Protein Kinase C Activation Stimulates Plasma Membrane Ca2+ Pump in Cultured Vascular Smooth Muscle Cells

PKC inhibition markedly enhances Ca2+ signaling and phosphatidylserine exposure downstream of protease-activated receptor-1 but not protease-activated receptor-4 in human platelets

BACKGROUND

Cytosolic calcium concentration is a critical regulator of platelet activation, and so platelet Ca(2+) signaling must be tightly controlled. Thrombin-induced Ca(2+) signaling is enhanced by inhibitors of protein kinase C (PKC), suggesting that PKC negatively regulates the Ca(2+) signal, although the mechanisms by which this occurs and its physiological relevance are still unclear.

OBJECTIVES

To investigate the mechanisms by which PKC inhibitors enhance thrombin-induced Ca(2+) signaling, and to determine the importance of this pathway in platelet activation.

METHODS

Cytosolic Ca(2+) signaling was monitored in fura-2-loaded human platelets. Phosphatidylserine (PS) exposure, a marker of platelet procoagulant activity, was measured by annexin V binding and flow cytometry.

RESULTS

PKC inhibition by bisindolylmaleimide-I (BIM-I) enhanced α-thrombin-induced Ca(2+) signaling in a concentration-dependent manner. PAR1 signaling, activated by SFLLRN, was enhanced much more strongly than PAR4, activated by AYPGKF or γ-thrombin, which is a potent PAR4 agonist but a poor activator of PAR1. BIM-I had little effect on α-thrombin-induced signaling following treatment with the PAR1 antagonist, SCH-79797. BIM-I enhanced Ca(2+) release from intracellular stores and Ca(2+) entry, as assessed by Mn(2+) quench. However, the plasma membrane Ca(2+) ATPase inhibitor, 5(6)-carboxyeosin, did not prevent the effect of BIM-I. PKC inhibition strongly enhanced α-thrombin-induced PS exposure, which was reversed by blockade of PAR1.

CONCLUSIONS

Together, these data show that when PAR1 is stimulated, PKC negatively regulates Ca(2+) release and Ca(2+) entry, which leads to reduced platelet PS exposure.

Gene expression and functional activity of sodium/calcium exchanger enhanced in vascular smooth muscle cells of spontaneously hypertensive rats

Effects of hypertension on the function of the Na+/Ca2+ exchanger (NCX) were investigated by analyzing vascular smooth muscle cells (VSMCs) from spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto (WKY) rats. Angiotensin II-induced 45Ca2+ efflux from VSMCs mediated by NCX was enhanced by up to 3-fold in SHR compared with WKY, whereas ionomycin-induced Ca efflux mediated by NCX was not different between SHR and WKY. The decline rate from the peak value of intracellular 45Ca2+ concentration ([Ca2+]i) mobilized by angiotensin II was decelerated by removal of extracellular sodium (Na+o) in SHR but not in WKY. Gene expressions of NCX subtype 1 and angiotensin II receptor type1A assessed by quantitative RT-PCR were increased by 1.3- and 1.5-fold, respectively in SHR compared with WKY. NCX protein was also increased 1.6-fold in SHR compared with WKY. MEK inhibitor, PD98059, partly blocked the Nao-dependent acceleration of the [Ca2+]i recovery rate and tyrosine kinase inhibitor, genistein, diminished it in SHR. Genistein decreased angiotensin II-induced Nao- dependent 45Ca2+ efflux. However, angiotensin II did not enhance the tyrosine phosphorylation of NCX. These results suggest that acceleration of Ca2+ efflux from VSMCs of SHR was at least partly due to the enhancement of functional activity of NCX via increased gene expression and tyrosine phosphorylation in connection with hypertension.

8Br-cGMP mediates relaxation of tracheal smooth muscle through PKA

In this study, guinea pig tracheal smooth muscle pre-contracted with histamine was relaxed by the addition of 100microM 8Br-cGMP, a non-hydrolyzable and cell-permeable analog for cGMP. This effect was not sensitive to cGMP-dependent protein kinase (PKG) inhibitors, whereas it was partially blocked by cAMP-dependent protein kinase (PKA) inhibitors. The relaxation observed was also reverted up to 50+/-8.5% by iberiotoxin, a selective inhibitor of large conductance, calcium-activated potassium channels (BK(Ca)). Our results indicate that there exists a crosstalk mechanism between cAMP and cGMP signaling pathways which lead to relaxation of guinea pig tracheal smooth muscle and also that BK(Ca) channels are involved to a certain extent in this phenomenon.

Regulation of vascular tone in animals overexpressing the sarcolemmal calcium pump

The mechanisms governing vascular smooth muscle tone are incompletely understood. In particular, the role of the sarcolemmal calcium pump PMCA (plasma membrane calmodulin-dependent calcium ATPase), which extrudes Ca2+ from the cytosol, and its importance compared with the sodium/calcium exchanger remain speculative. To test whether the PMCA is a regulator of vascular tone, we generated transgenic mice overexpressing the human PMCA4b under control of the arterial smooth muscle-specific SM22alpha promoter. This resulted in an elevated systolic blood pressure compared with littermate controls. In PMCA-overexpressing mice, endothelium-dependent relaxation of norepinephrine-preconstricted aortic rings to acetylcholine did not differ from wild type controls (76 +/- 8% versus 79 +/- 8% of maximum relaxation; n = 12, n.s.). De-endothelialized aortas of transgenic mice exhibited stronger maximum contraction to KCl (100 mmol/liter) compared with controls (86 +/- 6% versus 68 +/- 7% of reference KCl contraction at the beginning of the experiment; p <0.05). Preincubation of de-endothelialized vessels with the nitric oxide synthase (NOS) inhibitor l-NAME (l-N(G)-nitroarginine methyl ester) (10-5 mol/liter) resulted in a stronger contraction to KCl (p <0.05 versus without l-NAME), thus unmasking vasodilatory effects of inherent NO production. Maximum contraction to KCl after preincubation with l-NAME did not differ between PMCA mice and controls. In analogy to the results in PMCA-overexpressing mice, contractions of de-endothelialized aortas of neuronal NOS-deficient mice to KCl were significantly increased compared with controls (151 +/- 5% versus 131 +/- 6% of reference KCl contraction; p <0.05). In conclusion, our data suggest a model in which the sarcolemmal Ca2+ pump down-regulates activity of the vascular smooth muscle Ca2+/calmodulin-dependent neuronal NOS by a functionally relevant interaction. Therefore, the PMCA represents a novel regulator of vascular tone.

