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Labetalol (The S,R isomer) is a powerful a1 blocker (antagonist?). a1 is responsible for:
Common:
vasoconstriction of arteries to heart
vasoconstriction of veins
decrease motility of smooth muscle in GI
This confilicts with general (native) agonists such as
noradrenaline
adrenaline
Clonidine is a centrally acting alpha-androgenergic receptor agonist with more affinity for a2 than a1 (but still affinity). It closts a negative feedback loop that begins with descenting sympathetic nerves from the bran that control the production of catecholamines (epenephrine). This decreases stimulus to the adrenals, as controlled by the adrenal medulla, and lowers blood pressure (and heart rate).
Josh's (very ameturish) interpretation: Labetalol blocks adrenaline production, while Clonidine is (gently?) trying to moderate the body's 'need' for adrenaline. The system responsible for administering adrenaline (awakeness) calls (late) for some adrenaline, and labetalol blocks this request, resulting in somnolescence.