Inhibition of Bradykinin-Induced Phosphoinositide Hydrolysis and Ca Mobilisation by Phorbol Ester in Rat Cultured Vascular Smooth Muscle Cells

Regulation of the increase in inositol phosphate (IP) production and intracellular Ca²⁺ concentration ([Ca²⁺]_i by protein kinase C (PKC) was investigated in cultured rat vascular smooth muscle cells (VSMCs). Pretreatment of VSMCs with phorbol 12-myristate 14-acetate (PMA, 1 μM) for 30 min almost abolished the BK-induced IP formation and Ca²⁺ mobilisation. This inhibition was reduced after incubating the cells with PMA for 4 h, and within 24 h the BK-induced responses were greater than those of control cells. The concentrations of PMA giving a half-maximal (pEC₅₀) and maximal inhibition of BK induced an increase in [Ca²⁺]_i, were 7.8 ± 0.3 M and 1 μM, n = 8, respectively. Prior treatment of VSMCs with staurosporine (1 μM), a PKC inhibitor, inhibited the ability of PMA to attenuate BK-induced responses, suggesting that the inhibitory effect of PMA is mediated through the activation of PKC. Paralleling the effect of PMA on the BK-induced IP formation and Ca²⁺ mobilisation, the translocation and downregulation of PKC isozymes were determined by Western blotting with antibodies against different PKC isozymes. The results revealed that treatment of the cells with PMA for various times, translocation of PKC-, βI, βII, δ, ε, and ζ isozymes from the cytosol to the membrane were seen after 5 min, 30 min, 2 h, and 4 h of treatment. However, 24-h treatment caused a partial downregulation of these PKC isozymes in both fractions. Treatment of VSMCs with 1 μM PMA for either 1 or 24 h did not significantly change the K_D and B_max of the BK receptor for binding (control: K_D = 1.7 ± 0.2 nM; B_max = 47.3 ± 4.4 fmol/mg protein), indicating that BK receptors are not a site for the inhibitory effect of PMA on BK-induced responses. In conclusion, these resuts demonstrate that translocation of PKC-, βI, βII, δ, ε, and ζ induced by PMA caused an attenuation of BK-induced IPs accumulation and Ca²⁺ mobilisation in VSMCs.     

The phorbol ester TPA increases the affinity of exocytosis for calcium in ‘leaky’ adrenal medullary cells

Exposure of ‘leaky’ bovine adrenal medullary cells to the phorbol ester TPA causes a shift in the calcium-activation curve to lower calcium concentrations without altering the levels of secretion at the extremes of the activation curve. These results are consistent with a role for protein kinase C in exocytosis.     

Regulation of 5-hydroxytryptamine-induced signal transduction in canine cultured aorta smooth muscle cells by phorbol ester.

Regulation of the increase in inositol phosphates (IPs) production and intracellular Ca2+ concentration ([Ca2+]i) by protein kinase C (PKC) was investigated in cultured canine aorta smooth muscle cells (ASMCs). Stimulation of ASMCs by 5-hydroxytryptamine (5-HT) led to IPs formation and caused an initial transient [Ca2+]i peak followed by a sustained elevation of [Ca2+]i in a concentration-dependent manner. Pretreatment of ASMCs with phorbol 12-myristate 13-acetate (PMA) for 30 min almost abolished the 5-HT-induced IPs formation and Ca2+ mobilization. This inhibition was reduced after long-term incubating the cells with PMA. Prior treatment of ASMCs with staurosporine or GF109203X, PKC inhibitors, inhibited the ability of PMA to attenuate 5-HT-induced responses, suggesting that the inhibitory effect of PMA is mediated through the activation of PKC. In parallel with the effect of PMA on the 5-HT-induced IP formation and Ca2+ mobilization, the translocation and down-regulation of PKC isozymes were determined by Western blotting with antibodies against different PKC isozymes. The results revealed that treatment of ASMCs with PMA for various times, translocation of PKC-alpha, betaI, betaII, delta, epsilon, theta, and zeta isozymes from the cytosol to the membrane was seen after 5-min, 30-min, 2-h, and 4-h treatment. However, 24-h treatment caused a partial down-regulation of these PKC isozymes. In conclusion, these results demonstrate that translocation of PKC-alpha, betaI, betaII, delta, epsilon, theta, and zeta induced by PMA caused an attenuation of 5-HT-induced IPs accumulation and Ca2+ mobilization in ASMCs.     