Antisense-inhibition of plasma membrane Ca2+ pump induces apoptosis in vascular smooth muscle cells

The effect of antisense oligodeoxynucleotides (ODNs) of plasma membrane Ca(2+)-pumping ATPase (PMCA) on rat aortic vascular smooth muscle cells (VSMCs) in primary culture was examined. More than 80% of the PMCA expressed in cultured VSMCs was the PMCA-1B subtype. Exposed to antisense ODNs against PMCA-1, not only the expression of the PMCA protein but also mRNA of PMCA-1B was diminished in a concentration-dependent manner. Extracellular Na(+)-independent (45)Ca(2+) efflux catalyzed via PMCA was inhibited with antisense ODNs. Both the resting and ionomycin- or ATP-stimulated levels of intracellular Ca(2+) were increased by antisense ODNs. Furthermore, prolonged treatment with antisense ODNs caused apoptosis in VSMCs. The occurrence of apoptosis was inhibited by FK506, a potent immunosuppressant. These results demonstrate that the PMCA was specifically inhibited by antisense ODNs and suggest that PMCA plays an important role in regulation of intracellular Ca(2+) concentrations, especially at the resting condition to prevent an occurrence of apoptosis that may be induced through the activation of calcineurin.

Effect of arachidonic acid on duodenal enterocyte ATPases

Duodenal ion transport processes are supported by ATPase enzymes in basolateral membranes of the enterocyte. In vivo studies have shown that long term n-6 poly-unsaturated fatty acid (PUFA) supplementation in rats causes increases in intestinal Ca absorption, coupled with a higher total calcium balance and bone calcium content. The present in vitro study was undertaken to test the effect of arachidonic acid (AA), a highly unsaturated (and thus physiologically potent) member of the n-6 PUFA family, on ATPases in enterocyte basolateral membranes isolated with a sorbitol density gradient procedure. This paper presents results which show that AA inhibits Na+,K+-ATPase in a dose-dependent manner (-67% of basal activity at a concentration of 30 microg/ml, P < 0.005) but that this effect is not mediated by protein kinase C, as shown by the use of the protein kinase C blocker calphostin (0.5 microM). Indomethacin (IDM) at 0.1 mM, a cyclo-oxygenase blocker, could also not reverse the inhibitory effect of AA on Na+,K+-ATPase. Ca2+-ATPase, on the other hand, is not affected significantly (-10%, P > 0.05) by arachidonic acid at 30 microg/ml.

Upregulation of duodenal calcium absorption by poly-unsaturated fatty acids: events at the basolateral membrane

Poly-unsaturated fatty acids, especially of the n-3 series, have a beneficial effect in treatment of osteoporosis in the elderly. Duodenal calcium absorption is a particularly vulnerable aspect of the development of this disease. It has been shown that the process of calcium transport through the rat duodenal enterocyte takes place in essentially three steps: entry of calcium through channels in the brush border (apical membrane), transcellular transport through the cytoplasm by calbindin and extrusion at the basolateral membrane by Ca(2+)-ATPase and a Ca(2+)-Na(+)exchanger which is driven by Na(+), K(+)-ATPase. This paper presents a hypothesis that poly-unsaturated fatty acids can modulate both Ca(2+)-ATPase and Na(+), K(+)-ATPase activity either by a direct action on the enzyme or by phosphorylation processes via protein kinases A and C and thus exert their positive influence on calcium absorption in this manner.

The mechanism of the decrease in cytosolic Ca2+ concentrations induced by angiotensin II in the high K(+)-depolarized rabbit femoral artery

1. Using front-surface fluorometry of fura-2-loaded strips, and measuring the transmembrane 45Ca2+ fluxes of ring preparations of the rabbit femoral artery, the mechanism underlying a sustained decrease in the cytosolic Ca2+ concentration ([Ca2+]i) induced by angiotensin II (AT-II) was investigated. 2. The application of AT-II during steady-state 118 mM K(+)-induced contractions caused a sustained decrease in [Ca2+]i following a rapid and transient increase in [Ca2+]i, while the tension was transiently enhanced. 3. When the intracellular Ca2+ stores were depleted by thapsigargin, the initial rapid and transient increase in [Ca2+]i was abolished, however, neither the sustained decrease in [Ca2+]i nor the enhancement of tension were affected. 4. Depolarization with 118 mM K+ physiological salt solution containing 1.25 mM Ba2+ induced a sustained increase in both the cytosolic Ba2+ concentration ([Ba2+]i) level and tension. However, the application of 10(-6) M AT-II during sustained Ba(2+)-contractions was found to have no effect on [Ba2+]i, but it did enhance tension. 5. After thapsigargin treatment, AT-II neither decreased nor increased the enhanced Ca2+ efflux rate induced by 118 mM K(+)-depolarization, whereas AT-II did increase the enhanced 45Ca2+ influx and the 45Ca2+ net uptake induced by 118 mM K(+)-depolarization. 6. Pretreatment with calphostin-C, partially, but significantly inhibited the decrease in [Ca2+]i induced by AT-II. 7. These findings therefore suggest that AT-II stimulates Ca2+ sequestration into the thapsigargin-insensitive Ca2+ stores, and thus induces a decrease in [Ca2+]i in the high external K(+)-stimulated rabbit femoral artery.

Increased Ca2+ sensitivity as a key mechanism of PKC-induced constriction in pressurized cerebral arteries

The effects of activating protein kinase C (PKC) with indolactam V (Indo-V) and 1,2-dioctanoyl-sn-glycerol (DOG) on smooth muscle intracellular Ca2+ concentrations ([Ca2+]i) and arterial diameter were determined using ratiometric Ca2+ imaging and video edge detection of pressurized rat posterior cerebral arteries. Elevation of intraluminal pressure from 10 to 60 mmHg resulted in an increase in [Ca2+]i from 74 +/- 5 to 219 +/- 8 nM and myogenic constriction. Application of Indo-V (0.01-3 microM) or DOG (0.1-30 microM) induced constriction and decreased [Ca2+]i to 140 +/- 11 and 127 +/- 12 nM, respectively, at the highest concentrations used. In the presence of Indo-V, the dihydropyridine Ca2+-channel-blocker nisoldipine produced nearly maximum dilation and decreased [Ca2+]i to 97 +/- 7 nM. In alpha-toxin-permeabilized arteries, the constrictor effects of Indo-V and DOG were not observed in the absence of Ca2+. Both PKC activators significantly increased the degree of constriction of permeabilized arteries at different [Ca2+]i. We conclude that 1) Indo-V- or DOG-induced constriction of pressurized arteries requires Ca2+ influx through voltage-dependent Ca2+ channels, and 2) PKC-induced constriction of pressurized rat cerebral arteries is associated with a decrease in [Ca2+]i, suggesting an increase in the Ca2+ sensitivity of the contractile process.

