NATURAL SODIUM CHANNEL POISONS

Image retrieved from https://en.wikipedia.org/wiki/Sodium_channel

Lyndsay EssonConnor GibneySamantha NailDylan Radke

PHM142 Fall 2015Coordinator: Dr. Jeffrey HendersonInstructor: Dr. David Hampson

What Is A Sodium Channel ? Integral membrane protein Large alpha subunit which contains 4 domains.

Each domain consists of 6 transmembrane regions Allows the movement of Na+ ions across cell

membranes

Blumenthal, K. M., Seibert, A. L. (2003). Voltage-gated sodium channel toxins: poisons, probes, and future promise. Cell Biochemistry and Biophysics, 38 (2), 215-238. Image retrieved from: https://en.wikipedia.org/wiki/User:Guvanasen/Sandbox

Role In Physiology Crucial in propagation of action

potentials CLOSED at rest OPENED during depolarization INACTIVATED during repolarization

Costill, D. L., Kenney, W. L., Wilmore, J. H. (2011). Physiology of sport and exercise: 5th edition. Human Kinetics

Images retrieved from: Stanfield, C. S., Germann, W. J.(2007). Principles of human physiology. Benjamin Cummings.

Functional Sites on Sodium Channel Site 1: Binds toxins that physically occlude the

pore Site 2: Binds toxins on cytoplasmic and

intramembranous regions causing persistent activation

Site 3: Toxin binding causes increased open probability

Site 4: Binding causes a shift of channel activation to more hyperpolarized values.

Blumenthal, K. M., Seibert, A. L. (2003). Voltage-gated sodium channel toxins: poisons, probes, and future promise. Cell Biochemistry and Biophysics, 38 (2), 215-238.

Tetrodotoxin (TTX)

Also known as fugu toxin One of the most common sodium

channel poisonsCan be found in several species including

the california newt, pufferfish, starfish and angel fish

Poisoning by TTX most commonly occurs through consumption of improperly prepared pufferfish

The TTX binding site TTX is a site-1 toxin of the sodium channel

Blocks the channel and prevents sodium ions from crossing the membrane, blocking neurotransmission

Binding site is extracellular for TTX Positive charges of TTX coordinate with highly conserved negative

residues on the sodium channel which are organize into two rings around the poor, between S5 and S6 Inner ring residues: Glu-387, Glu 945, Asp-1426, Asp-1717 Outer ring residues: Asp-384, Glu-942, Lys-1422, Ala-1714

Blumenthal, K.M, Seibert, A.L., (2003) Voltage-gated sodium channel toxins. Cell Biochemistry and Biophysics 38:215-237Terlau, H., Heinemain, S.H., Stuhmer, W., Pusch, M., Conti, F., Imoto, K., Numa, S., (1991) Mapping the site of block by tetrdotoxin and saxitoxin of sodium channel II. Federation of European Biochemical Societies. 1(2):93-96

Symptoms of TTX poisoning TTX poisoning occurs in 2 phases following

ingestionPhase 1: numbness of the mouth begins about

20 minutes after ingestion, followed by loss of sensation in the face and limbs

Phase 2: Paralysis begins to set in and movement becomes difficult and the victims ability to breath is compromised.

Symptoms progress until respiratory and cardiac function cease and the victim dies, approximately 8 hours after ingestions depending on the dose

U.S. FDA, Centre for Food Safety and Applied Nutrition. Tetrodotoxin, Foodborne Pathogenic Microorganisms and Natural Toxins Handbook. Accessed November 10th 2015

Batrachotoxin (BTX) Isolated from the Poison

Dart Frog Potent Cardiotoxic and

Neurotoxic Steroidal Alkaloid

Irreversibly binds to the sodium channels and forces them to remain open

Causes muscle contractions, violent convulsions, fibrillation, arrhythmias, heart failure, and death.

https://commons.wikimedia.org/wiki/File:Batrachotoxin2.svg

http://offclouds.com/296476-poison-dart-frog.html

A Model of the Binding

Li, H-L., Hadid, D., Ragsdale, D. (2002) The Batrachotoxin Receptor on the Voltage-Gated Sodium Channel is Guarded by the Channel Activation Gate. Mol Pharmacol. 61: 905-912

Method of Action

Modifies the Sodium Channel in Five Ways1. Shifts the Threshold for Action Potentials

