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From the Department of Pharmacology and Therapeutics, Faculty of Medicine, University of British Columbia, Canada.
Correspondence to Dr Casey van Breemen, The University of British Columbia, Department of Pharmacology and Therapeutics, Faculty of Medicine, 2176 Health Sciences Mall, Vancouver, BC V6T 1Z3 Canada.
Abstract In a freshly isolated endothelial cell preparation from rabbit aorta, the regulation of the acetylcholine (ACh)sensitive intracellular Ca2+ store and the effects of the Ca2+-induced Ca2+ release agonists ryanodine and caffeine were studied using fura 2 imaging fluorescence microscopy. ACh (10 µmol/L) caused a transient release of Ca2+ from an intracellular store, presumably via an inositol tris-phosphatesensitive mechanism. This ACh response could be repeated in the presence of extracellular Ca2+ but was obtained only once in Ca2+-free bathing solution, which shows that a depleted intracellular Ca2+ store can be rapidly refilled from the extracellular space. Refilling can be prevented by the endoplasmic reticulum Ca2+-ATPase inhibitor cyclopiazonic acid (10 µmol/L), implying that Ca2+ enters the cytoplasm before accumulation in the endoplasmic reticulum. Ionomycin (10 µmol/L) caused a large Ca2+ release even after the ACh-releasable store had been emptied, indicating the existence of other ACh-insensitive stores, perhaps including the mitochondria. In one third of the cells studied, ACh induced oscillations in [Ca2+]i that were dependent on extracellular Ca2+. Also investigated were the effects of caffeine and ryanodine. In this cell preparation neither caffeine nor ryanodine induced a Ca2+ transient but instead slowly increased [Ca2+]i. It was observed that both caffeine and ryanodine were able to slowly deplete the ACh-sensitive store. These results indicate the presence of functional ryanodine receptors in native endothelial cells and demonstrate overlap between the caffeine and agonist-sensitive Ca2+ stores. We also found that caffeine was able to directly inhibit the process of ACh-induced Ca2+ release. It is hypothesized that endothelial endoplasmic reticulum contains both inositol tris-phosphate receptors and ryanodine receptors but that the former class are more densely distributed.
Key Words: ryanodine caffeine Ca2+-induced Ca2+ release acetylcholine
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