Tumour-promoting phorbol esters inhibit agonist-induced phosphatidate formation and Ca flux in human platelets

Tumour-promoting phorbol esters (phorbol-12-myristate-13-acetate, PMA; phorbol-12,13-dibutyrate, PDBu) but not 4β-phorbol, activate protein kinase C. Using human platelets pre-labelled with quin2 or ³²PO₄ we examined the effects of these compounds on human platelet cytosolic free Ca²⁺ ([Ca²⁺]_j) and on [³²]phosphatidic acid ([³²P]PtdOH). PMA and PDBu, but not 4β-phorbol inhibited thrombin-, PAF- and vasopressin-induced elevation of [Ca²⁺], and [²⁺P]PtdOH formation. It is suggested that protein kinase C may act to terminate the transduction processes that link receptor occupancy to cellular activation.     

Heparin recovers AT1 receptor and its intracellular signal transduction in cultured vascular smooth muscle cells

Although vascular smooth muscle cells (VSMCs) are widely used in cardiovascular research, their phenotypic change under various culture conditions is problematic to evaluate the experimental results obtained. The levels of angiotensin (Ang) type 1/2 (AT1/AT2) receptors as well as contractile and structural proteins are degraded through culture passages. The present study demonstrated that heparin recovered Ang receptors and differentiation markers, such as desmin, SM-22 and smooth muscle alpha-actin in VSMCs at the ninth passage. Heparin also potenciated Ang II-induced activation for ERK1/2 and p38. These results suggest a potential value of heparin-treated VSMCs as the model for analysis of Ang-mediated signal transduction under physiological condition.     

Cyclic GMP-dependent protein kinase activation in canine tracheal smooth muscle by methacholine and sodium nitroprusside

Methacholine (3 μM) and sodium nitroprusside (300 μM) increased cGMP-dependent protein kinase activity ratios (activity without cGMP divided by activity with 2 μM cGMP) in canine tracheal smooth muscle from a control value of 0.47 to 0.55 and 0.71, respectively. This correlates with 3-fold and 6-fold increases in cGMP concentrations in response to methacholine and sodium nitroprusside, respectively. Addition of charcoal to the homogenizing buffer prior to homogenization had no significant effect on the cGMP-dependent protein kinase response to either agent, suggesting that activation of the enzyme was not occurring as a result of cGMP release during homogenization. In order to limit cGMP dissociation from cGMP-dependent protein kinase during the assay procedure, it was necessary to perform assays at a reduced temperature (0°C) and with an abbreviated incubation time (2.5 min). When assayed at 30°C, activated cGMP-dependent protein kinase rapidly lost activity. This inactivation occurred whether the enzyme had been activated exogenously, by exposing a supernatant fraction of canine trachealis to 0.1 μM cGMP, or endogenously, by treating intact canine trachealis with methacholine or sodium nitroprusside. By assaying instead at 0°C, the inactivation of cGMP-dependent protein kinase was minimized. Therefore, the activity ratio obtained by this new modified assay provided an estimate of the endogenous activation state of cGMP-dependent protein kinase. The data indicate that cGMP responses in canine trachealis to both methacholine and sodium nitroprusside are functionally linked to activation of cGMP-dependent protein kinase and are consistent with the hypothesis that cGMP, via cGMP-dependent protein kinase activation, regulates smooth muscle contractility.     

Muscarinic and Quisqualate Receptor-Induced Phosphoinositide Hydrolysis in Primary Cultures of Striatal and Hippocampal Neurons. Evidence for Differential Mechanisms of Activation

Several neurotransmitters activate polyphosphoinositide (PPI) hydrolysis in CNS neurons as the first step of a transmembrane signalling cascade that may lead to neuronal ircuit modulation. Muscarinic and quisqualate rejceptor-riggered PPI hydrolysis was investigated in neuronal primary cultures. A clear increase in inositol phosphates (Ins-Ps) was detected as early as 15 s after the agonist addition;! at this time, the increases of inositol 1,4,5-trisphosphate (measured by HPLC) were relatively larger with respect to the other Ins-Ps. Ins-P accumulation was maintained in part in a. Ca²⁺-free medium, excluding that Ca²⁺ entry is the fundamental step of the receptor-induced PPI hydrolysis. Acute dell pre-treatment with phorbol dibutyrate, an activator of protein kinase C, was able to inhibit 50% of the response to carbachol, and almost completely the quisqualate effect, suggesting a negative feedback modulation by the enzyme. Finaljly, per tussis toxin failed to inhibit muscarinic responses, whereas it blocked ±70% of the quisqualate stimulation. The two receptors therefore appear coupled to phosphodiesterase by two different G proteins. The comparison of the results obtained by stimulating the two receptor systems suggests that the generation of the same intracellular signal at two distinct receptor types may occur by different coupling mechanisms, and be differently regulated even in the same neuronal preparations.     