Closer to Depolarization by 30-50 mV

2. No Fast Inactivation of Na+ Channels

3. No Slow Inactivation of Na+ Channels

4. Reduces Single Channel Conductance

5. Alters Ion Selectivity

Honerjager, P., Reiter, M. (1977) The Cardiotoxic Effect of Batrachotoxin. Naunyn-Schmiedeber’s Archives of Pharmacology. 299(3): 239-252

Management of Sodium Channel Poisoning

Tricyclic antidepressants often cause cardiac toxicity via sodium channel blockageIe. Flecainide, Imiprine

Hypertonic sodium salts most widely used treatment for this cardiac toxicity Sodium bicarbonate or lactate

Di Grande, A., et al. "Management of sodium-channel blocker poisoning: the role of hypertonic sodium salts." Eur Rev Med PharmacolSci 14 (2010): 25-30.

Mechanisms of Action

Hypertonic salts work by sodium loading and acting as alkalinizing agents

Thought to antagonize sodium channel blockage by; ○ increasing the unbinding rate○ Electrostatic repulsion○ interacting with the inactivated state intracellularly

to decrease concentrations of the drug

* Mechanism and therapeutic efficacy still poorly understood

Bou-Abboud, Elias, and Stanley Nattel. "Molecular mechanisms of the reversal of imipramine-induced sodium channel blockade by alkalinization in human cardiac myocytes." Cardiovascular research 38.2 (1998): 395-404.

Di Grande, A., et al. "Management of sodium-channel blocker poisoning: the role of hypertonic sodium salts." Eur Rev Med PharmacolSci 14 (2010): 25-30.

Therapeutic Benefits of TTX

Use of TTX as an

Analgesic agent

Defined role in relieving

normal and neuropathic pain Via interference of action potential generation in

nervous system Has been tested against cancer pain clinically Potential role in acute and inflammatory pain

Further clinical research still required Nieto, Francisco Rafael, et al. "Tetrodotoxin (TTX) as a therapeutic agent for pain." Marine drugs 10.2 (2012): 281-305.

Summary Slide Hetero-oligomeric protein; 4 domains that each contain 6

transmembrane regions Role in the human body is propagation of action potentials TTX is a site 1 sodium channel poison which prevents

neurotransmission Positively charged TTX binds to negative residues of aspartic and

glutamic acid on the extracellular pore of the sodium channel Causes increasing paralysis as the toxin moves through the body until

cardiac depression causes death BTX is a site 2 sodium channel poison which prevents the

sodium channel from closing. Modifies the Sodium Channel is 5 different ways.

Some sodium channel poisoning can be treated with hypertonic sodium salts

TTX can be used therapeutically for pain relief

References Blumenthal, K. M., Seibert, A. L. (2003). Voltage-gated sodium channel toxins:

poisons, probes, and future promise. Cell Biochemistry and Biophysics, 38 (2), 215-238.

Bou-Abboud, Elias, and Stanley Nattel. "Molecular mechanisms of the reversal of imipramine-induced sodium channel blockade by alkalinization in human cardiac myocytes." Cardiovascular research 38.2 (1998): 395-404.

Costill, D. L., Kenney, W. L., Wilmore, J. H. (2011). Physiology of sport and exercise: 5th edition. Human Kinetics

Di Grande, A., et al. "Management of sodium-channel blocker poisoning: the role of hypertonic sodium salts." Eur Rev Med PharmacolSci 14 (2010): 25-30.

Li, H-L., Hadid, D., Ragsdale, D. (2002) The Batrachotoxin Receptor on the Voltage-Gated Sodium Channel is Guarded by the Channel Activation Gate. Mol Pharmacol. 61: 905-912

Honerjager, P., Reiter, M. (1977) The Cardiotoxic Effect of Batrachotoxin. Naunyn-Schmiedeber’s Archives of Pharmacology. 299(3): 239-252

Nieto, Francisco Rafael, et al. "Tetrodotoxin (TTX) as a therapeutic agent for pain." Marine drugs 10.2 (2012): 281-305.

Terlau, H., Heinemain, S.H., Stuhmer, W., Pusch, M., Conti, F., Imoto, K., Numa, S., (1991) Mapping the site of block by tetrdotoxin and saxitoxin of sodium channel II. Federation of European Biochemical Societies. 1(2):93-96

U.S. FDA, Centre for Food Safety and Applied Nutrition. Tetrodotoxin, Foodborne Pathogenic Microorganisms and Natural Toxins Handbook. Accessed November 10 th 2015