Posted by SLS on April 24, 2004, at 21:14:35
In reply to Re: MAOI causes hypotension - What's the mechanism? » djmmm, posted by SLS on April 24, 2004, at 19:25:41
OK. It looks like we were both wrong - at least according to what little I could find on the Net. I wish I could have found something more definitive, though. Maybe you'll have better luck.
I get the impression that the mechanisms by which MAOIs produce hypotension are poorly understood. One piece that I found stated that it definitely was not due to NE alpha-1 antagonism. The following is an excerpt from:
http://www.acnp.org/g4/GN401000046/CH046.html
----------------------------------------------------------
SIDE EFFECTS OF MAOIS
Orthostatic HypotensionOrthostatic hypotension is a common side effect of the irreversible MAOIs, particularly phenelzine (Table 4). Clinically, the development of orthostatic symptoms is gradual and appears generally after 2–3 weeks of treatment. The relationship of the time course of this response to the etiology of orthostasis remains unclear. Some have suggested it may represent a compensatory down-regulation of peripheral ganglionic effects in response to central sympathetic stimulation. Others have proposed that inhibition of amine metabolism results in an artificial, supraphysiologic elevation of amines with few or no pressor effects and the resultant replacement of amines with greater pressor effects from intracellular amine stores. Gradual accumulation of octopamine in adrenergic neurons, for instance, may be the result of MAO inhibition and resultant alternate hydroxylation of tyramine to octopamine. It has been suggested that octopamine may replace NE from intra-axonal storage granules. Octopamine released upon sympathetic stimulation may act as a 'false neurotransmitter' with minimal activity at a- or b-adrenergic receptors. The result is a functional block of sympathetic neurotransmission, accompanied by decreased ability to regulate blood pressure in response to postural changes.
----------------------------------------------------------
- Scott
poster:SLS
thread:339531
URL: http://www.dr-bob.org/babble/20040423/msgs/339682.html