Ultrastructural aspects of activation and contraction of vascular smooth muscle

Ultrastructural studies of potential intracellular calcium storage sites and of the organization of contractile proteins in vascular smooth muscle are reviewed. The sarcoplasmic reticulum (SR) is a system of closed tubules present in every smooth muscle examined. The volume of the SR varies in different smooth muscles (from approximately 2.0 to 7.5% of cytoplasmic volume) and correlates with the ability of a given smooth muscle to contract in calcium-free media. The SR accumulates the divalent cation strontium and forms couplings with the surface membrane. Mitochondria are other potential sites of calcium accumulation in vascular smooth muscle, as indicated by the respiration supported accumulation of calcium (with a Km of approximately 17 muM) by isolated mitochondria and by the energy dependent accumulation of barium by mitochondria in situ. The presence of barium in mitochondrial granules in tissues contracted with barium and the presence of strontium and calcium in appropriately incubated preparations have been verified with electron probe microanalysis. Calcium has also been demonstrated in mitochondrial granules in frozen dried cardiac muscle sections and in cultured vascular smooth muscle cells. This technique appears suitable for eventual quantitation of mitochondrial calcium content in vascular and other smooth muscles. Thin (actin, 50-80 A), thick (myosin, approximately 155 A), and intermediate (approximately 100 A) filaments are present in suitably fixed vascular smooth muscle. In rabbit portal anterior mesenteric vein intermediate high voltage stereo electron microscopy shows the myosin filaments to be tapered and approximately 2.2 mum long: significantly longer than the myosin filaments in vertebrate striated muscle. Actin filaments insert on dense bodies. The ultrastructural findings are compatible with physiological evidence suggesting the contributions of intracellular organelles to the regulation of cytoplasmic free calcium levels and the operation of a sliding filament mechanism of contraction in vertebrate smooth muscle.