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Charged tetracaine as an inactivation enhancer in batrachotoxin-modified Na+ channels
1Department of Anesthesia Research Laboratories, Harvard Medical School, Boston, Massachusetts.
Abstract:
Two distinct types of local anesthetics (LAs) have previously been found to block batrachotoxin (BTX)-modified Na+ channels: type 1 LAs such as cocaine and bupivacaine interact preferentially with open channels, whereas type 2 LAs, such as benzocaine and tricaine, with inactivated channels. Herein, we describe our studies of a third type of LA, represented by tetracaine as a dual blocker that binds strongly with closed channels but also binds to a lesser extent with open channels when the membrane is depolarized. Enhanced inactivation of BTX-modified Na+ channels by tetracaine was determined by steady-state inactivation measurement and by the dose-response curve. The 50% inhibitory concentration (IC50) was estimated to be 5.2 microM at -70 mV, where steady-state inactivation was maximal, with a Hill coefficient of 0.98 suggesting that one tetracaine molecule binds with one inactivated channel. Tetracaine also interacted efficiently with Na+ channels when the membrane was depolarized; the IC50 was estimated to be 39.5 microM at +50 mV with a Hill coefficient of 0.94. Unexpectedly, charged tetracaine was found to be the primary active form in the blocking of inactivated channels. In addition, external Na+ ions appeared to antagonize the tetracaine block of inactivated channels. Consistent with these results, N-butyl tetracaine quaternary ammonium, a permanently charged tetracaine derivative, remained a strong inactivation enhancer. Another derivative of tetracaine, 2-(di-methylamino) ethyl benzoate, which lacked a 4-butylamino functional group on the phenyl ring, elicited block that was approximately 100-fold weaker than that of tetracaine. We surmise that 1) the binding site for inactivation enhancers is within the Na+ permeation pathway, 2) external Na+ ions antagonize the block of inactivation enhancers by electrostatic repulsion, 3) the 4-butylamino functional group on the phenyl ring is critical for block and for the enhancement of inactivation, and 4) there are probably overlapping binding sites for both inactivation enhancers and open-channel blockers within the Na+ pore.
Insights
Tetracaine acts as a novel local anesthetic, blocking sodium channels in both closed and open states, with charged forms being most effective. This dual-action mechanism offers new insights into local anesthetic drug design.
Area of Science:
- Pharmacology
- Neuroscience
- Molecular Biology
Background:
- Local anesthetics (LAs) are classified by their interaction with sodium channels: Type 1 (e.g., cocaine) bind open channels, Type 2 (e.g., benzocaine) bind inactivated channels.
- Understanding LA binding mechanisms is crucial for developing safer and more effective pain management therapies.
Purpose of the Study:
- To characterize tetracaine as a novel, third type of local anesthetic with unique binding properties to batrachotoxin (BTX)-modified sodium channels.
- To elucidate the molecular mechanisms underlying tetracaine's interaction with sodium channel states and identify key structural determinants for its activity.
Main Methods:
- Utilized steady-state inactivation measurements and dose-response curves to assess tetracaine's effect on BTX-modified Na+ channels.
- Investigated the influence of membrane potential and external sodium ions on tetracaine's blocking efficacy.
- Synthesized and tested tetracaine derivatives to probe structure-activity relationships.
Main Results:
- Tetracaine functions as a dual blocker, binding strongly to closed channels and to a lesser extent to open channels at depolarized potentials.
- Tetracaine significantly enhances sodium channel inactivation, with an IC50 of 5.2 microM at -70 mV, suggesting a 1:1 binding stoichiometry.
- Charged tetracaine is the primary active form, and external Na+ ions antagonize its blocking effect through electrostatic repulsion.
- The 4-butylamino group on the phenyl ring is essential for tetracaine's potent block and inactivation enhancement.
Conclusions:
- Tetracaine represents a distinct class of local anesthetics that modulate sodium channel gating through dual-state binding.
- The binding site for tetracaine is likely within the sodium channel pore, with potential overlap with open-channel blockers.
- Structural modifications, particularly the 4-butylamino group, are critical for potent sodium channel blockade and inactivation enhancement.