Abstract
The purpose of this study was to test whether the elevated intracellular Ca++ level ([Ca++]i) resulting from store-operated Ca++ entry was associated with vascular smooth muscle contraction. Cyclopiazonic acid (CPA), a selective inhibitor of sarcoplasmic reticulum Ca++-ATPase, concentration-dependently (1–10 μM) elevated [Ca++]i in rat aorta, as indicated by an increase in the fura-2 340/380 ratio. Simultaneous measurement of contraction demonstrated that 1 and 10 μM CPA induced insignificant and variable amounts of contraction, respectively. Verapamil (10 μM) had relatively little effect on the 1 and 10 μM CPA-elevated [Ca++]i. In contrast, Ni++ (0.1 mM), in the presence of verapamil, abolished the 1 μM CPA-elevated [Ca++]i. Ni++ (0.1 mM) also partially decreased the 10 μM CPA-elevated [Ca++]i and, furthermore, abolished the associated contraction. A higher Ni++ concentration (1 mM) abolished the 10 μM CPA-elevated [Ca++]ithat remained after verapamil and 0.1 mM Ni++. Phorbol dibutyrate (10 nM), a protein kinase C activator, potentiated contractions to 1 and 10 μM CPA in the presence of verapamil. Ni++ (0.1 mM) abolished the enhanced contractions, and decreased the elevated [Ca++]i. These results suggest that 1) elevated [Ca++]i due to store-operated Ca++ entry is dissociated from contraction; 2) the elevated [Ca++]i is restricted to at least two noncontractile compartments that can be differentiated by their relative sensitivities to blockade by low (0.1 mM) and higher (1 mM) Ni++ concentrations, and 3) [Ca++]i elevation within the compartment sensitive to blockade by 0.1 mM Ni++ can be coupled to contraction via protein kinase C activation.
Footnotes
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Send reprint requests to: Dr. Robert M. Rapoport, Department of Pharmacology and Cell Biophysics, University of Cincinnati College of Medicine, 231 Bethesda Ave., P.O. Box 670575, Cincinnati, OH 45267-0575.
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↵1 This work was supported in part by grants from the Department of Veterans Affairs (R.M.R.), NIH H123240 (R.J.P.) and a predoctoral fellowship from Ege University (M.T.).
- Abbreviations:
- CPA
- cyclopiazonic acid
- BHQ
- 2,5-di-(t-butyl)-1,4-hydroquinone
- fura-2/AM
- fura-2 acetoxymethyl ester
- PDB
- phorbol 12,13-dibutyrate
- R340/380
- ratio of emitted fluorescence intensities at 510 nm of fura-2 excited at 340 and 380 nm as presently determined in intact tissue
- Sf2/Sb2
- ratio of emitted fluorescence intensities of fura-2 excited at 380 nm in the presence of EGTA (Ca++-free), and in the presence of ionomycin (Ca++-saturated) as presently determined in intact tissue
- U46619
- 9,11-dideoxy-9α,11α-methanoepoxy prostaglandin F2α
- [Ca++]i
- intracellular Ca++
- SR
- sarcoplasmic reticulum
- Received May 29, 1997.
- Accepted December 23, 1997.
- The American Society for Pharmacology and Experimental Therapeutics
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