Regulation of calcium channels in aortic muscle cells by protein kinase C activators (diacylglycerol and phorbol esters) and by peptides (vasopressin and bombesin) that stimulate phosphoinositide breakdown

Myoinositol improves sperm parameters and serum reproductive hormones in patients with idiopathic infertility: a prospective double-blind randomized placebo-controlled study

Male infertility is a multifactorial disorder that affects a significant percentage of couples. Its etiology and pathogenesis remain elusive in about one-third of the cases; this is referred to as idiopathic infertility. Inositols mediate the sperm processes involved into oocyte fertilization, such as penetration of the ovum cumulus oophorus, binding with the zona pellucida and the acrosome reaction. The aim of this double-blind, randomized, placebo-controlled trial was to evaluate the efficacy and safety of myoinositol (the most abundant form of inositols present in nature) treatment in men with idiopathic infertility. To accomplish this, we evaluated the effects of myoinositol on sperm parameters and reproductive hormones at baseline and after 3 months of treatment in men with idiopathic infertility. No adverse reaction was observed. Myoinositol significantly increased the percentage of acrosome-reacted spermatozoa, sperm concentration, and total count and progressive motility compared to placebo. In addition, myoinositol rebalanced serum luteinizing hormone, follicle-stimulating hormone, and inhibin B concentrations. The clinical improvement of idiopathic infertile patients should encourage myoinositol use for the treatment of this disorder, even though its detailed mechanisms at the testicular level remain still unclear.

Arg-vasopressin facilitates calcium channel currents in osteoblasts

The hypothalamic nonapeptide and neurohypophyseal hormone arg-vasopressin (AVP), also known as antidiuretic hormone, is best known for its effects on water reabsorption in kidney. Osteoblasts play a major role in bone formation, employing intracellular Ca(2+) as a second messenger to modulate hormonal responses and as a cofactor for mineralization. Voltage-dependent Ca(2+) channels (VDCCs) mediate the influx of Ca(2+) in response to membrane depolarization. The purpose of this study was to investigate the effects of AVP on VDCC currents in osteoblasts using a patch-clamp recording method. An application of 1μM AVP facilitated VDCC currents in osteoblasts. To our knowledge, the data presented here demonstrate for the first time that AVP facilitates VDCCs in osteoblasts.

Neurohormonal regulation of cardiac ion channels in chronic heart failure

Alteration of neurohormonal homeostasis is a hallmark of the pathophysiology of chronic heart failure (CHF). In particular, overactivation of the renin-angiotensin-aldosterone system and the sympathetic catecholaminergic system is consistently observed. Chronic overactivation of these hormonal pathways leads to a detrimental arrhythmogenic remodeling of cardiac tissue due to dysregulation of cardiac ion channels. Sudden cardiac death resulting from ventricular arrhythmias is a major cause of mortality in patients with CHF. All the drug classes known to reduce mortality in patients with CHF are neurohormonal blockers. The aim of this review was to provide an overview of how cardiac ion channels are regulated by hormones known to play a central role in the pathogenesis of CHF.

Moclobemide attenuates anoxia and glutamate-induced neuronal damage in vitro independently of interaction with glutamate receptor subtypes

Recent data suggested the existence of a bidirectional relation between depression and neurodegenerative diseases resulting from cerebral ischemia injury. Glutamate, a major excitatory neurotransmitter, has long been recognised to play a key role in the pathophysiology of anoxia or ischemia, due to its excessive accumulation in the extracellular space and the subsequent activation of its receptors. A characteristic response to glutamate is the increase in cytosolic Na(+) and Ca(2+) levels which is due mainly to influx from the extracellular space, with a consequent cell swelling and oxidative metabolism dysfunction. The present study examined the in vitro effects of the antidepressant and type-A monoamine oxidase inhibitor, moclobemide, in neuronal-astroglial cultures from rat cerebral cortex exposed to anoxia (for 5 and 7 h) or to glutamate (2 mM for 6 h), two in vitro models of brain ischemia. In addition, the affinity of moclobemide for the different glutamate receptor subtypes and an interaction with the cell influx of Na(+) and of Ca(2+) enhanced by veratridine and K(+) excess, respectively, were evaluated. Moclobemide (10-100 microM) included in the culture medium during anoxia or with glutamate significantly increased in a concentration-dependent manner the amount of surviving neurons compared to controls. Moclobemide displayed no binding affinity for the different glutamate receptor subtypes (IC(50)>100 microM) and did not block up to 300 microM the entry of Na(+) and of Ca(2+) activated by veratridine and K(+), respectively. These results suggest that the neuroprotective properties of moclobemide imply neither the glutamate neurotransmission nor the Na(+) and Ca(2+) channels.

