Is the 5-Hydroxytryptamine 7 Receptor Constitutively Active in the Vasculature? A Study in Veins/Vein.

The 5-hydroxytryptamine 7 (5-HT7) receptor is reported to have considerable constitutive activity when transfected into cells. Constitutive activity-receptor activity in the absence of known agonist-is important for understanding the contributions of a receptor to (patho)physiology. We test the hypothesis that the 5-HT7 receptor possesses constitutive activity in a physiological situation. Isolated veins from male and female Sprague Dawley rats were used as models for measuring isometric force; the abdominal vena cava possesses a functional 5-HT7 receptor that mediates relaxation, whereas the small mesenteric vein does not. Compounds reported to act as inverse agonists were investigated for their ability to cause contraction (moving a constitutively active relaxant receptor to an inactive state, removing relaxation). Compared with a vehicle control, clozapine, risperidone, ketanserin, and SB269970 caused no contraction in the isolated male abdominal vena cava. By contrast, methiothepin caused a concentration-dependent contraction of the male but not female abdominal vena cava, although with low potency (-log EC50 [M] = 5.50 +/- 0.45) and efficacy (similar to 12% of contraction to endothelin-1). Methiothepin-induced contraction was not reduced by the 5-HT7 receptor antagonist (SB269970, 1 mu M, not active in the vena cava). These same compounds showed little to no effect in the isolated mesenteric vein. We conclude that the 5-HT7 receptor in the isolated veins of the Sprague Dawley rat does not possess constitutive activity. We raise the question of the physiological relevance of constitutive activity of this receptor important to such diverse physiological functions as sleep, circadian rhythm, temperature, and blood pressure regulation.