Cantharidin enhances norepinephrine-induced vasoconstriction in an endothelium-dependent fashion

In this study we characterized the effects of the protein phosphatase (PP) type 1 and type 2A inhibitor cantharidin (Cant) and its structural analogs cantharidic acid and endothall on PP activity, force of contraction, and myosin light chain phosphorylation in rat aorta. All compounds inhibited PP activity in homogenates of rat aorta with a rank order of potency of Cant = cantharidic acid > endothall. However, only Cant increased force of contraction and myosin light chain phosphorylation in intact isolated rat aortic rings. Based on these findings, we investigated the effects of Cant on alpha-adrenoceptor-mediated vasoconstriction. Cant (1 and 3 microM) enhanced norepinephrine-induced contraction in endothelium-intact rat aorta. In contrast, Cant did not affect norepinephrine-induced contraction in endothelium-denuded rat aorta. We suggest that inhibition of PP1 and/or PP2A activities by Cant enhances vascular contractility in endothelium-intact rat aorta by increasing the phosphorylation state of endothelial regulatory proteins.