Myosin phosphorylation-independent contraction induced by phorbol ester in vascular smooth muscle

In isolated rat aorta, carotid artery, tail artery, rabbit aorta and mesenteric artery, but not in ear artery, 1 microM 12-deoxyphorbol 13-isobutyrate (DPB) induced a sustained contraction. However, DPB increased cytosolic Ca++ concentration ([Ca++]i) only in rat aorta and carotid artery. Similar results were obtained with phorbol 12,13-dibutyrate, although the inactive phorbol ester, 4-alpha-phorbol 12,13-dibutyrate, was ineffective. In rat aorta, DPB-induced contraction was followed by an increase in 20 kDa myosin light chain (MLC) phosphorylation. Both contraction and MLC phosphorylation stimulated by DPB were greater than those due to high K+ for a given increase in [Ca++]i. A Ca++ channel blocker, verapamil, decreased the DPB-induced increments in [Ca++]i and MLC phosphorylation to their respective resting levels, although contraction was inhibited only slightly. In the absence of external Ca++ (with 0.5 mM ethyleneglycol bis(beta-aminoethyl-ether)tetraacetic acid), DPB induced sustained contraction without increasing [Ca++]i or MLC phosphorylation. This contraction was followed by an increase in stiffness and force recovery after a shortening step. These results suggest that the contraction induced by DPB in rat aorta is due to increase in [Ca++]i followed by MLC phosphorylation and Ca++ sensitization of MLC phosphorylation. In the presence of verapamil or in the absence of external Ca++, DPB may increase cross-bridge cycling by activating an unknown mechanism that is not dependent on an increase in MLC phosphorylation.