Regulation of the cardiac L-type calcium channel in L6 cells by arginine-vasopressin

L-type Ca2+ channel activity was measured in L6 cells as nifedipine-sensitive barium (Ba2+; 5 mM) influx in a depolarizing salt solution containing 140 mM KCl. Addition of AVP (arginine-vasopressin) during Ba2+ uptake reduced the rate of Ba2+ influx by 60-100%; this was followed by a gradual restoration of the initial rate of Ba2+ uptake. Blockade of PKC (protein kinase C) by pretreatment with 10 muM bisindolylmaleimide did not affect the initial inhibition of Ba2+ influx, but completely abolished the recovery phase. The effect of AVP was half-maximal at 10 nM AVP and was blocked by the V1a receptor antagonist d-(CH2)(5)-Tyr(Me)-AVP. Activation of G(alphas) by isoprenaline or cholera toxin antagonized the actions of AVP on Ba2+ uptake. This protection persisted in the presence of the PKA (protein kinase A) inhibitor KT5720, and was not mimicked by agents that increase cAMP. Inhibition of Ba2+ influx was also elicited by ATP and ET (endothelin 1) with an order of effectiveness ET<ATP<AVP. Each of these agents has been reported to act through G(q)-coupled receptors. We conclude that activation of G(q)-coupled receptors produces a rapid inhibition of the cardiac L-type Ca2+ channel, which is subsequently overcome by activation of PKC.

Effects of the new ethacrynic acid derivative SA9000 on intraocular pressure in cats and monkeys

To evaluate the pharmacological characteristics of the new ethacrynic acid (ECA) derivative SA9000, we examined its ocular hypotensive effects in cats and cynomolgus monkeys, its corneal toxicity in rabbits, and its binding affinities for forty-three receptors, ion channels, and second messenger systems. A 20 microl injection into the anterior chamber of eye (intracameral injection) of 0.1 mM SA9000 significantly reduced intraocular pressure (IOP) 3.8 mmHg in cats. A 10 microl intracameral injection of 1 mM SA9000 significantly reduced IOP 7 mmHg in living monkeys without evidence of in vivo (or in vitro) toxicity. The ocular hypotensive effect of SA9000 in monkeys was greater than that of ECA. The morphology of corneal endothelial and epithelial cells in rabbit eyes after intracameral injection of SA9000 was observed using electron microphotography. SA9000 at 2 mM did not induce any abnormalities, indicating that it has no corneal toxicity at a concentration higher than the minimum needed for an ocular hypotensive effect (1 mM). SA9000 at 0.01 mM showed negligible binding affinity for, or inhibition of, forty-three different receptors, ion channel proteins, and second messenger systems. These findings indicate that SA9000 has the potential to be both effective and safe as an ocular hypotensive drug, although the mechanism of action remains unclear.

Conventional-type protein kinase C contributes to phorbol ester-induced inhibition of rat myometrial tension

1 Phorbol ester decreases muscle tension in the rat myometrium, and the effect is more potent in late-pregnant myometrium than in nonpregnant myometrium. In the present study, we have examined the contribution of protein kinase C (PKC) isoforms to the phorbol ester-induced inhibition of tension in rat uterine smooth muscle. 2 Thymeleatoxin (THX), a selective activator of conventional-type PKC (cPKC), and 12-deoxyphorbol 13-isobutyrate (DPB), an activator of pan PKC, inhibited the tension induced by high K(+), and inhibitions were significantly increased in pregnant myometrium compared to nonpregnant myometrium. The inhibition by DPB and THX of high K(+)-induced tension was significantly attenuated when PKC was downregulated by long-term pretreatment with THX and inhibited by Go6976, a cPKC inhibitor. 3 Of the cPKCs, PKC alpha is predominantly expressed in the rat myometrium, as detected by Western blot analysis. The expression of PKC alpha gradually increases from the beginning of gestation, reaching a maximum at day 21 of pregnancy. Treatment with DPB induced PKC alpha to translocate from the cytosol to the membrane in the pregnant myometrium. PKC epsilon and PKC zeta, other dominant PKC isoforms in the rat myometrium, decrease during gestation, reaching a minimum in late pregnancy. 4 These results suggest that cPKC may be at least partly involved in the PKC-mediated inhibition of muscle tension in the rat myometrium.

Smooth muscle uses another promoter to express primarily a form of human Cav1.2 L-type calcium channel different from the principal heart form

Several different first exons and amino termini have been reported for the cardiac Ca channel known as alpha(1C) or Ca(V)1.2. The aim of this study was to investigate whether the expression of this channel is regulated by different promoters in smooth muscle cells and in heart in humans. Ribonuclease protection assay (RPA) indicates that the longer first exon 1a is found in certain human smooth muscle-containing tissues, notably bladder and fetal aorta, but that it is not expressed to any significant degree in lung or intestine. On the other hand, all four smooth muscle-containing tissues examined strongly express transcripts containing exon 1b, first reported cloned from human fibroblast cells. In addition, primary cultures of human colonic myocytes and coronary artery smooth muscle cells express predominantly transcripts containing exon 1b. The promoter immediately upstream of exon 1b was cloned, and it displays functional promoter activity when luciferase-expressing constructs were transfected into three different cultured smooth muscle cells: primary human coronary artery smooth muscles cells, primary human colonocytes, and the fetal rat aorta-derived A7r5 cell line. These results indicate that expression in smooth muscle is primarily driven by a promoter different from that which drives expression in cardiac myocytes.

