Caveolar systems and sarcoplasmic reticulum in coronary smooth muscle cells of the mouse.

Vascular smooth muscle cells in mouse heart contain prominent membrane systems (“sarcotubules”). One of these systems consists of vesicular structures whose unit membranes are continuous with the sarcolemma and which occur either as single caveolae, more complex tubules, or branched chains of fused caveolae. Such caveolar systems are both analogous to, and homologous with, the T or T-axial tubular systems of striated muscle cells. A second system of membranes, the sarcoplasmic reticulum (SR), comprises tubules and saccules that often come into close association with the sarcolemma but apparently are not open to the extracellular space. In addition to forming “couplings” with the sarcolemma, the SR often comes into close contact with mitochondria and caveolae. The ultrastructural complexity of these membrane systems in coronary vascular smooth muscle equals or surpasses that of smooth muscle cells in the large blood vessels that have been extensively studied by other investigators.