Adenosine inhibits the renal plasma-membrane (Ca2+ + Mg2+)-ATPase through a pathway sensitive to cholera toxin and sphingosine

Adenosine, a potent autacoid produced and released in kidneys, affects nearly all aspects of renal function, and an increase in cytosolic calcium has been implicated in adenosine effects. The aim of this work was to investigate whether adenosine modifies the calcium pump present in basolateral membranes of kidney proximal tubule cells. Adenosine exerts a biphasic influence on (Ca2+ + Mg2+)-ATPase activity. Inhibition occurs up to 0.1 microM and then gradually disappears as the adenosine concentration increases to 100 microM, an effect mimicked by the adenosine analog N6-cyclohexyladenosine, which preferentially binds to A1-type receptors. In contrast, the A2 receptor agonist 5', N-ethylcarboxamideadenosine is ineffective. The A1 receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine blocks the inhibitory effect of 0.1 microM adenosine and stimulates (Ca2+ + Mg2+)-ATPase activity in the presence of 1 mM adenosine, a concentration high enough to occupy the low-affinity A2 receptors. Inhibition by adenosine increases as medium ATP is lowered to micromolar concentrations, is maintained in the presence of pertussis toxin, and is completely abolished with 0.1 microM cholera toxin or 1 microM sphingosine. The inhibitory effect of adenosine can be reproduced by guanosine 5'-[gamma-thio]triphosphate, inositol 1,4, 5-trisphosphate or the diacylglycerol analog 12-O-tetradecanoylphorbol 13-acetate. In conjunction with the selectivity for its analogs and for its receptor agonist, the concentration profile of adenosine effects indicates that both inhibitory (A1) and stimulatory (A2) receptors are involved. The results obtained with the toxins indicate that a pathway that is modulated by G-proteins, involves a phospholipase C and a protein kinase C, and is affected by local variations in adenosine concentrations participates in the regulation of the (Ca2+ + Mg2+)-ATPase resident in basolateral membranes of kidney proximal tubules.

Mechanisms involved in the cellular calcium homeostasis in vascular smooth muscle: calcium pumps

The regulation of cytosolic Ca2+ homeostasis is essential for cells, and particularly for vascular smooth muscle cells. In this regulation, there is a participation of different factors and mechanisms situated at different levels in the cell, among them Ca2+ pumps play an important role. Thus, Ca2+ pump, to extrude Ca2+; Na+/Ca2+ exchanger; and different Ca2+ channels for Ca2+ entry are placed in the plasma membrane. In addition, the inner and outer surfaces of the plasmalemma possess the ability to bind Ca2+ that can be released by different agonists. The sarcoplasmic reticulum has an active role in this Ca2+ regulation; its membrane has a Ca2+ pump that facilitates luminal Ca2+ accumulation, thus reducing the cytosolic free Ca2+ concentration. This pump can be inhibited by different agents. Physiologically, its activity is regulated by the protein phospholamban; thus, when it is in its unphosphorylated state such a Ca2+ pump is inhibited. The sarcoplasmic reticulum membrane also possesses receptors for 1,4,5-inositol trisphosphate and ryanodine, which upon activation facilitates Ca2+ release from this store. The sarcoplasmic reticulum and the plasmalemma form the superficial buffer barrier that is considered as an effective barrier for Ca2+ influx. The cytosol possesses different proteins and several inorganic compounds with a Ca2+ buffering capacity. The hypothesis of capacitative Ca2+ entry into smooth muscle across the plasma membrane after intracellular store depletion and its mechanisms of inhibition and activation is also commented.

Protein kinases A and C phosphorylate purified Ca2+-ATPase from rat cortex, cerebellum and hippocampus

The plasma membrane Ca2+-ATPase (PMCA), the enzyme responsible for the maintenance of intracellular calcium homeostasis, is regulated by several independent mechanisms. In this paper we report that the protein kinases A and C differentially activate the Ca2+-ATPase purified from synaptosomal membranes of rat cortex, cerebellum and hippocampus. The effect of protein kinases was more pronounced for the cortical enzyme, whereas cerebellar and hippocampal Ca2+-ATPases were activated to a lesser degree. The preparation of Ca2+-ATPase contained the phosphoamino acids, i.e., P-Ser and P-Thr, indicating that the enzyme was purified in phosphorylated state. The phosphorylation of Ca2+-ATPase by PKA and PKC increased the amount of phosphoamino acids, but in a region-dependent manner. Using the specific antibodies against N-terminal portion of four main PMCA isoforms we have characterized the isoforms composition of Ca2+-ATPase purified from the nervous endings of examined brain areas. Our results indicate that the activity of calcium pump is related to its phosphorylated state, and that the phosphorylation is region-dependent. Moreover, the differences observed could be related to the composition of PMCA isoforms in the different brain areas. Phosphorylation of the plasma membrane Ca2+-ATPase appears to be a mechanism to control its activity. The results support also the possible involvement of PKA and PKC.

Protein kinase C: modulation of vasopressin-induced calcium influx and release in A7r5 vascular smooth muscle cells

This study was guided by the hypothesis that specific isoforms of protein kinase C may participate in modulating increases in intracellular Ca2+ that are induced by stimulation of vascular smooth muscle cells with vasopressin. Immunoblot analysis revealed that A7r5 vascular smooth muscle cells expressed conventional (alpha), novel (delta and epsilon), and atypical (iota/lambda and mu) isoforms of protein kinase C. Stimulation of fura-2-loaded cells with 20 nM vasopressin induced a rapid transient increase in the intracellular concentration of calcium that was followed by a slowly declining component which was above baseline throughout the period of observation. Cell fractionation studies showed that the calcium response was associated with (a) transient translocation of the alpha and delta isoforms of protein kinase C from the cytosolic fraction to the particulate-membrane fraction, (b) sustained translocation of the epsilon isoform, and (c) no translocation of iota/lambda or mu isoforms. Ratiometric and isobestic fluorescence analysis showed that vasopressin-induced Ca2+ influx and release were markedly inhibited in cells that were preincubated with either 1 microM phorbol 12-myristate 13-acetate, or 10 microM 1, 2 dioctanoyl-sn-glycerol, two structurally different activators of protein kinase C. In contrast, vasopressin-induced increases in intracellular Ca2+ were not significantly altered following preincubation with either 1 microM 4alpha-phorbol or 4alpha-phorbol 12,13-didecanoate, analogs of phorbol 12-myristate 13-acetate that do not activate protein kinase C. Moreover, the inhibitory effects of phorbol 12-myristate 13-acetate were prevented by treatment with 1 microM GF109203X, a potent inhibitor of protein kinase C. Taken together, these results show that direct activation of protein kinase C can negatively modulate vasopressin-induced Ca2+ influx and release in cultured vascular smooth muscle cells. They also show that stimulation with vasopressin induces translocation of specific isoforms of protein kinase C, an observation suggesting that one or more of these isoforms may participate in modulation of vasopressin-induced increases in intracellular Ca2+.

