POSSIBLE MECHANISMS FOR THE VENULAR CONSTRICTION ELICITED BY RUSCUS EXTRACT ON HAMSTER-CHEEK POUCH

We investigated the influence of a-adrenoceptors blockers and calcium blockers on the effects of the venotonic agent Ruscus extract on the d iameter of arterioles (ID 10-70 mu m) and venules (ID 20-135 mu m) of hamster cheek pouch microvasculature in vivo. For microcirculatory mea surements, the preparations were placed under an intravital microscope coupled to a closed-circuit TV system. The TV monitor display was use d to obtain arteriolar and venular internal diameter recordings (alway s at the same site) by an image shearing device. All drugs were applie d topically. Ruscus extract was tested in different concentrations and in combination with prazosin (alpha(1)-adrenoceptor antagonist), rauw olscine (alpha(2)-adrenoceptor antagonist), or diltiazem (calcium bloc ker). Topical application of Ruscus extract elicited concentrationdepe ndent responses in the studied vessels: arterioles remained unchanged in the concentration range tested, whereas venules remained unchanged or constricted depending on the concentration used. The observed venul ar constriction could be blocked by low concentrations (10(-9) M)) of prazosin or diltiazem and by high concentrations (> 10(-6) M) of rauwo lscine. Our results suggest that the venular constriction elicited by Ruscus extract in vivo, at the microcirculatory level, is mediated by calcium and by alpha-adrenoceptors and further support data previously reported on larger vessels and on patients with venous insufficiency. -