Serotonin relaxes porcine pial veins

The effect of serotonin [5-hydroxytryptamine (5-HT)] on pial venous tone of the pig was examined using in vitro tissue bath techniques. Isolated pial venous rings exhibited spontaneous rhythmic contractions (SRC) on mechanical stretching and/or applications of several vasoactive substances, including norepinephrine. On the other hand, KCl induced sustained active muscle tone (SAT) without SRC. The SRC induced by mechanical stretching were not affected by tetrodotoxin, nitro-L-arginine, alpha- and beta-adrenergic, histaminergic, and muscarinic receptor antagonists, indicating that the SRC in porcine pial veins are of myogenic origin. The SRC induced by stretching or applications of vasoactive substances and SAT induced by KCl were inhibited by 5-HT in a concentration-dependent manner. The inhibition was prevented by methysergide and methiothepin but not by ketanserin, propranolol, 3 alpha-tropanyl-1H-indole-3-carboxylic acid ester, hemoglobin, or nitro-L-arginine. The SRC and SAT were inhibited by 5-carboxamidotryptamine (5-CT), 8-hydroxy-2-di-N-propylaminotetralin HBr (8-OHDPAT), 1-[3-(trifluoromethyl)phenyl]piperazine (TFMPP), and 5-methoxytryptamine (5-MT), but not by sumatriptan, alpha-methylserotonin, or 2-methylserotonin. On the other hand, 5-CT, 8-OHDPAT, TFMPP, 5-MT, and sumatriptan constricted the porcine pial arteries exclusively. In 15% of pial venous preparations examined, 5-HT at low concentrations induced ketanserin-sensitive constrictions. These results indicate that the porcine pial venous smooth muscle contains multiple subtypes of 5-HT receptors. The 5-HT inhibition of SRC and SAT is predominant and is mediated by 5-HT1-like receptors, which, however, do not seem to correspond to 5-HT1A, 5-HT1B, 5-HT1C, 5-HT1D, 5-HT1E, or 5-HT1F receptor subtypes.(ABSTRACT TRUNCATED AT 250 WORDS)