The plasma membrane calcium pump, its role and regulation: new complexities and possibilities

Significant progress has been achieved in elucidating the role of the plasma membrane Ca2(+)-ATPase in cellular Ca2+ homeostasis and physiology since the enzyme was first purified and physiology since the enzyme was first purified and cloned a number of years ago. The simple notion that the PM Ca2(+)-ATPase controls resting levels of [Ca2+]CYT has been challenged by the complexity arising from the finding of four major isoforms and splice variants of the Ca2+ pump, and the finding that these are differentially localized in various organs and subcellular regions. Furthermore, the isoforms exhibit differential sensitivities to Ca2+, calmodulin, ATP, and kinase-mediated phosphorylation. The latter pathways of regulation can give rise to activation or inhibition of the Ca2+ pump activity, depending on the kinase and the particular Ca2+ pump isoform. Significant progress is being made in elucidating subtle and more profound roles of the PM Ca2(+)-ATPase in the control of cellular function. Further understanding of these roles awaits new studies in both transfected cells and intact organelles, a process that will be greatly aided by the development of new and selective Ca2+ pump inhibitors.

High-performance liquid chromatographic analysis of staurosporine in vivo. Its translocation and pharmacokinetics in rats

The protein kinase inhibitor staurosporine (Stsp) has been used extensively to study physiological functions, biochemical mechanisms, and cancer therapy. Using an HPLC assay for Stsp developed in our laboratory, we find that only 0.7% of Stsp remains in circulating blood of rats 5 min after injection. In vitro, Stsp is adsorbed to red blood cells (RBC) weakly and reversibly. In vivo, all but 1.2-2.5% of Stsp injected is adsorbed by the heart and lungs in one passage through them, indicating that the endothelium acts as a major Stsp sink. Following initial adsorption, pharmacokinetic studies demonstrated that Stsp had a half-life of 51.6 min in plasma and 75.3 min in RBC. Thus, plasma Stsp was in the cancer therapy range of 1-10 ng/ml for 2.7 h following a bolus injection. This data indicates that a bolus injection of Stsp must be followed by a continuous infusion of low Stsp concentration for several days to produce the G1 arrest in cells necessary to stop cell proliferation.

Protein kinase C activation stimulates calcium transport in adrenal zona glomerulosa cells

Adrenal zona glomerulosa (ZG) cells produce aldosterone in response to angiotensin II and extracellular potassium through different mechanisms which involve changes in cytosolic free calcium (Cai). Protein kinase C (PKC) activation is part of the angiotensin II signalling cascade but its effects on Cai are unknown. PKC activation with 1 microM phorbol 12-myristate 13-acetate (PMA) and 8 mM Ko significantly increased the rate of calcium influx (P < 0.001). Both the PKC- and the Ko-induced calcium influx occurred via a nifedipine-sensitive pathway. When both were combined, PKC activation and 8 mM Ko were not additive over either agent alone. PKC activation and 8 mM Ko also stimulated calcium efflux (P < 0.01). When combined together PKC activation and 8 mM Ko had additive effects on calcium efflux (P < 0.05). PKC activation did not increase Cai nor the exchangeable calcium pool in contrast to 8 mM Ko which significantly increased both (P < 0.001). Thus, PKC activation in ZG cells induces a pattern of calcium transport characterized by accelerated calcium recycling across the cell membrane without increasing cell calcium content.

Protein kinase C phosphorylates the "a" forms of plasma membrane Ca2+ pump isoforms 2 and 3 and prevents binding of calmodulin

Phosphorylation by protein kinase C of the "a" and "b" variants of plasma membrane Ca2+ pump isoforms 2 and 3 was studied. Full-length versions of these isoforms were assembled and expressed in COS cells. Whereas the "a" forms were phosphorylated easily with PKC, isoform 2b was phosphorylated only a little, and isoform 3b was not phosphorylated at all. Phosphorylation of isoforms 2a and 3a did not affect their basal activity, but prevented the stimulation of their activity by calmodulin and their binding to calmodulin-Sepharose. This indicated that phosphorylation prevented activation of these isoforms by preventing calmodulin binding. Based on these results, phosphorylation of the pump with PKC would be expected to increase free intracellular Ca2+ levels in those cells where isoforms 2a and 3a are expressed.

Cellular and molecular mechanisms of pulmonary vascular remodeling

In many organs and tissues, the cellular response to injury is associated with a reiteration of specific developmental processes. Studies have shown that, in response to injury, vascular wall cells in adult organisms express genes or gene products characteristic of earlier developmental states. Other genes, expressed preferentially in adult cells in vivo, are down-regulated following injurious stimuli. Complicating matters, however, are recent observations demonstrating that the vascular wall is comprised of phenotypically heterogeneous subpopulations of endothelial cells, smooth muscle cells, and fibroblasts. It is unclear how specific subsets of cells respond to injury and thus contribute to the vascular remodeling that characterizes chronic pulmonary hypertension. This review discusses vascular development in the lung and the cellular responses occurring in pulmonary hypertension; special attention is given to heterogeneity of responses within cell populations and reiteration of developmental processes.

The plasma membrane calcium pump: recent developments and future perspectives

The Ca2+ pump of the plasma membrane (PMCA) is regulated by a number of agents. The most important is calmodulin (CaM), which binds to a domain located in the C-terminal portion of the pump, removing it from an autoinhibitory site next to the active site. The CaM-binding domain is preceded by an acidic sequence which contains a hidden signal for endoplasmic reticulum (ER) retention. Chimeras of the PMCA and endoplasmic reticulum (SERCA) pumps have revealed the presence of a strong signal for ER retention in the first 45 residues of the SERCA pump. Four gene products of the PMCA pump are known: two of them (1 and 4) are ubiquitously expressed, two (2 and 3) are specific for nerve cells and may be induced by their activation. Mutagenesis work has identified four residues in three of the transmembrane domains of the pump which may be components of the trans-protein Ca2+ path. The mutation of two of these residues alters the membrane targeting of the pump.

Protein kinase C activates the plasma membrane Ca2+ pump isoform 4b by phosphorylation of an inhibitory region downstream of the calmodulin-binding domain

The carboxyl-terminal region of the plasma membrane Ca2+ pump isoform 4b contains two autoinhibitory regions which keep the pump inactive in the absence of activators such as calmodulin. One of these regions is approximately coterminous with the calmodulin-binding domain, while the second region is downstream (Verma, A. K., Enyedi, A., Filoteo, A. G., and Penniston, J. T. (1994) J. Biol. Chem. 269, 1687-1691). The carboxyl-terminal region has also been identified as the site for phosphorylation of this isoform by protein kinase C (Wang, K. K. W., Wright, L. C., Machan, C. L., Allen, B. G., Conigrave, A. D., and Roufogalis, B. D. (1991) J. Biol. Chem. 266, 9078-9085). Using constructs lacking various numbers of residues at the carboxyl terminus, we studied the degree of phosphorylation by protein kinase C and the resultant activation of Ca2+ transport. The results showed that the most specific and easy phosphorylation occurred in a region of about 20 residues which is downstream of the calmodulin-binding domain, and that the downstream inhibitory domain had also about the same size and location. Phosphorylation partially activated the pump by removing only the inhibition due to this region. Binding of calmodulin to the calmodulin-binding domain activated the pump more fully by removing the inhibition due to both regions, regardless of the state of phosphorylation at the downstream inhibitory region.

