Final answer:
Alpha-1 adrenergic receptors are located in smooth muscle cells of various blood vessels and tissues, and they cause smooth muscle contraction and vasoconstriction by activating the Gq protein, which increases intracellular calcium levels.
Step-by-step explanation:
Alpha-1 adrenergic receptors (or α1-adrenergic receptors) are G protein-coupled receptors found in various body tissues where they play a significant role in mediating physiological responses to the catecholamines, adrenaline and noradrenaline.
These receptors are predominantly found in smooth muscle cells of blood vessels, particularly those in the skin, gastrointestinal system, kidney (renal artery), and the brain.
When activated by endogenous ligands like norepinephrine or exogenous substances such as certain medications, α1-receptors trigger a myriad of effects, including smooth muscle contraction resulting in vasoconstriction, which affects blood pressure and organ perfusion.
Upon activation, α1-receptors couple with a G protein, namely Gq, which in turn activates phospholipase C (PLC). This leads to an increase in inositol triphosphate (IP3) and diacylglycerol (DAG), causing a rise in intracellular calcium levels and subsequent smooth muscle contraction.
This action is crucial for various bodily functions such as decreasing the motility of the gastrointestinal tract, constricting vessels, stimulating sweat glands, and contributing to glycogenolysis and gluconeogenesis from adipose tissue and the liver.