Protein kinase C and cerebral vasospasm

Twenty-five years after the discovery of protein kinase C (PKC), the physiologic function of PKC, and especially its role in pathologic conditions, remains a subject of great interest with 30,000 studies published on these aspects. In the cerebral circulation, PKC plays a role in the regulation of myogenic tone by sensitization of myofilaments to calcium. Protein kinase C phosphorylates various ion channels including augmenting voltage-dependent Ca2+ channels and inhibiting K+ channels, which both lead to vessel contraction. These actions of PKC amplify vascular reactivity to different agonists and may be critical in the regulation of cerebral artery tone during vasospasm. Evidence accumulated during at least the last decade suggest that activation of PKC in cerebral vasospasm results in a delayed but prolonged contraction of major arteries after subarachnoid hemorrhage. Most of the experimental results in vitro or in animal models support the view that PKC is involved in cerebral vasospasm. Implication of PKC in cerebral vasospasm helps explain increased arterial narrowing at the signal transduction level and alters current perceptions that the pathophysiology is caused by a combination of multiple receptor activation, hemoglobin toxicity, and damaged neurogenic control. Activation of protein kinase C also interacts with other signaling pathways such as myosin light chain kinase, nitric oxide, intracellular Ca2+, protein tyrosine kinase, and its substrates such as mitogen-activated protein kinase. Even though identifying PKC revolutionized the understanding of cerebral vasospasm, clinical advances are hampered by the lack of clinical trials using selective PKC inhibitors.

Control of vascular tone in the syndromes of Bartter and Gitelman

Bartter's and Gitelman's syndromes can be used as models to gain insight into the mechanisms responsible for maintaining/controlling vascular tone. In fact, the study of patients with these syndromes provides important insights into mechanistic details of the most relevant pathways of vascular tone. So far, several experimental findings in patients with these syndromes point to G protein abnormalities and suggest that the intracellular signaling systems that involve the G protein complex transducing components may be defective, leading to altered vascular reactivity. These results are also of particular interest because the derangements found in Bartter's and Gitelman's syndromes are the mirror images of those involved in the pathophysiology of hypertension.

Ca2+ signalling in rat vascular smooth muscle cells: a role for protein kinase C at physiological vasoconstrictor concentrations of vasopressin

Physiological vasoconstrictor concentrations of Arg8-vasopressin (AVP, 10-100 pM) stimulate oscillations (spikes) in cytosolic free Ca2+ concentration ([Ca2+]i) in A7r5 rat vascular smooth muscle cells. These Ca2+ spikes are dependent on L-type voltage-sensitive Ca2+ channels and increase in frequency with increasing AVP concentration. The signal transduction pathway responsible for this effect was examined in fura-2-loaded A7r5 cell monolayers. The serine/threonine phosphatase inhibitor calyculin A (5 nM) sensitized A7r5 cells to AVP, resulting in the stimulation of Ca2+ spiking by 1-10 pM AVP. Calyculin A alone did not stimulate Ca2+ spiking. The protein kinase C (PKC) activator 4beta-phorbol 12-myristate 13-acetate (PMA, 100 pM to 200 nM), also stimulated Ca2+ spiking and this effect was additive with a submaximal concentration of AVP (50 pM). The PKC inhibitors Ro-31-8220 (1 microM) and calphostin C (250 nM) completely blocked the stimulation of Ca2+ spiking by either PMA or AVP. alpha, beta, gamma, delta, epsilon, zeta and &lamdda; isoforms of PKC were detected in A7r5 cells by Western blot analysis. Time-dependent redistribution of PKC-alpha, -delta and -epsilon isoforms between the membrane and cytosolic fractions occurred in response to 100 pM AVP. Pretreatment for 24 h with 1 microM PMA downregulated expression of PKC-alpha and -delta, but not PKC-epsilon, and prevented the Ca2+-spiking responses to either 1 nM PMA or 100 pM AVP. Neither the release of intracellular Ca2+ by 1 microM AVP nor the increase in [Ca2+]i in response to elevated extracellular [K+] was prevented by the PMA pretreatment. We conclude that PKC activation is a necessary step in the signal transduction pathway linking low concentrations of AVP to Ca2+ spiking in A7r5 cells.

Design and biological evaluation of non-peptide analogues of omega-conotoxin MVIIA

Omega-conotoxin MVIIA, a highly potent antagonist of the N-type voltage sensitive calcium channel, has shown utility in several models of pain and ischemia. We report a series of three alkylphenyl ether based analogues which mimic three key amino acids of the toxin. Two of the compounds have been found to exhibit IC50 values of 2.7 and 3.3 microM at the human N-type voltage sensitive calcium channel.