Increased inhibitory effect of phorbol ester on cytosolic Ca2+ level and contraction in rat myometrium after gestation

Activation of voltage-dependent Ca2+ channels by high K+ (40 mM) increased the cytosolic Ca2+ level ([Ca2+]i) (estimated by fura-PE3 fluorescence ratio) and force in myometrium isolated from pregnant (21 days after gestation) and non-pregnant (estrus) rats. 12-Deoxyphorbol 13-isobutyrate (DPB, 1 mM) decreased the high (K+)-stimulated [Ca2+]i and force in a concentration-dependent manner. The inhibitory effect was stronger in the pregnant myometrium than in the non-pregnant myometrium. In the pregnant myometrium, the increase in Ca2+ permeability by ionomycin (1 microM) greatly increased [Ca2+]i and force, which were only partially inhibited by verapamil (10 microM). DPB (1 microM) inhibited the verapamil-insensitive component of the increases in [Ca2+]i and muscle tension. Oxytocin (100 nM) and thapsigargin (1 microM) also induced a verapamil-insensitive increase in [Ca2+]i and force, and DPB (1 microM) inhibited these increments. Ca2+ sensitivity of contractile elements, estimated from the relationships between Ca2+ and muscle force in intact and alpha-toxin permeabilized muscle, was not significantly changed by DPB (1 microM). In summary, DPB inhibits the increase in [Ca2+]i more strongly in myometrium isolated from pregnant rats than that from non-pregnant rats without any change in the [Ca2+]i/tension relationship. Since DPB decreased [Ca2+]i-rise induced by three different mechanisms, DPB may activate Ca2+ extrusion, rather than to inhibit a specific influx pathway, to decrease [Ca2+]i.

Okadaic acid as a probe for regulation in vitro of Mg(2+), Ca(2+)-ATPase activity in rat cortical and cerebellar synaptosomal membranes

The in vitro effect of okadaic acid on basal phorbol 12-myristate 13-acetate (PMA)-, and cyclic adenosine monophosphate (cAMP)-stimulated Mg(2+)-dependent Ca(2+)-adenosine triphosphatase (ATPase) activity in synaptosomal membranes isolated from rat brain cortex and cerebellum was investigated. The basal activity was enhanced by okadaic acid in both examined regions. This inhibitor differed in the regulation of Mg2+, Ca(2+)-ATPase activity in PMA- and cAMP-incubated membranes. Stimulation by calmodulin (CaM) of basal Mg2+, Ca(2+)-ATPase activity declined in cortex and cerebellum after treatment with okadaic acid. The presence of PMA or cAMP decreases the stimulatory effect of CaM. These results suggest that Mg2+, Ca(2+)-ATPase activity in the rat-brain synaptosomal membrane may be regulated in vitro by dephosphorylation processes.

Carbachol but not acetylcholine inhibits contraction by the protein kinase C-dependent and -independent pathways in the smooth muscle of guinea pig taenia caeci

In the intestinal smooth muscle of guinea pig taenia caeci, acetylcholine and carbachol induced a transient contraction followed by a sustained contraction. The magnitudes of the transient and sustained contractions were similar when muscle was stimulated with acetylcholine (0.1 microM-1 mM) or a lower concentration (0.1 microM) of carbachol. However, higher concentrations of carbachol (1 - 100 microM) induced significantly smaller sustained contraction than the transient contraction. In the 45 mM KCI-stimulated strips, addition of 100 microM carbachol induced a transient increase followed by a sustained decrease in the contractile tension. In contrast, acetylcholine (0.1 microM-1 mM) showed only weak inhibitory effect on the high K(+)-induced contraction either in the absence or presence of a cholinesterase inhibitor, 0.5 microM diisopropylfluorophosphate. The same concentration of diisopropylfluorophosphate shifted the concentration-response curve for acetylcholine to lower concentrations. In the muscles pretreated with 3 microM phorbol 12-myristate 13-acetate for 24 hr to desensitize protein kinase C, sustained contractions induced by higher concentrations of carbachol (1-100 microM) were significantly greater than those in the strips without the treatment with phorbol ester. However, the transient contraction and the contraction induced by a lower concentration (0.1 microM) of carbachol were not changed by the treatment with phorbol ester. Pretreatment with phorbol ester attenuated the inhibitory effect of carbachol on the high K(+)-induced contraction. These results suggest that the inhibitory effects of carbachol is composed of two phases: protein kinase C-independent transient inhibition and protein kinase C-dependent sustained inhibition.

Mechanisms for Ca signaling in vascular smooth muscle: resolved from 45Ca uptake and efflux experiments

Established cell lines are now widely used in experiments concerning vascular smooth muscle (VSM) function; however, considerable evidence suggests that cultured VSM cells are functionally different from VSM cells in intact blood vessels. In order to test the hypothesis that calcium signaling mechanisms are comparable in these two preparations, we developed a new method for high resolution 45Ca efflux studies in A7r5 cells. Briefly, this method involves plating cells in the lumen of a tubular glass efflux chamber and, after loading the cells with 45Ca, perfusing the chamber with a physiological saline solution and collecting the effluent. Using this method we found that the plasma membrane in cultured cells is not rate limiting for calcium efflux, since the efflux curves from both permeabilized and intact cells are kinetically the same. We also found the plasma membrane is not rate limiting in whole aortic segments by using a depolarizing solution followed by dihydropyridine solution. Thus, we demonstrated that the data obtained from cells or tissues with intact membranes reveal information about the intracellular stores (sarcoplasmic reticulum and mitochondria). Combining efflux data with a detailed kinetic model of cellular Ca transport allows least-squares estimation of the rate constants for release and uptake of Ca2+ by intracellular stores with a high degree of confidence (CV < 25%) as well as the Ca2+ contents and transmembrane fluxes associated with these stores. Quantitative comparison of results obtained from A7r5 cells with those we previously obtained for rabbit aortic segments reveals marked similarities and suggests that A7r5 cells serve as excellent model experiments for VSM cell Ca2+ homeostasis.

The plasma membrane calcium pump--a physiological perspective on its regulation

This review focuses on the physiological role of the plasma membrane Ca(2+)+ Mg(2+)-dependent adenosine triphosphatase (PM Ca(2+)-ATPase) in cellular signalling. Particular attention has been paid to the regulation of the PM Ca(2+)-ATPase (PM Ca2+ pump) by calmodulin, proteases, protein kinases, acidic phospholipids and oligomerization in intact cells. We also review recent work investigating the possible regulation of the PM Ca2+ pump by G proteins and agonists. The source of adenosine triphosphate (ATP) and Ca2+ in fueling and activating the Ca2+ pump is discussed, as well as the possible role of the PM Ca(2+)-ATPase in subplasma membrane Ca2+ regulation. The physiological implication of the localisation of the PM Ca2+ pump in caveolae is also considered.

