Different effect of serotonin on intracellular calcium ion dynamics in the smooth muscle cells between rat posterior ciliary artery and vorticose vein

Abstract

5-hydroxytriptamine (5-HT: serotonin) is an important transmitter that causes vessel constriction, although few studies have examined the effect of 5-HT on venous smooth muscles. The intracellular Ca²⁺ concentration ([Ca²⁺]_i) plays an essential role in stimulus-response coupling in numerous tissue/cells including vascular smooth muscle cells. The present study was performed to examine whether differences between arteries and veins in the response to 5-HT can be detected under confocal microscope with respect to [Ca²⁺]_i dynamics. In posterior ciliary arteries of rats, 5-HT induced a [Ca²⁺]_i increase. The 5-HT-induced responses were caused by both Ca²⁺ influx and mobilization. Agonist and antagonist experiments revealed that arterial smooth muscles possess 5-HT_{1a, 1b, 2} (Gprotein-coupled type) and 5-HT₃ (ion channel type) receptors, and that 5-HT₂ in particular plays a major role in these responses. For vorticose veins, the 5-HT-induced responses were also caused by both Ca²⁺ influx and mobilization. However, the cAMP dependent pathway (5-HT_4-7) was found to be significant in vasocontraction with respect to 5-HT in these vessels. Thus, Ca²⁺ mobilization was induced by 5-HT₂ and 5-HT_4-7 in a vessel-dependent manner, whereas Ca²⁺ influx universally was induced by 5-HT₃. These results indicate that the posterior ciliary arteries and vorticose veins in the same tissue might differ greatly in their responses to stimulus.