The effect of magnesium on the response of smooth muscle to 5-hydroxytryptamine.

1 The responses to 5-hydroxytryptamine (5-HT) of rabbit isolated mesenteric artery and vein and longitudinal smooth muscle of guinea-pig ileum were examined in Krebs solution containing 0, 1.2 or 2.4 mM Mg2+. 2 When the concentration of Mg2+ was raised the spontaneous contractile activity of rabbit mesenteric vein was depressed. The responses to 5-HT in rabbit mesenteric artery and vein and guinea-pig ileum were greater in the absence of Mg2+. The initial fast component of 5-HT-induced contractions in rabbit mesenteric vein was reduced more consistently than the subsequent slow component by increasing the Mg2+ concentration. 3 Exposure of mesenteric vein to Ca-free solution containing ethyleneglycoltetra-acetic acid (EGTA) promptly abolished 5-HT contraction in normal-Mg but not in low-Mg Krebs solution. 4 In mesenteric veins, no difference was observed in either the 'lanthanum-resistant' uptake of 45Ca or total tissue Ca, measured by atomic absorption spectrophotometry, after 60 min exposure to either low-Mg or normal-Mg Krebs solution. On the other hand, after 5 min exposure, the 'lanthanum-resistant' uptake of 45Ca was greater in the absence of Mg2+ than in the presence of higher Mg2+ concentrations. 5 It is suggested that Mg2+ depressed the 5-HT response at least partly by reducing the availability of Ca2+ from a rapidly equilibrating intracellular pool.