Protein kinase C modulates cytosolic free calcium by stimulating calcium pump activity in Jurkat T cells

Although protein kinase C (PKC) activation has been shown to inhibit Ca2+ influx in T lymphocytes, the role of PKC on Ca2+ sequestration or extrusion processes has not been fully explored. We examined the effect of CD3 stimulation and PKC activators on cytosolic Ca2+ (Ca2+i) extrusion and 45Ca2+ efflux in human leukemic Jurkat T cells. Treatment of Fura-2 loaded cells with phorbol 12-myristate 13-acetate (PMA) or thymeleatoxin (THYM) resulted in a decrease in Ca2+i both in the presence and absence of extracellular Ca2+, whereas inactive phorbol esters had no effect. PKC activators added at the peak of a Ca2+i transient induced by anti-CD3 mAb, ionomycin or thapsigargin (TG) stimulated the rate and extent of return of Ca2+i to basal levels by 17-53%. PKC stimulation of the Ca2+i decline was not enhanced by the presence of Na+, indicating that PKC activators increase Ca2+ pump activity rather than a Na+/Ca2+ exchange mechanism. As CD3 receptor activation enhanced the Ca2+i decline in TG-treated cells, antigen-mediated activation of phospholipase C (PLC) signaling includes enhanced Ca2+ extrusion at the plasma membrane. The effect of PKC activators on parameters of Ca2+i extrusion were further explored. PMA significantly increased the rate of Ca2+ extrusion in TG-treated cells from 0.28 +/- 0.02 to 0.35 +/- 0.03 s-1 (mean +/- SEM) and stimulated the initial rate of 45Ca2+ efflux by 69% compared to inactive phorbol ester treated cells. The effects of PKC activation on the Ca2+i decline were eliminated by PKC inhibitors, PKC down regulation (24 h PMA pretreatment), ATP-depletion and conditions that inhibited the Ca2+ pump. In contrast, pretreatment of cells with okadaic acid enhanced the PMA-stimulated response. We suggest that Jurkat T cells contain a PKC-sensitive Ca2+ extrusion mechanism likely to be the Ca2+ pump. In lymphocytes, receptor/PLC-linked PKC activation modulates Ca2+i not only by inhibiting Ca2+ influx but also by stimulating plasma membrane Ca2+i extrusion.

Growth factor-induced phosphorylation and activation of aortic smooth muscle Na+/Ca2+ exchanger

Although the Na+/Ca2+ exchanger is one of the major Ca2+ extrusion systems in excitable tissues, little is known about its regulation via protein phosphorylation. We now present evidence that the Na+/Ca2+ exchanger is phosphorylated in quiescent and growth factor-stimulated cultured aortic smooth muscle cells. The Na+/Ca2+ exchanger was isolated from 32P-labeled cells by immunoprecipitation with a specific polyclonal antibody. Phosphorylation of the exchanger was increased by up to 1.7-fold in response to platelet-derived growth factor-BB (PDGF-BB), alpha-thrombin, or phorbol 12-myristate 13-acetate (PMA). However, angiotensin II did not enhance the phosphorylation significantly. The extent of phosphorylation appeared to correlate with the growth factor-induced increase in cell 1,2-diacylglycerol. At least four phosphopeptides (P1 to P4) were detected by tryptic phosphopeptide map analysis of the phosphorylated exchanger, suggesting that phosphorylation occurred at multiple sites. PDGF-BB and PMA increased phosphorylation of the same phosphopeptides (in particular P1). Phosphorylated amino acids were exclusively serine residues in both quiescent and stimulated cells. We found that growth factors enhanced Na+/Ca2+ exchange activity and that there was a good correlation between the growth factor-induced stimulations of phosphorylation and exchange activity. PDGF-BB-induced activation of the exchanger was abolished by prior long treatment of cells with PMA. These results suggest that the Na+/Ca2+ exchanger is activated by protein kinase C-dependent phosphorylation in response to growth factors in vascular smooth muscle cells.

Signal transduction in vascular smooth muscle: diacylglycerol second messengers and PKC action

Agonist-stimulated phospholipid turnover can generate diacylglycerol (DAG), an intracellular second messenger that activates protein kinase C (PKC). DAG can be produced from the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) by a phosphoinositide-specific phospholipase C and by the degradation of phosphatidylcholine (PC) by a phospholipase C or the concerted actions of phospholipase D and phosphatidate phosphohydrolase. In vascular smooth muscle, agonist-stimulated DAG accumulation is biphasic; PIP2 hydrolysis produces a transient increase in DAG, which is followed by a sustained phase of DAG accumulation from PC degradation. Metabolism of DAG attenuates PKC activation and thus results in signal termination. The metabolic fates for DAG include 1) ATP-dependent phosphorylation to form phosphatidic acid (DAG kinase), 2) hydrolysis to release fatty acids and glycerol (DAG and monoacylglycerol lipases), 3) synthesis of triacylglycerol (DAG acyltransferase), and 4) synthesis of PC (choline phosphotransferase). Hydrolysis through the lipase pathway is the predominant metabolic fate of DAG in vascular smooth muscle. Activation of PKC in vascular smooth muscle modulates agonist-stimulated phospholipid turnover, produces an increase in contractile force, and regulates cell growth and proliferation. Further research is required to investigate cross talk between signal transduction mechanisms involving lipid second messengers. In addition, spatial considerations such as nuclear PKC activation and the influence of diradylglycerol generation on the duration of PKC activation are important issues.

Measurement of Ca2+ pump-mediated efflux in hypertension

Ca2+ homeostasis has been a prominent research area in the study of hypertension. There is convincing evidence that hypertension in spontaneously hypertensive rats is characterized by enhanced Ca2+ influx in various cell types. It is, however, still unclear whether hypertension is associated with reduced or enhanced Ca2+ efflux. Reduced Ca2+ efflux would augment the effects of enhanced Ca2+ influx. However, enhanced Ca2+ extrusion may occur as an adaptive process to minimize the effects of Ca2+ overload. This question remains unanswered because of inconsistent results obtained using a variety of experimental techniques. In this article we have reviewed the research findings and discuss existing and possible new techniques to assess Ca2+ efflux in hypertension, with particular attention to vascular smooth muscle. We have focused mainly on studies using the spontaneously hypertensive rat and discuss its appropriateness as a model for essential hypertension.

