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2006 ; 10
(4
): 960-90
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Caveolae in smooth muscles: nanocontacts
#MMPMID17125599
Popescu LM
; Gherghiceanu M
; Mandache E
; Cretoiu D
J Cell Mol Med
2006[Oct]; 10
(4
): 960-90
PMID17125599
show ga
Smooth muscle cell (SMC) caveolae have been investigated by quantitative and
qualitative analysis of transmission electron microscopy (TEM) images of rat
stomach, bladder and myometrium, guinea pig taenia coli, human ileum, and rat
aortic SMCs. Ultrathin (below 30 nm) serial sections were used for examination of
caveolar morphology and their connections with SMC organelles. Average caveolar
diameter was smaller in vascular SMCs (70 nm, n=50) than in visceral SMCs (77 nm,
n=100), but with the same morphology. Most of the caveolae, featured as
flask-shaped plasma membrane (PM) invaginations, opened to the extracellular
space through a 20 nm stoma (21 +/- 3 nm) having a 7 nm thick diaphragm. A small
percentage of caveolae (3%), gathered as grape-like clusters, did not open
directly to the extracellular space, but to irregular PM pockets having a 20-30
nm opening to the extracellular space. In visceral SMCs, caveolae were disposed
in 4-6 rows, parallel to myofilaments, whilst aortic SMCs caveolae were arranged
as clusters. This caveolar organization in rows or clusters minimizes the
occupied volume, providing more space for the contractile compartment. The
morphometric analysis of relative volumes (% of cell volume) showed that caveolae
were more conspicuous in visceral than in vascular SMCs (myometrium - 2.40%;
bladder - 3.66%, stomach - 2.61%, aorta - 1.43%). We also observed a higher
number of caveolae per length unit of cellular membrane in most visceral SMCs
compared to vascular SMCs (myometrium - 1.06/microm, bladder - 0.74/microm, aorta
- 0.57/microm, stomach - 0.48/microm). Caveolae increase the cellular perimeter
up to 15% and enlarge the surface area of the plasma membrane about 80% in SMCs.
Threedimensional reconstructions (15micro(3)) showed that most caveolae, in both
visceral and vascular SMCs, have nanocontacts with SR (87%), other with
mitochondria (10%) and 3% apparently have no contact with these organelles.
Usually, 15 nm wide junctional spaces exist between caveolae and SR, some of them
with nanostructural links between each other or with mitochondria: direct
contacts (space <2 nm or none) and molecular links, so called 'feet' (about 12 nm
electron dense structures between organellar membranes). Direct contacts possibly
allow molecular translocation between the two membranes. Electron-dense
'feet'-like structures suggest a molecular link between these organelles
responsible for intracellular Ca(2+) homeostasis (excitation-contraction coupling
or pharmaco-mechanical coupling). Close appositions (approximately 15 nm) have
also been observed between caveolae and perinuclear SR cisternae, suggesting that
caveolae might be directly implicated in excitation-transcription coupling.
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