Final answer:
The two types of adrenergic receptors are alpha and beta-adrenergic receptors, which are G protein-coupled receptors activated by catecholamines. Alpha receptors typically cause vasoconstriction while beta receptors stimulate heart muscle contraction and relaxation of smooth muscles.
Step-by-step explanation:
The two main types of adrenergic receptors are alpha (α)-adrenergic receptors and beta (β)-adrenergic receptors. Alpha-adrenergic receptors come in three subtypes: α1, α2, and a less commonly discussed α3, whereas beta-adrenergic receptors have three subtypes as well: β1, β2, and β3. These receptors are all G protein-coupled receptors (GPCRs), which are activated by the endogenous catecholamines, adrenaline (epinephrine) and noradrenaline (norepinephrine).
The alpha receptors are known for causing vasoconstriction of arteries and veins, while beta receptors are generally associated with increasing heart muscle contraction, smooth muscle relaxation, and glycogenolysis. The mechanism of action of these receptors involves the modulation of intracellular cyclic AMP (cAMP) levels: α1 receptors couple to Gq proteins and increase intracellular Ca2+, leading to smooth muscle contraction; α2 receptors couple to Gi proteins, decreasing cAMP activity; and β receptors couple to Gs proteins, increasing cAMP activity.