Abstract
We tested the assumption that nifedipine blocks L-type calcium current [ICa(L)] at +10 mV and unmasks Na+/Ca2+ exchange-triggered contractions in guinea pig isolated ventricular myocytes. Voltage-clamp pulses elicited ICa(L) at +10 mV and evoked contractions in myocytes superfused with Tyrode's solution (35°C). Nifedipine blocked ICa(L) with an IC50 of 0.3 μM; this decreased to 50 nM at a holding potential of −40 mV, indicating preferential block of inactivated L-type Ca2+ channels. Use-independent block ofICa(L) increased with concentration (10–100 μM) and application time when nifedipine was rapidly applied (t1/2 = ∼0.2 s) during rest intervals (5–30 s). The fraction of use-dependent block ofICa(L) diminished with increasing drug concentration. Nifedipine also acceleratedICa(L) inactivation on the first test pulse. The combination of 30 μM nifedipine/30 μM Cd2+ (Nif 30/Cd 30) was as effective as 100 μM nifedipine to suppressICa(L) on the first test pulse at +10 mV. The incidence of complete block of contractions, as for complete block of ICa(L), increased as a function of nifedipine concentration and application time. Neither nifedipine nor Nif 30/Cd 30 affected Na+/Ca2+ exchange current at +10 to +100 mV. Contractions at +100 mV, although as large as those at +10 mV, were delayed in onset and resistant to nifedipine or Nif 30/Cd 30. We conclude that nifedipine-sensitiveICa(L) triggers contractions at +10 mV, whereas nifedipine-resistant Na+/Ca2+ exchange current initiates those at +100 mV.
Footnotes
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Send reprint requests to: Achilles J. Pappano, Ph.D., Department of Pharmacology, MC-6125, University of Connecticut Health Center, 263 Farmington Ave., Farmington, CT 06030. E-mail:pappano{at}nso1.uchc.edu
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↵1 This study was supported by U.S. Public Health Service Grant HL-13339.
- Abbreviations:
- CICR
- calcium-induced calcium release
- SR
- sarcoplasmic reticulum
- ICa(L)
- L-type calcium channel current
- ICa(T)
- T-type Ca2+ channel current
- INa/Ca
- Na+/Ca2+ exchange current
- DHP
- dihydropyridine
- Nif 30/Cd 30
- 30 μM nifedipine plus 30 μM Cd2+
- E-C
- excitation-contraction
- Received November 24, 1999.
- Accepted April 14, 2000.
- The American Society for Pharmacology and Experimental Therapeutics
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