Phorbol dibutyrate induces contractions in bovine cerebral arteries by an extracellular calcium-independent mechanism

20-Hydroxyeicosatetraenoic acid-induced vasoconstriction and inhibition of potassium current in cerebral vascular smooth muscle is dependent on activation of protein kinase C

20-Hydroxyeicosatetraenoic acid (20-HETE), a cytochrome P450 metabolite of arachidonic acid, is a potent vasoconstrictor, and has been implicated in the myogenic activation of renal and cerebral arteries. We examined the role of protein kinase C (PKC) in the signal transduction pathway by which 20-HETE induces vasoconstriction and inhibition of whole-cell K+ current in cat cerebral vascular smooth muscle. 20-HETE induced a concentration-dependent constriction in isolated pressurized cat middle cerebral arteries (-29 +/- 8% at 1 microM). However, in the presence of an N-myristoylated PKC pseudosubstrate inhibitor peptide (MyrPsiPKC-I(19-27)), 20-HETE induced a concentration-dependent vasodilation (26 +/- 4% at 1 microM). In whole-cell voltage clamp studies, application of 20-HETE inhibited whole-cell K+ current recorded in cat cerebral vascular smooth muscle cells, an effect that was attenuated by MyrPsiPKC-I(19-27). Further evidence for the role of PKC activation in response to 20-HETE is the finding that 20-HETE increased the phosphorylation of myristoylated, alanine-rich PKC substrate in cultured cat cerebral vascular smooth muscle cells in a concentration- and PKC-dependent manner. These data provide evidence that PKC is an integral part of the signal transduction pathway by which 20-HETE elicits vasoconstriction of cerebral arteries and inhibition of whole-cell K+ current in cat cerebral vascular smooth muscle.

Methodological considerations for the measurement of protein kinase C translocation in intact smooth muscle

This study investigated several potential artifacts that may influence agonist-induced distribution of protein kinase C (PKC) activity between cytosolic and membrane fractions of intact smooth muscle. Protein kinase C activity in the membrane fraction prepared from rat aorta exposed to phorbol myristate acetate (PMA) was only partially extracted by 0.2% Triton (T)X-100, while 1% TX-100, or repeated 0.2% TX-100 extractions completely extracted PKC activity. Extraction of PKC activity from the membrane fraction with TX-100 concentrations of 0.2% or higher was problematic, however, since TX-100 concentrations as low as 0.04% nearly abolished Ca(2+)+ phosphatidyserine+diolein-induced phosphorylation of histone substrate in the PKC assay. Substitution of PMA for diolein, however, restored histone phosphorylation to the level observed in the absence of TX-100. Triton X-100 concentrations as low as 0.025% also abolished Ca(2+)-induced histone phosphorylation, while Ca(2+)+ phosphatidylserine-induced phosphorylation was little affected. In contrast to our previous demonstration that exposure of rat aorta to phorbol ester increased PKC activity in the membrane fraction in aorta washed in Ca(2+)-free solution following phorbol ester exposure (Chuprun et al., Am J Physiol 261:C675-C684, 1991; Bazan et al., Eur J Pharmacol-Molec Pharmacol Section 227:343-348, 1992), PMA decreased PKC activity in the initial 0.2% TX-100 extraction of the membrane fraction in the absence of tissue wash in Ca(2+)-free solution following PMA exposure. This study, along with our previous reports, suggest that partial PKC extraction from the membrane, and Ca(2+)-dependent homogenization-induced translocation of PKC from the cytosol to the membrane fraction, may complicate measurements of agonist-induced PKC translocation. The reliability of PKC assays in crude fractions may be increased in the presence of TX-100, due to the ability of TX-100 to inhibit Ca(2+)-induced phosphorylation, and through the substitution of PMA for diolein, which maximally stimulates PKC in the presence of detergent.

Intracellular Ca2+ release in cerebral arteries

Vasoconstricting agonists elevate the intracellular Ca2+ concentration and induce tension development in vascular smooth muscle cells by inducing both Ca2+ influx from the extracellular space and Ca2+ release from cellular stores. The relative importance of Ca2+ release has been found to vary between different sites in the vasculature. This review examines the role of Ca2+ release in the activation of cerebral arteries produced by several vasoconstricting stimuli. Although the activation of cerebral arteries by agonists such as 5-hydroxytryptamine and noradrenaline has typically been found to have little dependence on Ca2+ release, other vasoconstrictors such as thromboxane A2, which may be released from the endothelium by other agonists, appear to induce a substantial intracellular Ca2+ release in cerebral arteries. The limited efficacy of Ca2+ influx blockers in the treatment of delayed cerebrovascular constriction occurring as a result of subarachnoid haemorrhage suggests that intracellular mechanisms such as Ca2+ release and/or the activation of protein kinase C may be important determinants of vasoconstriction under pathological conditions.