Glucose upregulates plasminogen activator inhibitor-1 gene expression in vascular smooth muscle cells

We investigated the effects of high concentrations of glucose on plasminogen activator inhibitor-1 (PAI-1) gene expression in cultured rat vascular smooth muscle cells (VSMC). In response to a high glucose concentration (27.5 mM), PAI-1 mRNA increased within 2 h, peaked at 4 h, remained elevated for another 4 h, then decreased to basal levels at 24 h. On the other hand, mannose at the same concentration (22.5 mM mannose plus 5.5 mM glucose) as an osmotic control had little effect on PAI-1 mRNA expression. The expression of PAI-1 mRNA that was also increased by H(2)O(2), angiotensin II, or phorbol myristate acetate, was reversed by the MAPK kinase (MEK) inhibitor PD98059 or the specific protein kinase C (PKC) inhibitor GF109203X. High glucose appeared to activate MAPK and PKC in VSMC judging from Elk-1 and AP-1 activation, respectively. PD98059 inhibited and GF109203X prevented subsequent PAI-1 induction by glucose. These results suggest that glucose at high concentrations induces PAI-1 gene expression in VSMC at least partially via MAPK and PKC activation. This direct effect of glucose might have important implications for the increased plasma concentrations of PAI-1 and possibly atherosclerosis that are associated with diabetes.     

Phorbol Myristate Acetate Inhibition of Phospholipase C Activation by Carbachol in Slices of Rat Brain Cortex Is a Delayed and Indirect Effect

We examined the effect of phorbol esters on phospholipase C activation in rat brain cortical slices and membranes. There was little effect of concurrent addition of phorbol 12-myristate 13-acetate (PMA) with carbachol on phosphoinositide breakdown due to carbachol over a 1-h incubation of brain slices. However, if slices were preincubated for 3 h with 1 microM PMA or 200 microM sphingosine before addition of carbachol, there was a 35-50% inhibition of phosphoinositide breakdown. There was also a marked loss of protein kinase C (PKC) activity from both cytosol and membranes after a 3-h exposure to PMA. The loss in responsiveness to the muscarinic agonists in slices was not reflected in carbachol-stimulated phospholipase C activation using isolated membranes. However, the decrease in carbachol-induced phosphoinositide breakdown seen in slices after a 3-h exposure to PMA was abolished if the extracellular K+ concentration was elevated from 5.9 to 55mM. Because elevation of the K+ level induces depolarization and increases Ca2+ entry, we examined the effect of ionomycin, a Ca2+ ionophore. Ionomycin potentiated the effects of carbachol on phosphoinositide breakdown but was unable to reverse the effects of a 3-h incubation with PMA. Because apamin, an inhibitor of Ca2(+)-dependent K+ channels, mimicked the effects of exposure to PMA for 3 h, it is possible that these channels are involved in muscarinic cholinergic regulation of phosphoinositide breakdown in rat brain slices. These results support the hypothesis that prolonged PMA treatment in rat brain cortex has no direct effect on phospholipase C activation by muscarinic cholinergic stimulation.     

Muscarinic agonists acting through M2 acetylcholine receptors stimulate the migration of an NO-sensitive guanylyl cyclase to the plasma membrane of bovine tracheal smooth muscle

Muscarinic agonists acting on bovine tracheal smooth muscle (BTSM) induce two separate cGMP signals, one at 20?sec associated with NO-sensitive-soluble-guanylyl-cyclase (NO-sGC) and another at 60?sec, linked to natriuretic-peptide-GC. The 20-sec-cGMP novel cascade starts with mAChRs, via unknown components, activates an NO-sGC. To unravel this cascade, in crude membranes isolated from intact BTSM strips exposed to muscarinic agonists, we detected GC activities increments at 20?sec and 60?sec. The 20-sec-GC is a NO-sensitive-GC, identified as α₁β₁-heterodimer. In reconstitution experiments with purified plasma membranes and cytosol, muscarinic agonists induced an NO-sGC migration in a dose-dependent manner, being inhibited by muscarinic antagonists displaying an M₂AChR profile and blocked by PTX, suggesting the involvement of G_o/G_i proteins. The NO-sGC related to migration was isolated and identified as an α₁β₁-heterodimer. This work shows that muscarinic agonists in BTSM induce a massive and selective α₁β₁-NO-sGC migration from cytoplasm to plasma membranes being responsible for the 20-sec-cGMP signal.     