Potential role for triglycerides in signal transduction

We previously reported that endothelin-1 or platelet-derived growth factor promoted in aortic smooth muscle cells a rapid hydrolysis of 1-O-alkyl-2-acyl-sn-glycero-3-phosphoethanolamine (alkyl-PE) which was immediately converted into 1-O-alkyl-2,3-diacyl-sn-glycerol (alkyl-TG) within 5 s or 60 s respectively [C. Comminges et al. (1996) Biochem. Biophys. Res. Commun. 220, 1008-1013 and C. Comminges et al. (1997) Biochim. Biophys. Acta 1355, 69-80]. In this study, we show that this alkyl-PE hydrolysis is triggered by a transient activation of a specific phospholipase C (PLC) regulated by pertussis toxin-sensitive heterotrimeric G-proteins. Moreover, this PLC can be triggered through a Ca2+ influx depending on L-type Ca2+ channel activation, as suggested by the use of a specific 'activator' S(-)-BayK 8644 and of selective inhibitors such as nimodipine. Interestingly, low concentrations (10(-8)-10(-7)M) of alkyl-TG block the opening of L-type Ca2+ channels, whereas identical concentrations of DG do not alter L-type Ca2+ channels. This study thus unravels a hitherto unrecognized signaling pathway generating alkyl-TG as a novel lipid second messenger, potentially acting as a negative feedback regulator of L-type Ca2+ channels.

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.

Morphological transformation induced by activation of the mitogen-activated protein kinase pathway requires suppression of the T-type Ca2+ channel

Transformation of fibroblasts by various oncogenes, including ras, mos, and src accompanies with characteristic morphological changes from flat to round (or spindle) shapes. Such morphological change is believed to play an important role in establishing malignant characteristics of cancer cells. Activation of the mitogen-activated protein kinase (MAPK) pathway is a converging downstream event of transforming activities of many oncogene products commonly found in human cancers. Intracellular calcium is known to regulate cellular morphology. In fibroblasts, Ca2+ influx is primarily controlled by two types of Ca2+ channels (T- and L-types). Here, we report that the T-type current was specifically inhibited in cells expressing oncogenically activated Ras as well as gain-of-function mutant MEK (MAPK/extracellular signal-regulated kinase (ERK) kinase, a direct activator of MAPK), whereas treatment of ras-transformed cells with a MEK-specific inhibitor restored T-type Ca2+ channel activity. Using a T-type Ca2+ channel antagonist, we further found that suppression of the T-type Ca2+ channel by the activated MAPK pathway is a prerequisite event for the induction and/or maintenance of transformation-associated morphological changes.

Signaling mechanisms underlying the vascular myogenic response

The vascular myogenic response refers to the acute reaction of a blood vessel to a change in transmural pressure. This response is critically important for the development of resting vascular tone, upon which other control mechanisms exert vasodilator and vasoconstrictor influences. The purpose of this review is to summarize and synthesize information regarding the cellular mechanism(s) underlying the myogenic response in blood vessels, with particular emphasis on arterioles. When necessary, experiments performed on larger blood vessels, visceral smooth muscle, and even striated muscle are cited. Mechanical aspects of myogenic behavior are discussed first, followed by electromechanical coupling mechanisms. Next, mechanotransduction by membrane-bound enzymes and involvement of second messengers, including calcium, are discussed. After this, the roles of the extracellular matrix, integrins, and the smooth muscle cytoskeleton are reviewed, with emphasis on short-term signaling mechanisms. Finally, suggestions are offered for possible future studies.

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 activation of nitric oxide synthase by copper ion is mediated by intracellular Ca2+ mobilization in human pulmonary arterial endothelial cells

The aim of the study was to elucidate the vasodilatory mechanism due to Cu2+ by assessing nitric oxide (NO) production as determined by NOx (NO, NO2-, and NO3-) that is released from human pulmonary arterial endothelial cell (HPAEC) monolayers using a NO chemiluminescence analyzer, and also to assess Ca2+ movement using 45Ca and fura 2 in HPAEC. Cu2+ (10(-6)-10(-4) M) significantly increased NO production in a dose-dependent manner when extracellular Ca2+ was present. 45Ca influx into the adherent cells was dose-dependently enhanced by Cu(2+) (10(-6)-10(-4) M), but not by Mn(2+), Zn(2+) or Fe(2+). [Ca2+]i, measured by monitoring the fluorescence changes of fura 2, was significantly elevated in the presence of Cu2+. The increase in [Ca2+]i induced by Cu2+ was inhibited by either diethyldithiocarbamate (DDC) or the depletion of extracellular Ca2+. The dihydropyridine receptor agonist, BayK8644, significantly attenuated the Cu2+-induced increase in [Ca2+]i in a dose dependent manner and nitrendipine or nifedipine, the dihydropyridine receptor antagonists, dose-dependently inhibited a Cu2+-induced increase in [Ca2+]i. These results suggest that Cu2+ activates eNOS through the mechanism of [Ca2+]i elevation due to Ca2+ influx into HPAEC and that the Cu2+-induced [Ca2+]i elevation in HPAEC is likely due to activation of the dihydropyridine-like receptors.

