Dissociation between endothelium-mediated increases in tissue cGMP levels and modulation of aortic contractile responses.

Both methoxamine and clonidine elicited similar maximal contractions of rat isolated aorta in the absence of endothelium. These contractions were not associated with changes in tissue levels of cGMP or cAMP. In the presence of endothelium maximal methoxamine-induced contractions were not less than those elicited in the absence of endothelium but maximal clonidine-induced contractions were reduced to about 10% of those in the absence of endothelium. However, in the presence of endothelium both methoxamine and clonidine induced similar increases in tissue cGMP levels of about 1.5 to 2 fold; cAMP levels were unchanged. There is therefore a dissociation between endothelium-mediated inhibition of maximal contractile responses and increases in tissue levels of cGMP.