Evidence for extracellular localization of activator calcium in dog coronary artery smooth muscle as studied by the pyroantimonate method

Abstract

Correlated physiological and electron-microscopic studies were made on the source of calcium activating the contractile system (activator calcium) in dog coronary artery smooth muscle fibers. The magnitude of contracture tension induced by 100 mM K+ was dependent on external Ca2+ concentration and reduced or eliminated by factors known to reduce the Ca2+ spike or Ca2+ influx. Little or no mechanical response was elicited by treatments known to cause release of intracellularly stored calcium. These results indicated that the contractile system is mainly activated by the inward movement of extracellular calcium. In accordance with the physiological experiments, electronopaque pyroantimonate precipitate containing calcium was found in the lumina of caveolae, but not in any intracellular structures close to the plasma membrane, when the relaxed fibers were fixed in a 1% osmium tetroxide solution containing 2% potassium pyroantimonate. If the contracted fibers were fixed in the same solution, the pyroantimonate precipitate was diffusely distributed in the myoplasm in the form of numerous particles, while the precipitate in the caveolar lumina was scarcely seen. These findings are discussed in connection with the regulation of intracellular Ca2+ concentration in dog coronary artery smooth muscle.

Caveolae: where incoming and outgoing messengers meet.

“…For some time there has been speculation that caveolae might be a Ca2+ entry site, similar to the T-tubule of muscle cells (35). Histochemical methods have even indicated that high concentrations of Ca2+ are present at these sites, which suggests a storage role (36,37). Two recent reports now put these observations on a firmer footing: a form of the inositol 1,4,5-trisphosphate-regulated Ca2+ channel (38) and a Ca2+-pump ATPase have been localized to caveolae (39).…”

Endothelial Ca 2+ waves preferentially originate at specific loci in caveolin-rich cell edges

“…(ii) Substrates or signal transduction cascades required for the production of IP 3 may be concentrated at caveolae. In A431 cells, plasma membrane phosphatidylinositol 4,5-bisphosphate, a substrate for IP 3 , was found to be largely localized in caveolae, and this caveolar phosphatidylinositol 4,5-bisphosphate was specifically hydrolyzed by hormone-stimulated phospholipase C (45 (47). It is also known that caveolae, morphologically, are not a static component of the cell surface and that they can either be open in direct communication with the extracellular space or be closed as plasmalemmal vesicles to store trapped molecules such as Ca 2ϩ (20).…”

Potocytosis

“…They also appear to be involved in the export of molecules from the cell. Previously, we showed that the delivery of 5-methyltetrahy- (Kamen et al 1988), iron (Danielsen and van Deurs 1995), calcium (Kifor et al 1998;Lohn et al 2000;Sugi et al 1982;Suzuki and Sugi 1989), water (Page et al 1998), and retinol (Malaba et al 1995). B Receptors recycle and the ligand is delivered to the ER or other organelles: cholesterol (Fielding et al 1999;Graf et al 1999;Krieger 1999), SV40 virus Kartenbeck et al 1989;Pelkmans et al 2001;Stang et al 1997), advanced glycation end-product (AGE; Stitt et al 2000), CD59 (Deckert et al 1996), basic fibroblast growth factor (bFGF; Gleizes et al 1995), autocrine motility factor (AMF; Benlimame et al 1998), cholera toxin (CT; Henley et al 1998;Lencer et al 1999;Oh et al 1998;Orlandi and Fishman 1998;Parton 1994), and growth hormone (GH; Lobie et al 1999).…”