Altered regulation of L-type channels by protein kinase C and protein tyrosine kinases as a pathophysiologic effect in retinal degeneration

The effect of protein tyrosine kinases (PTK) on L-type calcium channels in cultured retinal pigmented epithelium (RPE) from rats with retinal dystrophy was investigated. Barium currents through Bay K 8644 (10(-6) M) sensitive L-type channels were measured using the patch-clamp technique. The current density of L-type currents is twice as high and the inactivation time constants are much slower than in cells from nondystrophic control rats. Application of the PTK blockers genistein, lavendustin A, and herbimycin A (all 5 x 10(-6) M) led to an increase of L-type currents. Intracellular application of pp60c-src (30 U/ml) via the patch pipette led to a transient decrease of L-type currents. The protein kinase A (PKA) and PKG blocker H9 (10(-6) M) showed no effect on L-type currents. However, the protein kinase C blocker chelerythrine (10(-5) M) reduced these currents. Up-regulation of PKC by 10(-6) M 4beta-phorbol-12 myristate-13 acetate (PMA) led to a decrease of L-type currents. Additional application of genistein led to a further decrease of these currents. However, intracellular application of pp60(c-src) in PMA-treated cells led to a transient increase of L-type currents. Investigating the calcium response to bFGF application showed that RPE cells from RCS rats used different pathways than control RPE cells to increase cytosolic free calcium. This different pathway does not involve the activation of L-type channels. The present study with RPE cells from rats with retinal dystrophy shows a changed integration of PTK and PKC in channel regulation. Considering the altered response to bFGF in RCS-RPE cells, this disturbed regulation of L-type channels by tyrosine kinases is involved in the etiology of retinal degeneration in RCS rats.

PKC activity modulates availability and long openings of L-type Ca2+ channels in A7r5 cells

The possibility that protein kinase C (PKC) could control the activity of L-type Ca2+ channels in A7r5 vascular smooth muscle-derived cells in the absence of agonist stimulation was investigated using the patch-clamp technique. Consistent with the possibility that L-type Ca2+ channels are maximally phosphorylated by PKC under these conditions, we show that 1) activation of PKC with the phorbol ester phorbol 12,13-dibutyrate was ineffective in modulating whole cell and single-channel currents, 2) inhibition of PKC activity with staurosporine or chelerythrine inhibited channel activity, 3) inhibition of protein phosphatases by intracellular dialysis of okadaic acid did not affect whole cell currents, and 4) the inhibitory effect of staurosporine was absent in the presence of okadaic acid. The inhibition of Ca2+ currents by PKC inhibitors was due to a decrease in channel availability and long open events, whereas the voltage dependence of the open probability and the single-channel conductance were not affected. The evidence suggests that in resting, nonstimulated A7r5 cells there is a high level of PKC activity that modulates the gating of L-type Ca2+ channels.

Mechanism of galanin-induced contraction of longitudinal smooth muscle of the rat jejunum

Intact and alpha-toxin-permeabilized longitudinal smooth muscle were mounted for measurement of force and myosin light chain phosphorylation. Galanin contracted intact jejunum with a half-maximum effective concentration of 9.2 +/- 0.1 nM. Neither atropine, hexamethonium, guanethidine, nor tetrodotoxin affected the contraction. The contraction was also unaffected by depletion of intracellular Ca2+ or by addition of thapsigargin; removal of extracellular Ca2+ or addition of nifedipine abolished the contraction. Galanin increased myosin light chain phosphorylation levels concomitantly with force. During continued tissue stimulation, force fell to suprabasal values, whereas myosin light chain phosphorylation levels remained elevated. Galanin increased Ca2+ sensitivity of contraction in alpha-toxin-permeabilized tissues, and this was reversed by either guanosine 5'-O-(2-thiodiphosphate) or pertussis toxin. These results suggest that galanin-induced contraction of longitudinal jejunal smooth muscle is dependent on a pertussis toxin-sensitive G protein that is apparently not coupled to the release of intracellular Ca2+ but to the influx of extracellular Ca2+ and involves an initial myofilament Ca2+ sensitization followed by Ca2+ desensitization.

Inhibition of Ca2+ uptake into A7r5 vascular smooth muscle cells by farnesol: lack of effect on membrane fluidity and Ca2+-ATPase activities

BACKGROUND

Previous studies have shown that farnesol, a 15-carbon nonsterol derivative of mevalonic acid, inhibits vasoconstriction. Because of its lipophilic properties, we hypothesized that farnesol increased membrane dynamics, thus reducing uptake of Ca2+ and contraction.

