CARDIOVASCULAR EFFECTS FROM STIMULATION OF 5-HYDROXYTRYPTAMINE RECEPTORS

The cardiovascular effects of 5-hydroxytryptamine (5-HT), consisting of bradycardia or tachycardia, hypotension or hypertension, and vasodilatation or vasoconstriction, are mediated by three main types of receptors called 5-HT1-like, 5-HT2, and 5-HT3. In intact animals 5-HT elicits a short-lasting bradycardia, accompanied by hypotension, via stimulation of 5-HT3 receptors located on sensory vagal nerve endings in the heart (Bezold-Jarisch reflex). The nature of 5-HT receptors mediating tachycardiac responses is species-dependent. Myocardial 5-HT1-like and 5-HT2 receptors subserve tachycardia in the cat and rat, respectively. Tachycardia in the dog and rabbit is due to a release of catecholamines effected via the 5-HT2 receptors on the adrenal medulla and the 5-HT3 receptors on postganglionic cardiac sympathetic nerve fibres, respectively. The receptors mediating tachycardia in the pig are unique as they do not resemble any of the three 5-HT receptors characterized so far. The blood pressure response to 5-HT is usually triphasic: initial short-lasting hypotension due to reflex bradycardia (via 5-HT3 receptors), a middle pressor phase (via 5-HT2 receptors), and a longer-lasting hypotension (via 5-HT1-like receptors). Vascular contraction by 5-HT is generally mediated by 5-HT2 receptors (located primarily on the large conducting vessels), though in some instances (e.g., dog saphenous vein, dog and human basilar artery, and porcine arteriovenous anastomoses) the contractile response is (also) mediated via 5-HT1-like receptors. Venous dilatation and arteriolar dilatation (leading to increased capillary ['nutrient'] blood flow) occur via 5-HT1-like receptors located mainly on the vascular smooth muscles but also on the endothelium; the smooth muscle and endothelial 5-HT1-like receptors seem to be heterogeneous. In addition, 5-HT can elicit vasodilatation and hypotension as a result of decreased sympathetic nervous tone by acting within the central nervous system and by inhibiting noradrenaline release by a presynaptic action. Both these effects also involve 5-HT1-like receptors that do not appear to be identical. Last, knowledge of the cardiovascular effects of 5-HT and the nature of the receptors involved should be helpful in developing 5-HT-related compounds that may be useful in the treatment of hypertension, migraine, and peripheral vascular diseases.