Possible mechanisms for the venular constriction elicited by Ruscus extract on hamster cheek pouch

We investigated the influence of alpha-adrenoceptors blockers and calcium blockers on the effects of the venotonic agent Ruscus extract on the diameter of arterioles (ID 10-70 microns) and venules (ID 20-135 microns) of hamster cheek pouch microvasculature in vivo. For microcirculatory measurements, the preparations were placed under an intravital microscope coupled to a closed-circuit TV system. The TV monitor display was used to obtain arteriolar and venular internal diameter recordings (always at the same site) by an image shearing device. All drugs were applied topically. Ruscus extract was tested in different concentrations and in combination with prazosin (alpha 1-adrenoceptor antagonist), rauwolscine (alpha 2-adrenoceptor antagonist), or diltiazem (calcium blocker). Topical application of Ruscus extract elicited concentration-dependent 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 venular constriction could be blocked by low concentrations (10(-9) M) of prazosin or diltiazem and by high concentrations (> 10(-6) M) of rauwolscine. 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.