OBJECTIVE

To characterize the effect of farnesol on cell membrane fluidity.

DESIGN

The study was conducted using A7r5 cells, a rat aortic vascular smooth muscle cell line. Inhibition of Ca2+ uptake by farnesol was first established in these cells. Then, the effect of farnesol on membrane dynamics was determined. Finally, to ascertain that activation of Ca2+ extrusion and reuptake processes by farnesol did not occur, Ca2+-ATPase activity was examined.

METHODS

Membrane fluidity in cell homogenates was estimated using two fluorescent dyes (1,6-diphenyl-1,3,5-hexatriene) and (1-[-(trimethylamino)-phenyl]-6-phenyl-1,3,5-hexatriene). Ca2+ uptake was determined by monitoring the changes in cytosolic Ca2+ concentration ([Ca2+]i) in fura-2-loaded cells after addition of KCI. Ca2+-ATPase activity was measured in 100000 x g cell fractions.

RESULTS

Farnesol reduced KCI-induced (Ca2+]i transients significantly (P < 0.001), but did not modify membrane dynamic properties [0.214+/-0.007 versus 0.218+/-0.007 (n = 10) and 0.142+/-0.002 versus 0.146+/-0.003 (n = 5) for 1 -[-(trimethylamino)-phenyl]-6-phenyl-1,3,5-hexatriene and 1,6-diphenyl-1,3,5-hexatriene anisotropies, respectively; NS]. Administration of up to 30 micromol/l farnesol did not affect Ca2+-ATPase activity.

CONCLUSION

Farnesol inhibits KCI-dependent rise of [Ca2+]i in A7r5 cells. This effect of farnesol is not related to a global change in plasma membrane lipid organization or to activation of Ca2+ pumps. Other mechanisms such as direct inhibition of voltage-dependent Ca2+ channels could therefore explain the biologic action of farnesol in the vascular tissue.

Oxidized LDLs but not native LDLs augment Ba2+ currents through L-type Ca2+ channels of the A7r5 smooth muscle-derived cell line

The whole-cell patch-clamp method was used on A7r5 smooth muscle-derived cell line, and Ba2+ currents through Ca2+ channels were recorded. The A7r5 cells showed voltage-dependent, long-lasting Ba2+ currents which were markedly inhibited by nifedipine (10 microM). The magnitude of the maximum Ba2+ current (IBa(max)) was augmented by an application of dbcAMP (1 mM), but not affected by TPA (80 nM). Noradrenaline (NA) at 100 microM caused an increase in the IBa(max) by 19.7% in the presence of phentolamine (10 microM). This effect was cancelled by Rp-cAMPs (10 microM). In the presence of propranolol (10 microM), NA tended to reduce the IBa(max). Application of Ox-LDLs at 100 microg protein/ml caused an increase in the IBa(max) by 15.7%, whereas native LDLs did not change the IBa(max). Rp-cAMPs was ineffective to the Ox-LDL action on the IBa(max). In the presence of Ox-LDLs, NA augmented the IBa(max) by 21.4% in the presence of phentolamine. These results suggest that Ox-LDLs activate L-type Ca2+ channels of A7r5 cells by a mechanism independent of cAMP/PKA signalling.

Activation by high potassium of a novel voltage-operated Ca2+ channel in rat spleen

1. High potassium produced a concentration-dependent contraction in rat isolated spleen. 2. The high potassium-induced contraction of rat spleen was abolished in Ca(2+)-free Krebs solution containing 1 mM EGTA, and the subsequent addition of 3 mM Ca2+ restored the high potassium-induced contraction to the control level. 3. Nifedipine, verapamil, diltiazem, Cd2+, Ni2+, Co2+, R-(+)-Bay K 8644 and pimozide inhibited and relaxed high potassium-induced contraction of rat spleen with IC50 and EC50 values much higher than those values in rat aorta. 4. In addition, high potassium-stimulated contraction of rat spleen was insensitive to omega-conotoxin GVIA, omega-conotoxin MVIIC and omega-agatoxin IVA. 5. The high potassium-induced contraction of rat spleen was also unaffected by tetrodotoxin (TTX), prazosin, chloroethylclonidine (CEC), yohimbine, propranolol, atropine, diphenhydramine, cimetidine, ketanserin, 3-tropanyl-indole-3-carboxylate, saralasin, indomethacin, nordihydroguaiaretic acid, GR32191B, domperidone, naloxone, chlorpromazine, suramin, (+/-)-2-amino-5-phosphonopentanoic acid, 6,7-dinitroquinoxaline-2,3-dione (DNQX), L-659,877, L-703,606, lorglumide, PD 135,158 N-methyl-D-glucamine, benextramine, amiloride, dantrolene, TMB-8, econazole, staurosporine and neomycin. 6. Forskolin and sodium nitroprusside relaxed high potassium-induced contraction of rat spleen with EC50 values of 0.55 +/- 0.04 and 20.0 +/- 2.7 microM, respectively. 7. It is concluded that high potassium may activate a novel, pharmacologically uncharacterized voltage-operated Ca2+ channel in rat spleen.

