Physiological Basis of Adrenergic Receptor

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Chapter: Medicinal Chemistry : Adrenergic Blockers

Blockade of vasoconstrictor α1 receptors reduces peripheral resistance and causes reduced cardiac output leads to decreased blood pressure.


PHYSIOLOGICAL BASIS OF ADRENERGIC RECEPTOR ANTAGONISTS

Blockade of vasoconstrictor α1 receptors reduces peripheral resistance and causes reduced cardiac output leads to decreased blood pressure. The α blockers abolish the pressor action of adrenaline, which then produces fall in the blood pressure due to β2 mediated vasodilatation called Dale’s vasomotor reversal. Pressor action of selective α-agonists is suppressed. Reflex tachycardia occurs due to the presynaptic α2 receptors.

Nasal stuffness and miosis result due to the blockade of α2 receptors. The therapeutic application of drugs coming under the adrenergic antagonists are receptor oriented, especially β blockers are used as antihypertensive, α adrenergic antagonists are used to control the peripheral vascular resistance.


Mechanism of Action of α-Adrenergic Blockers

α-Adrenergic receptor response in clinical relevance include α2 receptor mediated contraction of arterial and venous smooth muscle. α2 adrenergic receptors are involved in suppressing sympathetic output, increasing vagal tone, facilitating platelet aggregation, inhibiting the release of norepinephrine, and acetylcholine from nerve endings. Blockade of α1 receptors inhibits vasoconstriction induced by endogenous catecholamines. Vasodilatation may occur in both arteriolar resistance vessels and veins. α2 receptor regulates both central and peripheral sympathetic neurons. Acceleration of presynaptic α receptors inhibits the norepinephrine release. In some vascular beds, these drugs promote vasodilatation through the release of nitric oxide (endothelial relaxing factor). Phenoxybenzamine inhibits the uptake of catecholamine from the nerve terminals. Phentolamine and tolazoline are competitive α adrenergic antagonists and block the receptor for 5-HT and it causes the release of histamine from the mast cells, which is a potent vasodilator.


Mechanism of Action of β-Adrenergic Receptor Blockers

β adrenergic receptor antagonists slow the heart rate and decrease the myocardial contractility, these prolongs the systolic conduction and disturbs the ventricular fibres. Dimensions of the ventricle is decreased, oxygen consumption is decreased, and thereby decreases the heart rate and aortic pressure. In blood vessels, these drugs reduces the noradrenaline release from the sympathetic terminals and decrease the renin from kidney due to the blockade of β receptors.


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