Caveolar nanospaces in smooth muscle cells

Caveolae, specialized membrane nanodomains, have a key role in signaling processes, including calcium handling in smooth muscle cells (SMC). We explored the three-dimensional (3D) architecture of peripheral cytoplasmic space at the nanoscale level and the close spatial relationships between caveolae, sarcoplasmic reticulum (SR), and mitochondria, as ultrastructural basis for excitation-contraction coupling system and, eventually, for excitation-transcription coupling. About 150 electron micrographs of SMC showed that superficial SR and peripheral mitochondria are rigorously located along the caveolar domains of plasma membrane, alternating with plasmalemmal dense plaques. Electron micrographs made on serial ultrathin sections were digitized, then computer-assisted organellar profiles were traced on images, and automatic 3D reconstruction was obtained using the Reconstruct software. The reconstruction was made for 1 microm(3) in rat stomach (muscularis mucosa) and 10 microm(3) in rat urinary bladder (detrusor). Caveolae, peripheral SR, and mitochondria close appositions create coherent cytoplasmic nanoscale subdomains of about 15 nm distance. Apparently, 80% of caveolae establish close contacts with SR and about 10% establish close contacts with mitochondria in both types of SMC. Thus, our results showed that caveolae and peripheral SR build Ca(2+) release units at which mitochondria often could play a part. The couplings caveolae-SR occupy 4.19% of cellular volume in stomach and 3.10% in rat urinary bladder, while couplings caveolae-mitochondria occupy 3.66% and 3.17% respectively. We conclude that there are strategic caveolae-SR or caveolae-mitochondria contacts at the nanoscale level in the cortical cytoplasm of SMC, presumably responsible for vectorial control of free Ca(2+) cytoplasmic concentrations in definite nanospaces. This may account for selective activation of specific Ca(2+) signaling pathways.