Alpha Adrenoceptors and Agonists

Vocabulary flashcards covering alpha adrenoceptors, their G-protein pathways, second messengers, physiological effects, and agonist classification by selectivity and mechanism.

$\alpha_1$ Receptor

An adrenoceptor coupled to $G_q$ protein that signals via $\uparrow \text{PLC} \rightarrow \text{IP}_3 + \text{DAG} \rightarrow \uparrow \text{Ca}^{2+}$, mediating vasoconstriction, mydriasis, and contraction of GI sphincters and bladder-neck/urethra/prostate.

$\alpha_2$ Receptor

An adrenoceptor coupled to $G_i$ protein that signals via $\downarrow \text{Adenylyl cyclase} \rightarrow \downarrow \text{cAMP}$, causing presynaptic $\downarrow \text{NE}$ release, central $\downarrow$ sympathetic outflow, and $\downarrow \text{insulin}$.

$\alpha_1$-Selective Agonists

Adrenoceptor agonists that selectively activate $\alpha_1$ receptors, including phenylephrine, midodrine, and oxymetazoline.

$\alpha_2$-Selective Agonists

Adrenoceptor agonists that selectively activate $\alpha_2$ receptors, including clonidine, guanfacine, and dexmedetomidine.

Non-selective Alpha Agonists

Adrenoceptor agonists that act non-selectively on alpha receptors, including norepinephrine and epinephrine.

Direct-acting Agonists

Adrenoceptor agonists that bind and activate the receptor themselves.

Indirect-acting Agonists

Adrenoceptor agonists that $\uparrow \text{NE}$ availability by promoting its release or blocking its reuptake.

Mixed-acting Agonists

Adrenoceptor agonists that possess both direct and indirect actions, such as ephedrine.

Phenylephrine

An $\alpha_1$-selective adrenoceptor agonist.

Midodrine

An $\alpha_1$-selective adrenoceptor agonist.

Oxymetazoline

An $\alpha_1$-selective adrenoceptor agonist.

Clonidine

An $\alpha_2$-selective adrenoceptor agonist.

Guanfacine

An $\alpha_2$-selective adrenoceptor agonist.

Dexmedetomidine

An $\alpha_2$-selective adrenoceptor agonist.

Ephedrine

An adrenoceptor agonist with a mixed mechanism of action, having both direct and indirect actions.

Norepinephrine

A non-selective alpha adrenoceptor agonist.

Epinephrine

A non-selective alpha adrenoceptor agonist.

$G_q$ Signaling Pathway

The effector mechanism associated with $\alpha_1$ receptors, defined by $\uparrow \text{PLC} \rightarrow \text{IP}_3 + \text{DAG} \rightarrow \uparrow \text{Ca}^{2+}$.

$G_i$ Signaling Pathway

The effector mechanism associated with $\alpha_2$ receptors, defined by $\downarrow \text{Adenylyl cyclase} \rightarrow \downarrow \text{cAMP}$.

$\alpha_1$ and $\alpha_2$ Receptor Comparison

Summary table detailing the G-protein coupling, effector/2nd messengers, and principal physiologic effects for $\alpha_1$ and $\alpha_2$ receptors.

Alpha Agonists by Selectivity Overview

Classification table grouping alpha adrenoceptor agonists into $\alpha_1$-selective, $\alpha_2$-selective, and non-selective categories.

Alpha Agonists by Mechanism Overview

Classification table categorizing adrenoceptor agonists into direct, indirect, and mixed mechanisms of action.