cardiac SR. The cisternal regions appear scattered throughout the cell and frequently are found near mitochondria. There is little difference between the narrow tubules of free SR and the junctional SR which is connected to the T tubules. Sometimes the tubular junctional SR broadens and wraps around a T tubule. The SR is also connected to the sarcolemma at the intercalated disc and sometimes is found near gap junctions. Evidence of SR-mitochondria connections is presented.

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