This research will characterize the molecular mechanisms by which cardiac anti-arrhythmic drugs inhibit function of an intrinsic membrane protein: the cardiac sodium (Na) channel. Na+ influx causes the rapid initial upstroke of the cardiac action potential and allows impulse conduction. Na channels are the site of action of many antiarrhythmic drugs, but details cardiac Na channel function and of the mechanisms of inhibition of Na+ flux through channels are lacking. Because many details of Na channel block are derived from studies of non-mammalian nerves (squid and frog) much less is known about mammalian cardiac Na channels. Since it is now clear that there are multiple types of Na channel in the nervous system, and there are well know differences in cardiac and neuronal Na channels, it is more important than ever to study cardiac Na channels directly. Therefore, the aim of this project is to extensively characterize cardiac Na channels and their interactions with drugs and ions using the patch clamp technique. Experiments are designed to test the concepts developed in the modulated receptor hypothesis of Na channel blockers (Hille, 1977; Hondeghem and Katzung, 1977). This model has been used extensively to explain block of ion channels. The model states that Na channel blockers have unique affinities for each of the three primary states of the Na channel; and access to the receptor occurs via a hydrophobic and a hydrophilic pathway. Because of the importance of Na channels in cardiac excitability and the therapeutic importance of Na channel blockers, it is essential to develop a greater knowledge of the cardiac Na channel and to explore these constructs at the level of the channel protein. Enzymatically dissociated cells from guinea pig, mouse, rat and human myocardium will be used. Measurements will determine whether Na block involves a reduction of the current through a single channel or if the probability of channel opening is altered, and the effects of pH, Na and Ca on channel function and drug block will be determined. Many Na channel blockers (TTX, lidocaine, quinidine) also alter the action potential plateau and modulate repolarization; therefore, experiments will also determine if there is a second population of late opening Na channels or a different gating mode with a different sensitivity to antiarrhythmic agents.

Agency
National Institute of Health (NIH)
Institute
National Heart, Lung, and Blood Institute (NHLBI)
Type
First Independent Research Support & Transition (FIRST) Awards (R29)
Project #
1R29HL040608-01A1
Application #
3472044
Study Section
Pharmacology A Study Section (PHRA)
Project Start
1989-04-01
Project End
1994-03-31
Budget Start
1989-04-01
Budget End
1990-03-31
Support Year
1
Fiscal Year
1989
Total Cost
Indirect Cost
Name
Vanderbilt University Medical Center
Department
Type
Schools of Medicine
DUNS #
004413456
City
Nashville
State
TN
Country
United States
Zip Code
37203
Turgeon, J; Daleau, P; Bennett, P B et al. (1994) Block of IKs, the slow component of the delayed rectifier K+ current, by the diuretic agent indapamide in guinea pig myocytes. Circ Res 75:879-86
Makita, N; Bennett Jr, P B; George Jr, A L (1994) Voltage-gated Na+ channel beta 1 subunit mRNA expressed in adult human skeletal muscle, heart, and brain is encoded by a single gene. J Biol Chem 269:7571-8
Valenzuela, C; Bennett Jr, P B (1994) Gating of cardiac Na+ channels in excised membrane patches after modification by alpha-chymotrypsin. Biophys J 67:161-71
Snyders, D J; Tamkun, M M; Bennett, P B (1993) A rapidly activating and slowly inactivating potassium channel cloned from human heart. Functional analysis after stable mammalian cell culture expression. J Gen Physiol 101:513-43
Bennett, P B; Po, S; Snyders, D J et al. (1993) Molecular and functional diversity of cloned cardiac potassium channels. Cardiovasc Drugs Ther 7 Suppl 3:585-92
Bennett Jr, P B; Makita, N; George Jr, A L (1993) A molecular basis for gating mode transitions in human skeletal muscle Na+ channels. FEBS Lett 326:21-4
Roberds, S L; Knoth, K M; Po, S et al. (1993) Molecular biology of the voltage-gated potassium channels of the cardiovascular system. J Cardiovasc Electrophysiol 4:68-80
Po, S; Roberds, S; Snyders, D J et al. (1993) Heteromultimeric assembly of human potassium channels. Molecular basis of a transient outward current? Circ Res 72:1326-36
Po, S; Snyders, D J; Baker, R et al. (1992) Functional expression of an inactivating potassium channel cloned from human heart. Circ Res 71:732-6
Snyders, J; Knoth, K M; Roberds, S L et al. (1992) Time-, voltage-, and state-dependent block by quinidine of a cloned human cardiac potassium channel. Mol Pharmacol 41:322-30

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