Abstract
It has been shown that mibefradil (Ro 40-5967) exerts a selective inhibitory effect on T-type Ca2+ currents, although at higher concentrations it can antagonize high voltage-activated Ca2+ currents. The action of mibefradil on Ca2+channels is use- and steady-state-dependent and the binding site of mibefradil on L-type Ca2+ channels is different from that of dihydropyridines. By using conventional whole-cell and perforated patch-clamp techniques, we showed that mibefradil has an inhibitory effect on both T- and L-type Ca2+currents in insulin-secreting cells. However, the effect on L-type Ca2+ currents was time-dependent and poorly reversible in perforated patch-clamp experiments. By using mass spectrometry, we demonstrated that mibefradil accumulates inside cells, and furthermore, a metabolite of mibefradil was detected. Intracellular application of this metabolite selectively blocked the L-type Ca2+ current, whereas mibefradil exerted no effect. This study demonstrates that mibefradil permeates into cells and is hydrolyzed to a metabolite that blocks L-type Ca2+ channels specifically by acting at the inner side of the channel.
Footnotes
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Send reprint requests to: Ming Li, Ph.D., Department of Pharmacology, University of South Alabama, College of Medicine, Mobile, AL 36688. E-mail: mli{at}jaguar1.usouthal.edu
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1 This work was supported by National Institutes of Health Grant DK50151 and Juvenile Diabetes Foundation International Grant no. 197037.
- Abbreviations:
- dm-mibefradil
- (1S,2S)-2-[2-[[3-(2-benzimidazolyl)propyl]methylamino]ethyl]-6-fluoro-1,2,3,4-tetrahydro-1-isopropyl-2-napthyl hydroxy hydrochloride
- Received July 28, 1999.
- Accepted October 25, 1999.
- The American Society for Pharmacology and Experimental Therapeutics
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