Excitatory and inhibitory actions of norepinephrine on the Ba2+ current through L-type Ca2+ channels of smooth muscle cells of guinea-pig vas deferens

The effect of norepinephrine (NE) was examined on the whole-cell Ba2+ current through L-type Ca2+ channels of freshly isolated smooth muscle cells of guinea-pig vas deferens. The magnitude of maximum Ba2+ current [1Ba(max)] varied in different cells, although the capacitance of the cell membrane was similar (approximately 50 pF). Application of dbcAMP augmented 1Ba(max) by 37%, which was canceled by Rp-cAMPs, while PMA decreased the current by 32%, which was canceled by staurosporine. NE increased 1Ba(max) of the cells which originally showed relatively small 1Ba(max), and decreased the current of the cells which showed larger 1Ba(max). In the presence of phentolamine, NE increased 1Ba(max), and this effect was remarkable in cells showed smaller 1Ba(max). In the presence of propranolol, NE decreased 1Ba(max). The excitatory beta-adrenoceptor activation was canceled by Rp-cAMPs, and the inhibitory alpha-adrenoceptor effect was canceled by staurosporine. It is suggested that NE shows dual (excitatory and inhibitory) actions on the L-type Ca2+ channels of smooth muscle of guinea-pig vas deferens. The excitatory beta-adrenoceptor action mediated through cAMP/PKA is predominant in cells with lower density of the Ca2+ channels, while inhibitory alpha-adrenoceptor action mediated through PKC is predominant in cells with higher channel density.

Estrogen increases the expression of uterine protein kinase C isozymes in a tissue specific manner

The pattern of protein kinase C isozyme expression in uterine smooth muscle and ventricular cardiac muscle was examined in ovariectomized rats pretreated with estradiol-17 beta alone or with estradiol-17 beta and progesterone. Protein kinase C isozyme expression was examined in membrane and cytosolic subcellular fractions by immunoblot analysis using antisera specific for alpha, gamma, beta 1, beta 2, delta, epsilon, zeta, theta isozymes. All isozymes were detectable in positive control brain extracts. The predominant isozymes in the myometrium were delta and beta 2 while in the ventricle, beta 2 and zeta were the dominant forms. In unstimulated tissues, all isozymes except PKC-delta, were predominantly found in the cytosolic compartment. Both estrogen and progesterone increased membrane-associated isozyme expression 35-125% in uterine muscle. Neither estrogen nor progesterone treatment significantly affected protein kinase C expression in cardiac muscle. These data suggest that estradiol, which increases uterine muscle hypertrophy and contractility, may exert these effects by increasing membrane-associated protein kinase C expression in a tissue-specific manner.

Intracellular signaling pathway of substance P-induced superoxide production in human neutrophils

We examined the intracellular mechanisms of substance P-induced superoxide anion (O2-) production in human neutrophils. Addition of substance P (30 microM) caused O2- production and biphasic increases in intracellular Ca2+ concentrations ([Ca2+]i) (early transient and subsequent sustained components) associated with the formation of inositol 1,4,5-trisphosphate (IP3). O2- and [Ca2+]i were assayed by using ferricytochrome C and fura 2-AM, respectively. These responses were abolished by tachykinin NK1 receptor antagonists, [D- Pro9[spiro-gamma-lactam],Leu10,Trp11]physalaemin-(1-11) (GR82334) or [D-Arg1,D-Trp7,9,Leu11]substance P (spantide), and an intracellular Ca2+ chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM). Inhibition of IP3 formation by GTP-binding protein (G-protein) inactivators such as guanosine 5'-O-(2-thiodiphosphate) (GDP beta S) and islet-activating protein (IAP), or a phospholipase C inhibitor, 1-[6-[[17 beta-3-methoxyestra-1,3,5(10)- trien-17-yl]amino]hexyl]1 H-pyrrole-2,5-dione (U-73122), blocked the substance P-induced O2- production and biphasic increases in [Ca2+]i. An IP3 receptor antagonist, heparin, reduced both the substance P-induced O2- production and the transient increase in [Ca2+]i without any significant effects on the sustained increase in [Ca2+]i. Protein kinase C inhibitors, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7) and calphostin C, only slightly suppressed O2- production, and abolished the sustained increase in [Ca2+]i without any significant effects on the transient increase in [Ca2+]i. A Ca2+ entry blocker, nicardipine, completely inhibited the sustained increase in [Ca2+]i without affecting O2- production and the transient increase in [Ca2+]i. These results suggest that the tachykinin NK1 receptor/G-protein-linked IP3 formation with the resulting IP3-induced transient increase in [Ca2+]i is the main signal transduction pathway for substance P-stimulated O2- production in neutrophils.