Pathophysiology of voltage-gated K channels in vascular smooth muscle cells: Modulation by protein kinases

In this review, the pathological alteration and clinical relevance of voltage-gated K⁺ (Kv) channels and their specific regulation by protein kinase-dependent signaling in vascular smooth muscle cells are described, particularly focusing on the pulmonary vasculature. The physiological relevance, channel characteristics, pharmacological modulation, and expression of Kv channels vary between different arterial beds and between subdivisions of arteries within those vascular beds. Although detailed signaling cascades regulating Kv channels are not clearly elucidated, it is known that the Kv channels in vascular smooth muscle cells can be tightly regulated by protein kinases C (PKC) and A (PKA). Alterations in Kv channel expression and function has been noted in pathological and pathophysiological conditions including hypertension (pulmonary and systemic), in diabetes and in individuals subjected to prolonged hypoxia (high altitude living). Vascular Kv channels are potential therapeutic targets in diseases such as pulmonary arterial hypertension and, therefore, it is important to understand the specific pharmacological modulation of Kv channel isoforms in different vascular beds.     

Pharmacological and functional characterization of bradykinin receptors in rat cultured vascular smooth muscle cells

The pharmacological properties of bradykinin receptors were characterized in rat cultured vascular smooth muscle cells (VSMCs) using [3H]-bradykinin as a ligand. Analysis of binding isotherms gave an apparent equilibrium dissociation constant (K(D)) of 1.2 +/- 0.2 nM and a maximum receptor density (Bmax) of 47.3 +/- 4.4 fmol/mg protein. The specific binding of [3H]-bradykinin to VSMCs was inhibited by the B2 receptor-selective agonists (bradykinin and kallidin) and antagonists ([D-Arg0, Hyp3, Thi5, D-Tic7, Oic8]-bradykinin (Hoe 140) and [D-Arg0, Hyp3, Thi(5,8), D-Phe7]-bradykinin) with an order of potency as kallidin = bradykinin = Hoe 140 > [D-Arg0, Hyp3, Thi(5,8), D-Phe7]-bradykinin, but not by a B1 receptor-selective agonist (des-Arg9-bradykinin) and antagonist ([Leu8, des-Arg9]-bradykinin). Stimulation of VSMCs by bradykinin produced a concentration-dependent inositol phosphate (IP) accumulation, and initial transient peak of [Ca2+]i with half-maximal responses (pEC50) were 7.53 and 7.69, respectively. B2 receptor-selective antagonists (Hoe 140 and [D-Arg0, Hyp3, Thi(5,8), D-Phe7]-bradykinin) significantly antagonized the bradykinin-induced responses with pK(B) values of 8.3-8.7 and 7.2-7.9, respectively. Pretreatment of VSMCs with pertussis toxin (100 ng/ml, 24 h) did not alter the bradykinin-induced inositol phosphate accumulation and [Ca2+]i changes in VSMCs. Removal of external Ca2+ led to a significant attenuation of responses induced by bradykinin. Influx of external Ca2+ was required for the bradykinin-induced responses, since Ca2+-channel blockers, nifedipine, verapamil, and Ni2+, partially inhibited the bradykinin-induced IP accumulation and Ca2+ mobilization. These results demonstrate that bradykinin stimulates phosphoinositide hydrolysis and Ca2+ mobilization via a pertussis toxin-insensitive G-protein in rat VSMCs. Bradykinin B2 receptors may be predominantly mediating IP accumulation and subsequently induction of Ca2+ mobilization may function as the transducing mechanism for bradykinin-stimulated contraction of vascular smooth muscle.     

