Entrance of colloidal ThO2 tracer into the T tubules and longitudinal tubules of the guinea pig heart

The transverse-axial tubular system (TATS) of mouse myocardium: its morphology in the developing and adult animal

Invaginations of the sarcolemma that generate the transverse-axial tubular system (TATS) of the ventricular myocardial cells have begun to develop in the mouse by the time of birth. The formation of the TATS appears to be derived from the repetitive generation of caveolae, which forms "beaded tubules". Beaded tubules are retained in the adult, in which they frequently present a spiraled topography. Development of the TATS progresses so rapidly that complex systems are already present in the cardiac muscle cells of young mice; by 10-14 days of age, the ultrastructure is essentially identical to that of the adult. The mouse myocardial TATS is composed of anastomosed elements that are directed transversely and axially (longitudinally). Many tubules have an oblique orientation, however, and most elements of the TATS are highly pleiomorphic. In this respect the TATS of the mouse heart is relatively primitive in appearance in comparison with the more ordered TATS latticeworks typical of the ventricular cells of other mammals. Stereological analysis of the mouse TATS indicates that the volume fraction (VV) and surface density (SV) are considerably greater than previously reported (3.24% and 0.5028 micron-1, respectively). The most complex ramifications of the TATS are embodied in the subsarcolemmal caveolar system and the deeper tubulovesicular "labyrinths", both of which can be found in early postnatal and adult ventricular cells. In atrial cells, TATS development is initiated several days later than in the ventricular cells. The TATS of adult atrial myocardial cells is less prominent than the ventricular TATS and consists largely of axial elements; the incidence of the TATS, furthermore, is more pronounced in the left than in the right atrium.

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.

The presence of transverse and axial tubules in the ventricular myocardium of embryonic and neonatal guinea pigs

Developing transverse (T) tubules are found in embryonic guinea pig ventricular myocardium after approximately eight weeks of gestation. By the time of birth (nine weeks total gestation); longitudinally-oriented axial tubules connected to the T tubules also have formed, and the majority of cells closely resemble those of the adult. The form taken by the developing T and axial tubules suggests that they are generated in a manner similar to that for T tubules in chick and rat skeletal muscle, namely by repeated formation of caveolae.

Loss of the plateau of the cardiac action potential in hypertonic solutions

The effect of hypertonicity on the electrical properties of vertebrate myocardial cells was studied in ventricular muscle fibers of guinea pig, cat, frog, and chicken. The latter two species do not have a T-tubule system, whereas the former two do. In hypertonic solutions (2 x isotonic) produced by addition of sucrose or excess of NaCl, cell diameter decreased and there was a slight hyperpolarization and decrease in action potential overshoot. In guinea pig and cat, the hypertonic solution caused a decrease in input resistance and the plateau of the action potential to disappear in some of the cells; contractions of the entire ventricle also became depressed. These effects were reversed by returning the muscle fibers to isotonic solution. Addition of 5 mM SrCl(2) to the hypertonic solution also caused the plateau component and contraction to reappear. In frog and chick cells, loss of the plateau component and contraction never occurred in hypertonic solution, and input resistance increased. Urea and glycerol hyperosmolarity (2 x) caused no loss of the plateau component or contraction. If the frog and chicken ventricular, and guinea pig atrial myocardial cells (all of which lack T tubules) were to serve as an adequate control for possible effects of hypertonicity on the surface membrane and on contractile proteins, then the results suggest that swelling of the T tubules of mammalian myocardial cells leads to loss of the plateau component.

Sarcoplasmic reticulum and excitation-contraction coupling in mammalian smooth muscles

The sarcoplasmic reticulum (SR) was studied in the smooth muscles of rabbit main pulmonary artery, mesenteric vein, aorta, mesenteric artery, taenia coli, guinea pig mesenteric artery, and human uterus, and correlated with contractions of the smooth muscles in Ca-free media. SR volumes were determined in main pulmonary artery (5.1%), aorta (5%), portal-anterior mesenteric vein (2.2%), taenia coli (2%), and mesenteric artery (1.8%): because of tangentially sectioned membranes these estimates are subject to a correction factor of up to +50% of the values measured. Smooth muscles that contained a relatively large volume of SR maintained significant contractile responses to drugs in the virtual absence of extracellular calcium at room temperatures, while smooth muscles that had less SR did not. The unequal maximal contractions of main pulmonary artery elicited by different drugs were also observed in Ca-free, high potassium-depolarizing solution, indicating that they were secondary to some mechanism independent of changes in membrane potential or calcium influx. Longitudinal tubules of SR run between and are fenestrated about groups of surface vesicles separated from each other by intervening dense bodies. Extracellular markers (ferritin and lanthanum) entered the surface vesicles, but not the SR. The peripheral SR formed couplings with the surface membrane: the two membranes were separated by gaps of approximately 10 nm traversed by electron-opaque connections suggestive of a periodicity of approximately 20-25 nm. These couplings are considered to be the probable sites of electromechanical coupling in twitch smooth muscles. Close contacts between the SR and the surface vesicles may have a similar function, or represent sites of calcium extrusion. The presence of both thick and thin myofilaments and of rough SR in smooth muscles supports the dual, contractile and morphogenetic, function of smooth muscle.