Regulation of Ca2+ transport by platelet-derived growth factor-BB in rat vascular smooth muscle cells

The present study investigates the effects of platelet-derived growth factor (PDGF) isoform BB (PDGF-BB) on cytosolic Ca2+ concentration ([Ca2+]i), Ca2+ transport, and Ca2+ pools in rat vascular smooth muscle (VSM) cells. VSM cells from thoracic aorta of Milan normotensive rats were enzymatically dispersed, cultured in 10% serum medium, and made quiescent by 72 hours in 0.3% serum medium. [Ca2+]i, Ca2+ influx, Ca2+ efflux, and exchangeable cell Ca2+ pool were evaluated by ratiometric fluorescent and radioisotope techniques. Ca2+ transport showed time-dependent changes during stimulation with PDGF-BB. The initial early responses to this peptide were transient rise in [Ca2+]i, a 30% decrease in Ca2+ influx, and a 3.6-fold increase in the rate constant for active Ca2+ efflux. Stimulation of Ca2+ efflux and inhibition of Ca2+ influx were associated with a substantial 30% reduction in the cell Ca2+ pool. This initial stimulation of Ca2+ efflux is concomitant with Ca2+ mobilization into the cytosol and is due to activation of Na(+)-independent Ca2+ efflux via the Ca2+ pump. After a 10-minute stimulation, Ca2+ influx returned to the basal value, whereas Ca2+ efflux remained 2.2-fold above control values, leading to a decline in [Ca2+]i below basal levels and a further decrease in the cell Ca2+ pool. Nearly half of this late Ca2+ efflux appears to be driven by Na(+)-Ca2+ exchange, as evidenced by its external Na+ dependence. After a 120-minute stimulation with PDGF-BB, nifedipine-sensitive Ca2+ influx is increased 37% above basal levels, and Ca2+ efflux remains elevated. During prolonged stimulation by PDGF-BB, both Ca2+ influx and efflux are stimulated, resulting in a new intracellular Ca2+ homeostasis marked by the recovery of the cell Ca2+ pool but a lowered [Ca2+]i. These final events coincide with the initiation of cell proliferation in VSM cells by PDGF-BB.

Calcium ion homeostasis in smooth muscle

Ca2+ plays an important role in the regulation of smooth-muscle contraction. In this review, we will focus on the various Ca(2+)-transport processes that contribute to the cytosolic Ca2+ concentration. Mainly the functional aspects will be covered. The smooth-muscle inositol 1,4,5-trisphosphate receptor and ryanodine receptor will be extensively discussed. Smooth-muscle contraction also depends on extracellular Ca2+ and both voltage- and Ca(2+)-release-activated plasma-membrane Ca2+ channels will be reviewed. We will finally discuss some functional properties of the Ca2+ pumps that remove Ca2+ from the cytoplasm and of the Ca2+ regulation of the nucleus.

Characterization of 130 kDa protein from rat cerebellum synaptosomal membranes phosphorylated by PKC

1. The effect of endogenous PMA-stimulated phosphorylation of the protein in the molecular weight range of 130 kDa in rat cerebellum synaptosomal membranes was examined. 2. The 50% inhibition of the phosphorylation of 130 kDa protein by 5 microM polymyxin B was observed after 6 min of preincubation. 3. The sensitivity of 130 kDa protein for phosphorylation in the presence of exogenous protein kinase C suggests, that this protein could serve as a physiological substrate of protein kinase C. 4. Partial characterization of 130 kDa protein was performed. Upon incubation with [gamma-32P]ATP the 130 kDa protein formed Ca(2+)-dependent, hydroxylamine-sensitive phosphointermediate, which was inhibited by 50 microM vanadate, but not 0.5 mM vanadyl. 5. One-dimensional peptide mapping by proteolysis of 130 kDa protein with V8 protease was obtained.

Evidence for a role for protein kinase C in the modulation of bombesin-activated cellular signalling in human breast cancer cells

The phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) is a potent activator of protein kinase C (PKC) and is known to affect a variety of biochemical processes in human breast cancer cells. In the present study we have employed MCF-7 cells to investigate the effects of TPA on inositol lipid signalling, the putative pathway leading to PKC activation and intracellular Ca2+ mobilization. Phosphoinositide hydrolysis in MCF-7 cells was stimulated by bombesin (BN) as evidenced by increases in both inositol phosphate production and cytidine diphosphate diacylglycerol (CDP-DG) accumulation. Pretreatment of MCF-7 cells with TPA caused attenuation of both these BN-induced responses. This inhibitory action of TPA on inositol phosphate production was mimicked by diacylglycerol analogues and was reversed by staurosporine, H-7 and tamoxifen, all known inhibitors of PKC. Furthermore, putative down-regulation of PKC by prolonged TPA pretreatment also reversed the inhibitory action of TPA and enhanced BN-induced phosphoinositide hydrolysis. TPA also inhibited BN-induced increases in cytosolic Ca2+ concentration ([Ca2+]i) and caused a dose-dependent inhibition of epidermal growth factor (EGF) binding in MCF-7 cells. However, EGF receptor occupancy was unaffected by BN. These data support an inhibitory role for PKC in the regulation of phosphoinositide hydrolysis and [Ca2+]i in breast cancer cells and provide a potential mechanism for feedback regulation of this signalling pathway in these cells.

Time course of changes in concentration of intracellular free calcium in cultured cerebrovascular smooth muscle cells exposed to oxyhemoglobin

A culture of smooth muscle cells obtained from monkey middle cerebral arteries was developed to allow quantitative assessment of intracellular calcium and immunofluorescence analysis after various periods of exposure to oxyhemoglobin. Intracellular calcium concentration was examined for up to 7 days after a single exposure to oxyhemoglobin. Intracellular calcium concentrations were measured with the fluorescent dye fura-2 and were significantly elevated for 7 days after exposure to oxyhemoglobin (P less than 0.01). Less than 2 minutes after application of oxyhemoglobin, there was marked elevation of intracellular calcium from the control value of 75 +/- 2 nmol/L to 240 +/- 28 nmol/L (P less than 0.01 by analysis of variance). Intracellular calcium concentration of cells exposed for 24 hours to oxyhemoglobin and then grown in normal oxyhemoglobin-free medium fell close to normal levels on Days 3 and 7. On Day 3, the increase in intracellular calcium that followed repeated daily exposure to oxyhemoglobin was greater than that resulting from a single application of oxyhemoglobin (P less than 0.01 by Student's t test), but by Day 7 the elevation produced by these different approaches was similar. Smooth muscle cells exposed to oxyhemoglobin showed a reduction in immunoreactivity to alpha-actin. These data support the hypothesis that disruption of intracellular calcium regulation and calcium overloading may be important in the process of cell injury, which results in vasoconstriction and sometimes cell death, after exposure to oxyhemoglobin.

The plasma membrane calcium pump: a multiregulated transporter

Activation of many cells, especially nonexcitable cells, results in a Ca(2+) transient that is influenced in part by the kinetics of active extrusion of Ca(2+) across the plasma membrane. The molecular cloning of the plasma membrane Ca(2+)-pump has helped to clarify the relationship between its structure and function. The Ca(2+)-pump is controlled by multiple regulators, including calmodulin, phospholipids and various kinases. Longer term control is achieved through regulation of its gene expression, and the presence of a number of Ca(2+)-pump isoforms that differ in their regulatory domains provides potential functional diversity. In this review, we focus on the mechanisms that regulate the function of the Ca(2+)-pump, and their physiological significance.

