A phorbol ester has dual actions on the mechanical response in the rabbit mesenteric and porcine coronary arteries.

Abstract

SUMMARY1. To clarify the role of protein kinase C in the mechanical response, the effects of 12-o-tetradecanoylphorbol-13-acetate (TPA), an activator ofprotein kinase C, were investigated on intact and skinned smooth muscle preparations of the rabbit mesenteric artery.2. TPA (0-1 uM) showed dual actions (initial enhancement followed by inhibition during long exposure) on the K+-induced contraction. The enhancement was marked in the presence of 39 mM-K+ but inhibition was the predominant effect in the presence of 128 mM-K+.3. Addition of2-6 mM-Ca2+ to a Ca2+-free solution containing 2 mm-EGTA following application of A23187 (1 /LM), produced contraction. TPA showed the same dual actions on this Ca2+-induced contraction.4. In chemically skinned muscles, TPA increased the amplitude of Ca2+-induced contractions evoked by low concentrations of Ca2+ (0-1-0{3 /LM), but reduced those evoked by high concentrations of Ca2+ (1-10 /M). Both actions of TPA were facilitated in the presence of phosphatidylserine (PS).5. TPA with PS had no effect on the Ca2+-independent contraction evoked in relaxing solution containing 10 mM-EGTA and 4 mM-Mg ATP following application of adenosine-5-o-3-thiotriphosphate (ATPYS) and 0-3 ,uM-Ca2 .6. The amount of Ca2+ stored in cells estimated from the amplitude of the caffeine-induced contraction was not modified by application of TPA with PS in skinned or intact muscle tissues.7. The effects of TPA were investigated on the Ca2+ transient measured from the intensity of fluorescence of quin-2 in dispersed cell suspensions prepared from the porcine coronary artery. TPA had no effect on the Ca2+ transient in high K+ but enhanced the amplitude of the contraction.8. Amplitudes of the tonic response evoked by 39 mM-K+ in intact muscle tissues and the contraction induced by 0 3 /sM-Ca2+ in skinned muscle were much the same.TPA with PS enhanced the amplitudes of both contractions to the same extent. 9. From the above results, we concluded that TPA shows dual actions on the contractile machinery and may act on the regulatory systems of contractile proteins. 17-2 T. ITOH AND OTHERSBoth excitatory and inhibitory actions ofTPA depended on the concentration ofCa2 .However, the physiological action of protein kinase C as estimated from the action of TPA seems to be related to an excitatory action on the contractile machinery.

Mitogen-Activated Protein Kinases Partially Regulate Endothelin-1-Induced Contractions through a Myosin Light Chain Phosphorylation- Independent Patyway.

“…1B). Although Ca 2+ -and calmodulin-dependent phosphorylation of the 20 kDa MLC are widely believed to be essential for the initiation of smooth muscle contraction [16], phorbol ester, an activator of protein kinase C, also increases contraction, MLC phosphorylation, and the unloaded shortening velocity induced by low concentrations of [Ca 2+ ] i in skinned smooth muscle [3,14,20]. Therefore, the mechanism(s) responsible for smooth muscle contraction are not solely dependent on [Ca 2+ ] i .…”

Effects of pinacidil on contractile proteins in high K+‐treated intact, and in β‐escin‐treated skinned smooth muscle of the rabbit mesenteric artery

“…A phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), activates protein kinase C and enhances the Ca2+-induced contraction in skinned smooth muscle strips (Itoh et al, 1986b;Fujiwara et al, 1988). As shown in Ca2+.…”

“…DG or its analogue slowly enhances Ca2 +-induced contraction in skinned smooth muscles through an activation of protein kinase C (Nishizuka, 1984;Itoh et al, 1986b). The inhibition of protein kinase C (described above) by pinacidil may relate to the much stronger inhibition of the tonic contraction induced by 128 mm K+ than the phasic ones.…”

Actions of second messengers synthesized by various spasmogenic agents and their relation to mechanical responses in dog tracheal smooth muscle

“…Skinned tissues were prepared by use of saponin (40y4gml-', for 20min), as described previously (Itoh et al, 1986). To prevent deterioration of the Ca-sensitivity of the contractile proteins, 0.1 /IM calmodulin was present throughout the experiments.…”

“…To prevent deterioration of the Ca-sensitivity of the contractile proteins, 0.1 /IM calmodulin was present throughout the experiments. The tension-pCa relationship was obtained by cumulative application of solutions containing various Ca concentrations and buffered with 4 mm EGTA (Itoh et al, 1986).…”