Abstract
The mechanisms of Pb++ block and unblock of L-type Ca++ channel currents were measured using ventricular myocytes or the cloned channel. The cloned channel was expressed in either Xenopus laevis oocytes or human embryonic kidney cells (HEK 293, stable transfectants). The threshold for Pb++ block was 1 nM, and the apparent IC50value was 152 nM in oocytes and 169 nM in HEK 293 cells. Pb++ block was dependent on the composition of the external recording solution but not dependent on the subunit composition of the channel. Pb++ block was voltage dependent, with little block observed at negative test potentials using low concentrations of Pb++. Strong depolarizations (>+100 mV) reversed Pb++ block, allowing measurement of reblock kinetics. Reblock was fast (τ = 11 msec), as measured during a +20-mV test pulse. Simple washout did not completely reverse Pb++block, especially after exposure to concentrations of >100 nM. Full recovery could only be observed after treatment with heavy metal antidotes such as meso-2,3-dimercaptosuccinic acid, 2,3-dimercapto-1-propanesulfonic acid and EDTA. These results suggest that Pb++ blocks voltage-gated Ca++ channels by two mechanisms and that full reversal of lead block requires chelator treatment.
Footnotes
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Send reprint requests to: Edward Perez-Reyes, Ph.D., Department of Physiology, Loyola University Medical Center, Maywood, IL 60153. E-mail eperez{at}luc.edu
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↵1 This work was supported in part by the National Institutes of Health and the American Heart Association (E.P.R.). E.P.R. is an Established Investigator of the American Heart Association.
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↵2 E. Perez-Reyes and L. L. Cribbs, unpublished observations.
- Abbreviations:
- DMSA
- meso-2,3-dimercaptosuccinic acid
- DMPS
- 2,3-dimercapto-1-propanesulfonic acid
- TEA
- tetraethylammonium
- NMG
- N-methyl-glucamine
- HEPES
- 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid
- HEK
- human embryonic kidney
- Received September 4, 1996.
- Accepted March 7, 1997.
- The American Society for Pharmacology and Experimental Therapeutics
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