Clonidine and vasomotion physiology.

It is known that clonidine exerts a hyperpolarizing action and alpha 2-adrenergic activity. The experimental work investigates the conditions under which each action of clonidine is developed on vascular smooth muscle. Two parameters were studied in vitro on bovine aortic media, Ca2+ uptake and vascular tone. The Ca2+ uptake measurement was performed by incubating in Krebs' solution small slices of the preparation in the presence of 45Ca. Studies on vascular tone were performed on deendothelialized bovine aortic rings suspended in Krebs' solution. Low concentrations of clonidine (1 nM-1 microM) decrease Ca2+ uptake and relax the preparation, indicating dominance of the inhibiting action of clonidine may be due to an hyperpolarization. Clonidine 10 microM results in equalization of the opposing actions of Ca2+ uptake and vascular tone. When the preparation is stimulated by alpha 1-adrenergic against phenylephrine 1 microM, clonidine 1 nM-10 microM potentates the Ca2+ uptake and vascular contraction in a monophasic way, indicating that the depolarizing mechanisms connected with the alpha 1-adrenergic stimulation totally inhibit the relaxing action of clonidine possibly due to hyperpolarization. This action is restricted in the presence of yohimbine (alpha 2-adrenergic blocker).