Atropine can cause patients to become “as red as a beet” due to superficial vasodilation. How does atropine cause this flushing response? 

At toxic doses, and even occasionally therapeutic doses, atropine can cause dilation of cutaneous blood vessels resulting in an atropine flush. This reaction, making the patient “as red as a beet”, is well recognised as a classical sign of atropine overdose.

Atropine is a muscarinic receptor antagonist. Muscarinic receptors are involved in controlling the dilation of some blood vessels (see Sympathetic cholinergic innervation of blood vessels? Not in humans) but they are not known to be important for control of superficial cutaneous blood vessels.  It has therefore been something of a mystery why atropine should cause flushing. The mechanisms by which atropine causes this “anomalous vascular response” have therefore long been debated.

One hypothesis is that the response is due to the fact that atropine has alpha-adrenoceptor antagonist effects at very high doses (1). Antagonism of alpha adrenoceptors could block alpha-adrenoceptor-mediated vasoconstriction resulting in vasodilation and flushing. However, the more widely accepted explanation in recent years has been that the flushing is secondary to overheating due to block of sweating. Antagonism of M3 receptors on sweat glands will block the sweating response. This will cause overheating and compensatory superficial cutaneous vasodilation to increase heat loss.

Reference:
(1) Chang KC, Hahn KH. (1995) Is alpha-adrenoceptor blockade responsible for atropine flush? Eur J Pharmacol. 1995 Sep 25;284(3):331-4.