Three-dimensional electron microscopic reconstruction of intracellular organellar arrangements in vascular smooth muscle--further evidence of nanospaces and contacts.

The sarcoplasmic reticulum (SR) of smooth muscle is crucial for appropriate regulation of Ca(2+) signalling. In visceral and vascular smooth muscles the SR is known to periodically lie in close register, within a few nanometres, to the plasma membrane. Recent work has focussed on reconstructions of the ultrastructural arrangement of this so-called peripheral SR that may be important for the genesis of phenomena such as Ca(2+) sparks. Here, we turn our attention to vascular smooth muscle and explore the 3-dimensional (3D) ultrastructural positioning of SR found deeper in the cell that is involved in the propagation of Ca(2+) waves. We use digital reconstruction and volume rendering of serial electron microscopic sections from isolated resistance arteries, pressurized in vitro to mimic cellular geometric conformations anticipated in vivo, to map SR positioning. We confirm that these central portions of SR are in close register with mitochondria and the nucleus with all three organelles tightly enveloped by a myofilament/cytoskeletal lattice. Nanospacings between the SR and individual mitochondria are visible and in three dimensions as the SR contorts to accommodate these organelles. Direct connection of the SR and nuclear membranes is confirmed. Such 3D positioning of centrally located SR further informs us of its likely role in the manifestation of spatiotemporal Ca(2+) dynamics: signal encoding may be facilitated by spatially directed release of Ca(2+) to influence several processes crucial to vascular smooth muscle and resistance artery function including myofilament activation by Ca(2+) waves, mitochondrial respiration and gene transcription.