培养大鼠主动脉血管平滑肌细胞产生巨噬细胞集落刺激因子及其受体的表达

本研究用培养大鼠主动脉血管平滑肌细胞（ＶＳＭＣｓ）,结果如下：（１）用生物活性检测法发现ＶＳＭＣｓ无血清条件培养液可刺激巨噬细胞集落形成,其作用能被抗巨噬细胞集落刺激因子（ＭＣＳＦ）抗体抑制;（２）用免疫细胞化学技术证明ＶＳＭＣｓ存在ＭＣＳＦ受体;（３）用Ｎｏｒｔｈｅｒｎ ｂｌｏｔ技术证明ＶＳＭＣｓ有ＭＣＳＦ及其受本的ｍＲＮＡ表达,血清刺激使两者表达明显增强。本研究首次报道了培养大鼠主动脉ＶＳＭＣ     

Activation of muscarinic receptors in porcine airway smooth muscle elicits a transient increase in phospholipase D activity

Phospholipase D (PLD) is a phosphodiesterase that catalyses hydrolysis of phosphatidylcholine to produce phosphatidic acid and choline. In the presence of ethanol, PLD also catalyses the formation of phosphatidylethanol, which is a unique characteristic of this enzyme. Muscarinic receptor-induced changes in the activity of PLD were investigated in porcine tracheal smooth muscle by measuring the formation of [³H]phosphatidic acid ([³H]PA) and [³H]phosphatidylethanol ([³H]PEth) after labeling the muscle strips with [³H]palmitic acid. The cholinergic receptor agonist acetylcholine (Ach) significantly but transiently increased formation of both [³H]PA and [³H]PEth in a concentration-dependent manner (>105–400% vs. controls in the presence of 10^–6 to 10^–4 M Ach) when pretreated with 100 mM ethanol. The Ach receptor-mediated increase in PLD activity was inhibited by atropine (10^–6 M), indicating that activation of PLD occurred via muscarinic receptors. Activation of protein kinase C (PKC) by phorbol-12-myristate-13-acetate (PMA) increased PLD activity that was effectively blocked by the PKC inhibitors calphostin C (10^–8 to 10^–6 M) and GFX (10^–8 to 10^–6 M). Ach-induced increases in PLD activity were also significantly, but incompletely, inhibited by both GFX and calphostin C. From the present data, we conclude that in tracheal smooth muscle, muscarinic acetylcholine receptor-induced PLD activation is transient in nature and coupled to these receptors via PKC. However, PKC activation is not solely responsible for Ach-induced activation of PLD in porcine tracheal smooth muscle.     

Bradykinin B<Subscript>2</Subscript> receptor-mediated proliferation via activation of the Ras/Raf/MEK/MAPK pathway in rat vascular smooth muscle cells

It has been suggested that bradykinin (BK) plays an important role in regulating neointimal formation after vascular injury. However, implication of BK in the growth of rat vascular smooth muscle cells (VSMCs) is controversial. Therefore, we examined the mitogenic effect of BK on VSMCs associated with activation of mitogen-activated protein kinase (MAPK). Both [(3)H]thymidine incorporation and p42/p44 MAPK phosphorylation were activated by BK in time- and concentration-dependent manners. Pretreatment of these cells with neither pertussis toxin nor cholera toxin attenuated the BK-induced responses. Pretreatment of VSMCs with Hoe 140 (a selective B(2) receptor antagonist), U73122 (an inhibitor of phospholipase C), and BAPTA/AM (an intracellular Ca(2+) chelator) inhibited both [(3)H]thymidine incorporation and p42/p44 MAPK phosphorylation in response to BK. BK-induced [(3)H]thymidine incorporation and p42/p44 MAPK phosphorylation were inhibited by pretreatment of VSMCs with tyrosine kinase inhibitors (genistein and herbimycin A), protein kinase C (PKC) inhibitors (staurosporine, Go-6976, and Ro-318220), an MAPK kinase inhibitor (PD98059), and a p38 MAPK inhibitor (SB203580). Overexpression of the dominant negative mutants, H-Ras-15A and Raf-N4, suppressed p42/p44 MAPK activation induced by BK and PDGF-BB, indicating that Ras and Raf may be required for activation of these kinases. From these results, we concluded that the mitogenic effect of BK is mediated through activation of the Ras/Raf/MEK/MAPK pathway similar to that of PDGF-BB. BK-mediated MAPK activation was modulated by Ca(2+), PKC, and tyrosine kinase all of which are associated with cell proliferation in rat cultured VSMCs.     