Calcium signals in growth factor signal transduction

There is a substantial amount of information which has been obtained concerning the effects of growth factors on [Ca2+]i in proliferating cells. A number of different mitogens are known to induce elevations in [Ca2+]i and some characterization of the Ca2+ response to different classes of mitogens has been obtained. In addition, much is known about whether the Ca2+ response to a particular growth factor occurs as the result of an influx of external Ca2+ or a mobilization of internal Ca2+ stores. In addition, a considerable amount of information is available on the mechanism by which the Ins(1,4,5)P3-sensitive internal Ca2+ store takes up and releases Ca2+. However, there is still a large deficiency in our information concerning other Ca2+ stores in proliferating cells as well as in our knowledge of the mechanisms for regulating Ca2+ entry pathways. Much more data addressing these issues exists for other types of agonist-stimulated cells, and we have discussed much of it in this review article. While the wealth of data in nonproliferating cells provides some indications of what mechanisms might be involved in the growth factor-induced changes in [Ca2+]i, it is clear that much work must be done in proliferating cells to fully understand how external factors such as growth factors control [Ca2+]i. In addition, much work remains to be done in identifying the mechanisms for the internal control of [Ca2+]i as cells move through the cell cycle and in identifying the role that these changes in [Ca2+]i may play throughout the cell cycle.

Concentration-dependent effects of protein kinase C-activating and -nonactivating phorbol esters on myocardial contractility, coronary resistance, energy metabolism, prostacyclin synthesis, and ultrastructure in isolated rat hearts. Effects of amiloride

An extensive investigation of the cardiac actions of phorbol esters and the potential role of the Na(+)-H+ exchanger in those actions was carried out using isolated rat hearts. Sixty minutes of perfusion with 10(-9) M phorbol 12-myristate 13-acetate (PMA) or 10(-8) M phorbol 12,13-dibutyrate (PDBu) produced marked cardiac dysfunction associated with depressed contractility, coronary constriction, and elevated resting tension, the latter being particularly evident with PMA. These effects were also associated with disturbances in tissue levels of energy metabolites manifested primarily by a reduction in ATP and an elevation in lactate. Furthermore, both phorbols produced a sustained stimulation of the release of 6-ketoprostaglandin F1 alpha (6-keto PGF1 alpha), the hydrolysis product of prostacyclin (prostaglandin I2). Amiloride, an inhibitor of the Na(+)-H+ exchanger, significantly attenuated the loss in contractility and elevation in coronary pressure as well as the stimulated release of 6-keto PGF1 alpha but was without effect on elevations in resting tension or on changes in energy metabolites. Increasing concentrations of PMA or PDBu 10-fold resulted in a much more rapid and severe (greater than 80% loss in contractile function after 30 minutes) effect that was nonetheless qualitatively identical to that seen with the lower concentrations of phorbol. However, the effects were not prevented by amiloride. Surprisingly, 4 alpha-phorbol 12,13-didecanoate (alpha-PDD, 10(-6) M), which does not activate protein kinase C, was found to be a potent inhibitor of cardiac function (greater than 80% loss in contractility and 50% increase in resting tension) after 30 minutes of perfusion, although these effects were not associated with changes in levels of energy metabolites or with elevations in coronary pressure. Similarly, none of the actions of this compound were attenuated by amiloride. In contrast to the sustained effects of other phorbols on 6-keto PGF1 alpha release, the effect of alpha-PDD was transient (less than 10 minutes). In all hearts studied, the marked depression in contractile function caused by all phorbol esters occurred in the absence of any ultrastructural changes. 4 alpha-Phorbol (10(-6) M), which does not activate protein kinase C, was without effect on any parameter studied. Our results demonstrate very complex effects of phorbol esters on numerous parameters of cardiac function, including an amiloride-sensitive component that occurs at low concentrations. The latter observation suggests the involvement of Na(+)-H+ exchange activation, possibly occurring as a consequence of protein kinase C stimulation, in mediation of the effects of phorbol esters at low concentrations.(ABSTRACT TRUNCATED AT 400 WORDS)

Activation of the protein kinase C-mediated contractile system in canine basilar artery undergoing chronic vasospasm

We previously suggested that activation of the protein kinase C-mediated contractile system may participate in the occurrence of chronic cerebral vasospasm. In the present study, we compared segments of normal beagle basilar arteries in vitro with segments of arteries undergoing chronic vasospasm to determine the responsiveness to various agonists such as serotonin, prostaglandin F2 alpha, and phorbol 12,13-diacetate as well as to external Ca2+. We also compared the effects of W-7 (a calmodulin inhibitor), nicardipine (a calcium channel blocker), and H-7 (a protein kinase C inhibitor) on the spontaneous tonus of arterial segments stabilized at a resting tension of 3 g. Compared with normal segments, the responsiveness to each agonist in segments undergoing vasospasm was essentially unchanged whereas the the responsiveness to external Ca2+ was significantly decreased (p less than 0.001). In segments undergoing vasospasm the decrease in resting tension induced by W-7 was markedly diminished (p less than 0.01), that induced by nicardipine was unchanged, and that induced by H-7 was significantly increased (p less than 0.01). Our results indicate that spontaneous tonus due to activation of the protein kinase C system is significantly augmented in segments undergoing vasospasm. Thus this system, rather than the Ca2+/calmodulin system, appears to play a major role in the occurrence of chronic vasospasm.

Altered reactivity of hemolysate-treated cultured smooth-muscle cells from rabbit basilar artery determined by digital imaging microscopy

During culture, smooth-muscle cells obtained from rabbit basilar arteries were examined for contractile activity by means of differential interference microscopy with a video analysis system (digital imaging microscopy system). This system proved useful for observing the contraction and ultrastructural changes of the living cells. Hemolysate-treated cells showed augmented responses to 5-hydroxytryptamine and leukotriene C4, but not to KCl. This augmented response diminished gradually during the culture period. Both a phospholipase C blocking agent, 2-nitro-4-carboxyphenyl-n,n-diphenylcarbamate (NCDC), and a myosin light chain kinase blocking agent, 1-(5-chloronaphthalenesulfonyl)-1H-hexahydro-1,4-diazepine (ML-9), suppressed this augmented response. Protein kinase C activity of the cells, as measured by Western blot analysis, did not increase during the period of culture with hemolysate. The results obtained suggest that hemolysate had the following effects on the cells: 1) acute but gradual contraction of the cells; 2) augmentation of cellular responses to vasoactive agents; and 3) progressive contraction and morphological alteration of the cells. Possible mechanisms by which hemolysate exerts these effects are discussed, taking into consideration the interrelationship between these effects.

Mechanisms of vasoconstriction

The contractility of vascular smooth muscle cells is controlled in a complex manner by both extracellular and intracellular messages. The vascular endothelium does not simply act as a physical barrier between the blood and smooth muscle cells, it integrates intravascular signals and controls the contractility of underlying smooth muscle cells by way of release of paracrine factors with contracting or relaxing properties. Vasoconstrictors trigger a cascade of interacting intracellular signals that concur in initiating and maintaining contractions. Each step of these signalling pathways is a possible logical site for potential therapeutic interventions.