Apo-Prazo Mechanism of Action





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Pharmacology: Pharmacodynamics:The exact mechanism of the hypotensive action of prazosin is unknown. Prazosin causes a decrease in total peripheral resistance and was originally thought to have a direct relaxant action on vascular smooth muscle. Recent animal studies, however, have suggested that the vasodilator effect of prazosin is also related to blockade of postsynaptic alpha-adrenoceptors. The resulls of dog torelimb experiments demonstrate that the peripheral vasodilator effect of prazosin is confined mainly to the level of the resistance vessels (arterioles). Tolerance does not appear to develop and rebound elevation or blood pressure does not seem to occur following abrupt cessation of prazosin therapy. According to hemodynamic studies a fall in blood pressure is unaccompanied by a clinically significant change in heart rate or renal blood flow and glomerular filtration rate.
Prazosin circulated in human plasma highly protein-bound (97%), yet disappeared rapidly with a plasma half-life of 2 to 3 hours. After chronic administration, no apparent drug accumu­lation was observed nor were any obvious decreases in plasma concentrations noted. Secondary plasma drug peaks and shoulders suggested probable enterohepatic circulation.
Animal studies indicate that prazosin is extensively metabolized primarily by demethylation and conjugation and excreted mainly via bile and feces. Less extensive human studies sug­gest similar metabolism and excretion in man.
Plasma drug levels following oral administration of prazosin in humans were generally low and variable.
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