Pharmacological analysis of the cardiac effects of 5-HT and some 5-HT receptor agonists in the pithed rat

Summary— A pharmacological analysis of the effects of 5-HT on heart rate has been performed in the pithed rat. 5-HT induced a dose-dependent increase in heart rate whereas 5-HT, receptor agonists — 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), 5-methoxy-N,N-dimethyl-tryptamine (5-MeODMT), 5-methoxy 3-(1,2,3,6-tetrahydro-4-piridinyl) 1H indole (RU 24969) and 1-(m-trifluoromethylphenyl)-piperazine (TFMPP) — failed to increase heart rate. The increase in heart rate induced by the selective 5-HT₂ receptor agonist 1-(2,5-dimethoxy-4-iodo-phenyl)-2-aminopropane (DOI) was not significant. The dose-response curve to 5-HT for its tachycardic effects was shifted two-fold to the right by ketanserin and LY 53857 and nine-fold to the right by methiothepin. The effects of high doses of 5-HT (higher than 100 μg/kg iv) were antagonized by methiothepin, (-)propranolol, 2-{2-[4(O-methoxyphenyl)-piperazine-1-yl]-ethyl}4,4-dimethyl-1,3 (2H-4H) isoquinoline-dione (AR-C 239) and by pretreatment with reserpine. The 5-HT₁ receptor antagonists, pindolol and spiroxatrine, the 5-HT₃ receptor antagonist MDL 72222 and the α₂-adrenoceptor blocking agent idazoxan failed to antagonize the tachycardia induced by 5-HT. It is concluded that in the pithed rat, the tachycardia induced by 5-HT remained unexplained (implication of 5-HT₂ receptors probably different from the classical vascular 5-HT₂ receptor, or implication of 5-HT_1C receptors?). Moreover, at high doses (higher than 100 μ/kg iv), 5-HT may increase heart rate by releasing catecholamines.