Endothelin-1 induced contraction of rat aorta in Ca2(+)-free medium independent of phosphatidylinositol 4,5-bisphosphate (PIP2) breakdown

Dual role of PKC in modulating pharmacomechanical coupling in fetal and adult cerebral arteries

This study tested the hypothesis that protein kinase C (PKC) has dual regulation on norepinephrine (NE)-mediated inositol 1,4, 5-trisphosphate [Ins (1,4,5)P(3)] pathway and vasoconstriction in cerebral arteries from near-term fetal ( approximately 140 gestational days) and adult sheep. Basal PKC activity values (%membrane bound) in fetal and adult cerebral arteries were 38 +/- 4% and 32 +/- 4%, respectively. In vessels of both age groups, the PKC isoforms alpha, beta(I), beta(II), and delta were relatively abundant. In contrast, compared with the adult, cerebral arteries of the fetus had low levels of PKC-epsilon. In response to 10(-4) M phorbol 12,13-dibutyrate (PDBu; PKC agonist), PKC activity in both fetal and adult cerebral arteries increased 40-50%. After NE stimulation, PKC activation with PDBu exerted negative feedback on Ins(1,4,5)P(3) and intracellular Ca(2+) concentration ([Ca(2+)](i)) in arteries of both age groups. In turn, PKC inhibition with staurosporine resulted in augmented NE-induced Ins(1,4,5)P(3) and [Ca(2+)](i) responses in adult, but not fetal, cerebral arteries. In adult tissues, PKC stimulation by PDBu increased vascular tone, but not [Ca(2+)](i). In contrast, in the fetal artery, PKC stimulation was associated with an increase in both tone and [Ca(2+)](i). In the presence of zero extracellular [Ca(2+)], these PDBu-induced responses were absent in the fetal vessel, whereas they remained unchanged in the adult. We conclude that, although basal PKC activity was similar in fetal and adult cerebral arteries, PKC's role in NE-mediated pharmacomechanical coupling differed significantly in the two age groups. In both fetal and adult cerebral arteries, PKC modulation of NE-induced signal transduction responses would appear to play a significant role in the regulation of vascular tone. The mechanisms differ in the two age groups, however, and this probably relates, in part, to the relative lack of PKC-epsilon in fetal vessels.

Eicosanoid-induced Ca2+ release and sustained contraction in Ca(2+)-free media are mediated by different signal transduction pathways in rat aorta

1. The effects of 12-O-tetradecanoyl 4 beta-phorbol 13-acetate (beta-TPA) on the inositol 1,4,5-trisphosphate (IP3) production, Ca2+ release from the intracellular Ca2+ stores and sensitization of contractile apparatus, induced by prostaglandin F2 alpha (PGF2 alpha) and U46619, a thromboxane A2-mimetic, were studied, using fura-2-loaded and -unloaded rat thoracic aortic strips. 2. Both eicosanoids had characteristic patterns of responses in Ca(2+)-free, 2 mM EGTA-containing solution (Ca(2+)-free solution). They induced transient increases in intracellular Ca2+ concentration ([Ca2+]i) without corresponding transient contraction, but produced delayed, sustained contraction, where [Ca2+]i was returned to the basal level. 3. Treatment with beta-TPA for 60 min reduced the eicosanoids-induced IP3 production, suggesting that the treatment inhibits PIP2 breakdown. 4. The treatment also attenuated [Ca2+]i transient induced by the eicosanoids, but not by caffeine (an IP3-independent releaser of stored Ca2+), in fura-2-loaded preparations incubated in Ca(2+)-free solution. 5. In contrast in the presence of beta-TPA, the sustained contractions evoked by the eicosanoids in Ca(2+)-free solution were potentiated, suggesting that the sites of actions of beta-TPA and the eicosanoids may differ from each other. 6. PGF2 alpha and U46619 utilize different and parallel signal transduction pathways to release Ca2+ by IP3 produced by PIP2 breakdown (beta-TPA-sensitive), and to increase the sensitivity of contractile apparatus, in which protein kinase C may not be involved (beta-TPA-insensitive).

Effects of protein kinase C activation on norepinephrine-induced phosphatidylinositide hydrolysis in intact rat aorta

The purpose of this study was to investigate the role of protein kinase C in the regulation of alpha 1-adrenoceptor-mediated phosphatidylinositide hydrolysis in intact vascular smooth muscle. Phorbol myristate acetate (0.1 and 1 microM) and staurosporine inhibited and potentiated, respectively, norepinephrine-induced inositol phosphate formation in intact rat aorta. In contrast, 30 microM prostaglandin F2 alpha, which activated protein kinase C to a similar magnitude as 1 microM phorbol myristate acetate, was without effect on norepinephrine-induced inositol phosphate formation. These results suggest that protein kinase C activated in response to physiologic agonists, but not in response to phorbol esters, may be compartmentalized within the smooth muscle cell.

Activation of phospholipase D by endothelin-1 and other pharmacological agents in rabbit iris sphincter smooth muscle

The stimulation of phospholipase D (PLD) activity by endothelin-1 (ET1) was investigated in rabbit iris sphincter prelabelled with [3H]myristic acid. In the presence of 0.5% ethanol, ET1 caused a time- and dose-dependent increase in the production of [3H]phosphatidylethanol ([3H]PEt). Within 30 s the peptide increased PEt formation by 30% and after 5 min increased it by 140%. The EC50 value for ET1-stimulated PEt formation was found to be 30 nM. This value is appreciably lower than the EC50 we previously obtained for ET1-induced inositol trisphosphate production (45 nM), but considerably higher than that for arachidonic acid release (1 nM). PEt formation was significantly stimulated by prostaglandin F20, phorbol 12,13-dibutyrate (PDBu), chloroform, A23187 and A1F4-, but it was not affected by carbachol or the platelet-activating factor. PDBu-stimulated PEt formation was blocked by staurosporine and it was not potentiated by A23187. Staurosporine had no effect on ET1-stimulated PEt formation. Our data indicate that ET1 stimulation of PLD occurs independently of protein kinase C activation, phospholipase C activation and intracellular Ca2+ mobilization, and phospholipase A2 activation. In this tissue the ET1 receptor is probably coupled to the three phospholipases through several G-proteins, and this appears to be species and receptor type specific.