Inpocketings of the cell membrane (caveolae) in the rat myocardium

Three-dimensional electron microscopy of mitochondria and endoplasmic reticulum in the red muscle fiber of the rat diaphragm

The three-dimensional arrangement of mitochondria and endoplasmic reticulum in the red muscle fiber was studied both in thick sections of the rat diaphragm fixed in glutaraldehyde and impregnated with uranyl acetate followed by lead and copper citrate, and in thin sections of glutaraldehyde fixed tissue treated with ferrocyanide-reduced osmium. The mitochondria were located either at the periphery of the fiber, where they were spherical, or between the myofibrils, where they formed longitudinal columns of rectangular, slightly flattened elements. From both types of mitochondria, thin, elongated branches arose at right angles that formed transversely oriented mitochondrial pairs at the I band level. At the periphery of the fiber, the endoplasmic reticulum took the appearance of a subsarcolemmal network of tubular cisternae oriented parallel to the cell surface. In the juxtanuclear region, it was made up of spherical masses composed of tightly knitted tubules that were interconnected by more loosely anastomosed tubules. In between the myofibrils, it was composed of longitudinally oriented repetitive units whose structure varied according to their position in from of the A or I bands of the myofibrils. In front of the A band, the endoplasmic reticulum appeared as a single sheet of anastomotic tubules compressed between the adjacent myofibrils, whereas at the I band level, its tubular elements passed in front and behind the transverse expansions of the mitochondria to form an intricate ultilayered network in from of the Z line.

Quantitative studies on plasmalemmal folds and caveolae of rabbit ventricular myocardial cells

Plasmalemmal folds and caveolae were investigated by qualitative and quantitative analysis of electron micrographs obtained by freeze fracture and transmission electron microscopy (TEM) of rabbit right ventricular papillary muscles whose mean sarcomere lengths ranged from 1.64 to 2.28 micron. In passively extended muscles, folds were observed at sarcomere lengths of 2.3 micron and could be shown by extrapolation to become completely extended at a maximum sarcomere length of 2.8 micron. It was concluded that the plasmalemma does not contribute to resting tension in the physiological range of sarcomere lengths. Caveolae are present in both the external plasmalemmal envelope and T-tubular plasmalemma. They show no preferential distribution with respect to underlying myofibrillar striations or membrane folds and are nearly devoid of membrane particles in freeze-fractured material. The surface density of caveolar necks (4.0/micron2 apparent plasmalemmal area) is only 16-20% of that reported for frog skeletal muscle. Caveolae augment plasmalemmal area by 21-32%, assuming two or three caveolae per neck, respectively. Caveolar membrane does not serve as a reservoir of membrane to be recruited into external plasmalemma, at least over the physiological range of sarcomere lengths. In heart muscle they do not account for the T-tubular access resistance, and their function in this tissue remains unknown.

The plasma membrane of leading pacemaker cells in the rabbit sinus node. A qualitative and quantitative ultrastructural analysis

We studied the fine structure of the plasma membrane of electrophysiologically identified leading pacemaker cells from the rabbit sinus node, using both ultrathin sections of fixed tissue and replicas of freeze-cleaved material. We found that differences exist between sinus node and working myocardial membranes, but these are only quantitative. The caveolae or sarcolemmal invaginations are present in very large numbers; they increase the surface area of the plasma membrane by about 100%. The small macular nexuses that are present represent 0.2% of the membrane surface area. Nexuses are therefore about 10 times less numerous in leading sinus node cells than in working myocardium cells. A single equivalent electrical representation of the sinus node shows, nevertheless, that an appreciable electrical coupling may be expected.

Effect of stretch and contraction on caveolae of smooth muscle cells

The number of caveolae present at the surface of smooth muscle cells of guinea-pig taenia coli and visualized by freeze-fracture is about 35 per micron2. (By comparison, endothelial cells of intramuscular capillaries have about 73 caveolae per micron2). The packing density of smooth muscle caveolae is not significantly different in muscle strips isotonically contracted with carbachol or stretched and relaxed in a calcium-free solution, under a range of loads varying from 1 to 15 g. Also the diameter of the fractured necks of the caveolae appears unchanged in all the experimental conditions tested. The plasma membrane of smooth muscle cells often shows a ring of intramembranous particles rimming the opening of a caveola; on the other hand, particles are rare in the membrane of the caveolae themselves. The close relation between caveolae and sarcoplasmic reticulum is readily visualized in freeze-fracture preparations. Characteristic changes of the cell surface shape accompany the contraction and relaxation of the muscle. On rare occasions small aggregates of intramembranous particles are found and it is possible that they represent punctate gap junctions. However, the characteristic clusters of particles found in the circular musculature of the caecum and ileum are not seen in taenia coli.