Methylxanthines

2009-05-21T20:11:34Z

Kwells:

'''Source'''
Xanthines e.g. theophylline are used as bronchodilators (and very rarely for their positive inotropic effects on the cardiac muscle). There is also themethylated form, the methylxanthines e.g. theobromine (in chocolate) and caffeine. All of the xanthines are phosphodiesterase inhibitors (thus increasing the levels of cylcic AMP) and are also adenosine receptor antagonists.

The most common form of xanthine toxicity is theobromine (found in chocolate, cocoa, soft drinks and tea)however the ingestion of stimulants is another cause.

'''Mechanism of action'''
As phosphodiesterase inhibitors, the methylxanthines lead to an increase n cAMP. This in turn leads to a rise in intracellular calcium and also a decrease in calcium sequestration. Hence, there is increased activity of the cardiac and skeletal muscles. There is also competitive antagonism of the intracellular adenosine receptors. This causes CNS stimulation, vasoconstriction and tachycardia Finally, the methylxanthines tend to cause a degree of sensory cortex 'irritation' leading to excitation.

'''Toxicokinetics'''
It is readily absorbed, both orally and parentrally, and becomes widely distributed throughout the body.
Metabolism is in the liver and it is renally excreted. This is enahanced by acidic urine due to the relative alkalinity of the methylxanthines.

'''Clinical signs'''
-Increased urination
-Vomiting
-Diarrhoea
-Tachycardia (up to 300bpm)
-Tachypnoea (up to 150-200bpm)
-Arrhythmias
-Restlessness, hyperactivity, stiffness, twitching, hyperreflexia.
-Ataxia
-Seizures
-Hyperthermia

'''Toxicity'''
Lethal range in dogs = 100-250mg/kg
LD50 of caffeine and theobromine = 100-200mg/kg
LD50 of theophylline = 300mg/kg dogs, 700mg/kg cats

The highest concentration of theobromine is found in unsweetened baking chocolate and dark chocolate.

'''Diagnosis'''
Clinical signs and history of exposure

'''Treatment'''
-There is no specific antidote
-Attempt gastric decontamination:
- Emesis, gastric lavage if a small dog. N.B. avoid emesis if there is any evidence of hypereflexia or seizures.
- Activated charcoal (every 3-4hours to reduce the half life of the toxin)
- Cathartics
-Supportive therapy and reduction of side effects:
- Respiratory e.g. oxygen
- Cardiovascular --> need to control arrhythmias (lidocaine, propanolol) and correct any acid/base or electrolyte abnormalities.
- CNS --> may need to control any excitement or seizures using diazepam.
- Insulin will antagonise the effects of caffeine

Methylxanthines

2009-05-20T18:30:49Z

Kwells: New page: '''Source''' Xanthines e.g. theophylline are used as bronchodilators (and very rarely for its positive inotropic effects on the cardiac muscle). Also have the methylxanthines e.g. theobrom...

'''Source'''
Xanthines e.g. theophylline are used as bronchodilators (and very rarely for its positive inotropic effects on the cardiac muscle). Also have the methylxanthines e.g. theobromine (in chocolate) and caffeine. All xanthines are phosphodiesterase inhibitors i.e. will increase the levels of cylcic AMP and are also adenosine receptor antagonists.

The most common form of xanthine toxicity is theobromine (found in chocolate, cocoa, soft drinks and tea)

'''Clinical signs'''
-Increased urination
-Vomiting
-Diarrhoea
-Tachycardia
-Arrhythmias
-Restlessness
-Ataxia
-Seizures

'''Toxicity'''
Lethal range in dogs = 100-250mg/kg
LD50 of caffeine and theobromine = 100-200mg/kg
LD50 of theophylline = 300mg/kg dogs, 700mg/kg cats

The highest concentration of theobromine is found in unsweetened baking chocolate and dark chocolate.

'''Mechanism of action'''
By preventing the inactivation of cAMP there is an increase in intracellular calcium and also a decrease in calcium sequestration. Hence, it acts to stimulate the CNS and cardiac tissue, causing increased myocardial contractility.

'''Diagnosis'''
Clinical signs and history of exposure

'''Treatment'''
-There is no specific antidote
-Attempt gastric decontamination:
- Emesis, gastric lavage if a small dog
- Activated charcoal
- Cathartics
-Supportive therapy and reduction of side effects:
- Respiratory
- Cardiovascular --> need to control arrhythmias and correct any acid/base or electrolyte abnormalities.
- CNS --> may need to control any excitement or seizures using diazepam.

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Methylxanthines

Methylxanthines