CLONIDINE AND VASOMOTION PHYSIOLOGY

It is known that clonidine exerts a hyperpolarizing action and alpha(2 )-adrenergic activity. The experimental work investigates the conditio ns under which each action of clonidine is developed on vascular smoot h muscle. Two parameters were studied in vitro on bovine aortic media, Ca2+ uptake and vascular tone. The Ca2+ uptake measurement was perfor med by incubating in Krebs' solution small slices of the preparation i n the presence of Ca-45. Studies on vascular tone were performed on de endothelialized bovine aortic rings suspended in Krebs' solution. Low concentrations of clonidine (1 nM-1 mu M) decrease Ca2+ uptake and rel ax the preparation, indicating dominance of the inhibiting action of c lonidine may be due to an hyperpolarization. Clonidine IO mu M results in equalization of the opposing actions of Ca2+ uptake and vascular t one. When the preparation is stimulated by alpha(1)-adrenergic against phenylephrine I mu M, clonidine I nM-10 mu M potentates the Ca2+ upta ke and vascular contraction in a monophasic way, indicating that the d epolarizing mechanisms connected with the alpha(1)-adrenergic stimulat ion totally inhibit the relaxing action of clonidine possibly due to h yperpolarization. This action is restricted in the presence of yohimbi ne (alpha(2)-adrenergic blocker). (C) 1998 Pious Science. Ail rights r eserved.