Connection of microtubules, caveolae, mitochondria and sarcoplasmic reticulum in the taenia coli of guinea-pigs.

Microtubules were identified in the subsarcolemmal area of smooth muscle cell in the taenia coli of guinea-pigs. They may traverse myofibrils but more frequently they run parallel with the long axis of the cell. The microtubules were often in contact with sarcolemmal caveolae. A caveola appeared to contact with an end of a microtubule or with the outer membrane of a mitochondrion. Occasionally a long mitochondrion was located between two evaginations of sarcolemma. The direct contact of caveolae with a mitochondrion was typical in such long mitochondria. Granular and agranular sarcoplasmic reticulum (SR) were also in the vicinity of mitochondria. The SR was in contact with mitochondria and also with vesicles. Subsarcolemmal mitochondria were often in continuity with agranular SR or invaginations of sarcolemma. Low evaginations of sarcolemma appeared to be a cell contact device but nexas-like interdigitations, protrusions from the dark cell into the light cell, were also evident. These morphological findings may suggest a potential network of transport or conductance in this type of smooth muscle.