Regulation of the Release of Interleukin-6 from Human Astrocytoma Cells

Abstract: Recent evidence suggests that the level of interleukin-6 (IL-6) is elevated in Alzheimer's disease (AD) brains. IL-6 is produced by reactive glial cells and could potentially affect neuronal survival. Understanding the biochemical mechanism that regulates the production and release of IL-6 by astrocytic cells may help to identify potential targets for therapeutic intervention in AD. In the present study, glial fibrillary acidic protein-positive human U373MG astrocytoma cells were used as a model of reactive astrocytes. Production of IL-6 in response to drug treatment was monitored with an ELISA assay. Histamine (1–100 µ M ), substance P (SP; 1–100 n M ), and human interleukin-1β (IL-1β; 1–30 p M ) stimulated the release of IL-6 in a time- and concentration-dependent manner, with EC₅₀ values of 4.5 µ M , 8 n M , and 4.5 p M , respectively. The respective effects of histamine, SP, and IL-1β were effectively blocked by the histamine H₁, SP, and IL-1 receptor antagonists, supporting a receptor-mediated event for these agents. Both histamine and SP enhanced the formation of inositol phosphates and increase intracellular calcium levels, suggesting that the phosphatidyl-inositol bisphosphate/protein kinase C pathway may be involved in the IL-6 release process. Indeed, phorbol 12-myristate 13-acetate, a protein kinase C activator, also evoked IL-6 release from the U373MG cells. On the other hand, IL-1β, which produces a much more robust release of IL-6 than histamine or SP, has no effect on inositol phosphate formation or intracellular calcium levels. The biochemical mechanism of the release of IL-6 in response to IL-1β remains to be elucidated.     

Jagged-1/Notch3 signaling transduction pathway is involved in apelin-13-induced vascular smooth muscle cells proliferation

The apelin/apelin receptor （APJ, apelin-angiotensin receptor-like 1） system is a newly deorphanized G protein- coupled receptor system. Both apelin and APJ that are important regulatory factors are expressed in the cardio- vascular system. Our previous studies demonstrated that apelin-13 significantly stimulated vascular smooth muscle cell （VSMC） proliferation. In this paper, our data sug- gested that the Jagged-l/Notch3 signaling transduction pathway is involved in apelin-13-induced VSMC prolifer- ation by promoting the expression of Cyclin D1. Results indicated that apelin-13 stimulates the proliferation of VSMC and the expression of Jagged-1 and Notch3 in con- centration- and time-dependent manners. The increased expression of Jagged-1 and Notch3 induced by apelin-13 could be abolished by extracellular signal-regulated protein kinase （ERK） blockade. PD98059 （ERK inhibitor） can inhibit the activation of Jagged-I/Notch3 induced by apelin- 13. Down-regulation of Notch3 using small interfering RNA inhibits the expression of Cyclin DI and prevents apelin- 13-induced VSMC proliferation. In conclusion, Jagged-I/ Notch3 signaling transduction pathway is involved in VSMC proliferation induced by apelin-13.     

Inhibitory and stimulatory effects of phorbol ester on vasopressin-induced cellular responses in cultured rat aortic smooth muscle cells

In rat aortic smooth muscle cells, vasopressin (AVP) induces prostacyclin (PGI2) production, probably as the consequence of phospholipase C activation. Our study analyzes the effects of phorbol 12-myristate 13-acetate (PMA)-induced protein kinase C (PKC) activation on AVP-induced inositol 1,4,5-trisphosphate formation, cytosolic free Ca2+ concentration [( Ca2+]c), and PGI2 production. PMA rapidly decreased PKC activity in the cytosol of smooth muscle cells, while increasing it transiently in the membranes with a maximum around 20 min. Prior exposure of the cells to PMA resulted in a transient inhibition of both AVP-induced inositol 1,4,5-trisphosphate formation and [Ca2+]c rise. This was inversely correlated with membraneous PKC activity and partially reversed by the PKC inhibitor staurosporine. In contrast, pretreating the cells with PMA markedly potentiated A23187 or AVP-induced PGI2 production. Under those conditions, AVP-induced PGI2 production did not correlate either with PMA-induced membranous PKC activity or with AVP-induced PLC activation. However, this potentiating effect of PMA was reversed by staurosporine and was not mimicked by the 4 alpha-phorbol, an inactive analogue of PMA. Thus, the possibility is raised that, while inhibiting AVP-induced PLC activation, PMA-induced PKC activation increases the Ca2+ sensitivity of the cellular signaling system leading to PGI2 production.