Different pathways of calcium sensitization activated by receptor agonists and phorbol esters in vascular smooth muscle.

1. It has been shown that receptor agonists and activators of protein kinase C, phorbol esters, increase Ca2+ sensitivity of contractile elements in vascular smooth muscle. To discover if protein kinase C is involved in the agonist-mediated Ca2+ sensitization, we examined the effects of receptor agonists in the rat isolated aorta in which protein kinase C activity had been diminished by pretreatment with phorbol 12-myristate 13-acetate for 24 h. 2. In the aorta with protein kinase C activity, a high concentration (1 microM) of 12-deoxyphorbol 13-isobutyrate induced contraction and a low concentration (100 nM) potentiated high K(+)-induced contraction. In addition, prostaglandin F2 alpha induced greater contractions than high K+ at a given cytosolic Ca2+ level. The maximally effective concentrations of noradrenaline and endothelin-1 also induced greater contraction than high K+. In the aorta without protein kinase C activity, the contraction induced by 12-deoxyphorbol 13-isobutyrate and its potentiation of the high K(+)-induced contraction were abolished. However, prostaglandin F2 alpha, noradrenaline and endothelin-1 still induced a greater contraction than high K+. 3. In the aorta without protein kinase C activity, noradrenaline, endothelin-1 and prostaglandin F 2 alpha, but not 12-deoxyphorbol 13-isobutyrate, induced contractions in the presence of the Ca2+ channel blocker, verapamil, or in the absence of external Ca2+, by increasing Ca2+ sensitivity. 4. In the permeabilized preparations, inhibition of protein kinase C activity abolished the effect of potentiation of the Ca(2+)-induced contraction by 12-deoxyphorbol 13-isobutyrate although the potentiation of the contraction by prostaglandin F2 alpha did not change. 5. These results suggest that there are two pathways for Ca2+ sensitization in rat aorta; a protein kinase C-dependent pathway which is activated by phorbol esters, and a protein kinase C-independent pathway which is activated by receptor agonists.