Alpha-blockers include substances that competitively inhibit alpha-adrenergic receptors – phentolamine, tropodifen, hydrogenated derivatives of ergot alkaloids and other substances. Effect of alpha-blockers does not coincide completely with the blockade of nerve impulses coming to postganglionic fibers, since these substances block the stimulatory effects mainly related to the initiation of alpha-AR. Same inhibitory effects (eg relaxation of bronchial smooth muscle and intestine) are preserved.
Alpha-adrenergic receptors are uniformly distributed in the human body. There are two main subtypes of alpha-AR. This alpha 1 and alpha 2-AP. Alpha2 subtype is pre-synaptic and causes a decrease in production of norepinephrine through a negative feedback mechanism. Alpha1-subtype is located postsynaptically and is the target of conservative treatment of diseases of the urinary tract, to a greater degree of benign prostatic hyperplasia (BPH).
The use of non-selective alpha-blockers (acting on alpha 1 and alpha 2-AP) is limited due to the fact that these drugs block as the limit and postsynaptic alpha-AR. Keep in mind that block presynaptic alpha-AP gives physiological autoregulation mediator release of norepinephrine. As a result of violations of the negative feedback occurs excessive release of norepinephrine, the recovery of adrenergic transmission. The latter explains the lack of resistance block postsynaptic alpha 1-AR by using non-selective alpha-blockers.
Increased tachycardia is the result of increased release of norepinephrine. Through functioning of alpha2-AP stored negative feedback mechanism, and hence increased release of noradrenaline occurs. In this block the postsynaptic alpha 1-AR becomes more stable. Furthermore, there is no pronounced tachycardia. In view of these characteristics have been developed preparations that selectively block the action of postsynaptic (peripheral) alpha 1-AR, such as prazosin.
Alpha-blockers are given great role in the treatment of new-onset acute urinary retention. The greatest effect of combination therapy is observed in alpha-AB and urethral bladder drainage catheter for a few days. Experience of using doxazosin and tamsulosin in 273 patients aged 52 to 74 years in the preoperative preparation indicates that the inclusion of alpha-blockers in the preoperative preparation of the scheme can prevent the development of postoperative acute urinary retention.