Pharmacology/Alpha1 blockers

Alpha 1 receptors are located in:

  • Eye radial (i.e dilator) muscle
  • Arterioles and veins,
  • Male sex organs, Urinary bladder,
  • Kidney and Liver.

Alpha 1 receptors use Gq Coupled secondary mediators and cause increase in phospholipase C and calcium levels in the cell.

Alpha1 blockers:

  • Selective Blockers: Prazosin, Doxazocin, Terazocin
  • Non selective Alpha blockers (both alpha 1 and alpha 2 receptor blockers): Phentolamine(competitive inhibitor), Phenoxybenzamine (non competitive inhibitor)

Effects of Alpha1 receptor blockage:

  • Eye: Miosis
  • Arterioles and veins: Reduced sympathetic tone in blood vessels- This results in vasodilation, Decrease in preload and after load on heart, lowering of peripheral vascular resistance and thus a fall in blood pressure is noted. The drop in blood pressure causes a reflex tachycardia.

Alpha 1 receptors (GPCR) that are located on arteriole smooth muscle and coupled to IP3/DAG cause vasoconstriction and increase in blood pressure.

Prazosin: Prazosin can lower BP rapidly so we warn patients to relax for 1 hour after taking the first dose and for any increase in dose. Other side effects: Orthostatic hypotension, Nasal stuffiness, First dose syncope, Urinary incontinence and failure of ejaculation.

It is a smooth muscle relaxant so it is used in cases of benign prostatic hypertrophy, as it decreases the tone of urinary sphincters.