Modulation of Ca(2+)-activated K+ current by isoprenaline, carbachol, and phorbol ester in cultured (and fresh) rat aortic vascular smooth muscle cells

1. Effects of isoprenaline (ISO), carbachol, and phorbol ester on the outward K+ currents in single cultured (or fresh) rat aortic vascular smooth muscle (A7r5 and A-10) cells were examined using a whole-cell voltage-clamp (at room temperature 22 degrees C). 2. With 10 mM EGTA in the pipette solution, the delayed rectifier K+ current (IK) was activated by Ca2+ at pCa 7 more than at pCa 10, and was TEA (10 mM) and apamin (200 nM) sensitive, which represents a Ca(2+)-activated K+ current (IKCa). 3. In cultured A7r5 cells, isoprenaline (1 and 5 microM) and carbachol (0.1 and 1 microM) inhibited IKCa. Phorbol ester, 4-beta-phorbol-12, 13-dibutyrate (PDB), at 0.1 and 1 microM also inhibited IKCa, and increased the inhibitory effects induced by isoprenaline (1 microM). 4. In fresh aortic cells, these drugs, at the same concentrations, also produced the similar effects. 5. In A-10 cells, PDB (1 microM) enhanced the transient outward current (4-AP-sensitive), but ISO (1 microM) inhibited the current. 6. These results suggest that the IKCa current would be inhibited by cyclic nucleotides (cAMP and cGMP) and also by PK-C stimulation, and thereby be directly contributed to excitation-contraction coupling of the vascular smooth muscle cells.

Translocation of protein kinase C isoenzymes by elevated extracellular Ca2+ concentration in cells from a human giant cell tumor of bone

In this study we investigated the protein kinase C isoenzymes expressed by human osteoclast-like cells harvested from a giant cell tumor of bone (GCT23 cells), and by freshly isolated rat osteoclasts. Immunoblotting analysis revealed that the -alpha, -delta, and -epsilon, PKC isoforms, but not the -beta isoenzyme, are expressed by GCT23 cells. Immunofluorescence studies demonstrated that PKC-alpha, -delta, and -epsilon are homogeneously expressed by both mononuclear and multinucleated GCT23 cells, as well as by rat osteoclasts. Similar to authentic osteoclasts, GCT23 cells responded to an increase of extracellular Ca2+ concentration ([Ca2+]o) with a dose-dependent elevation of the cytosolic free Ca2+ concentration ([Ca2+]i). An increase of [Ca2+]o stimulated the translocation of PKC-alpha from the cytosolic to the particulate fraction, suggesting the involvement of this isoenzyme in the signal transduction mechanism prompted by stimulation of the [Ca2+]o sensing. By contrast, PKC-delta was not altered by exposure to elevated [Ca2+]o, whereas PKC-epsilon underwent reciprocal translocation, disappearing from the insoluble fraction and increasing in the cytosol. The effects of PKC on GCT23 cell functions were investigated by treatment with phorbol 12-myristate, 13-acetate (PMA). We observed that activation of PKC by PMA failed to affect adhesion onto the substrate, but down-regulated the [Ca2+]o-induced [Ca2+]i increases. The latter effect was specific, since it was reversed by treatment with the PKC inhibitors staurosporine and chelerythrine.

Identification of the Ca2+ current activated by vasoconstrictors in vascular smooth muscle cells

The noncontractile aortic cell line A7r5 was chosen to study the effect of the vasoconstrictor peptide vasopressin on transmembrane Ca2+ movements, using conventional whole-cell patch recording techniques. Conditions in which previously characterised vasoconstrictor-modulated currents were suppressed revealed a tiny inward current component (-18 +/- 2 pA, n = 50, at -61 mV in 110 mM CaCl2). The vasopressin-activated inward current was absent when Ca2+ was absent from the extracellular solution, and the current amplitude increased with [Ca2+] (0.01-110 mM), with an apparent dissociation constant for Ca2+ of 9.7 mM. It was highly selective for Ca2+ over monovalent cations (permeability ratio Ca/Cs greater than 17). It was not voltage gated, except that the current/potential characteristic showed some inwards rectification. Amplitudes of the evoked inward currents had the same order of magnitude in Sr2+ and Ca2+, whereas they were much smaller in Mn2+, suggesting that this pathway is highly permeable to Sr2+ but poorly permeable to Mn2+. Inward currents evoked in Ca2+ were inhibited by other cations with the following order of potency: La3+ > Cd2+ > Co2+ approximately Ni2+ approximately Mn2+. The channel producing this current corresponds most probably to the ionic pathway originally called the receptor-operated calcium channel, which produces a long-lasting, constrictor-induced plateau of increased intracellular free calcium concentration in smooth muscle.

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.

Protein kinase A increases availability of calcium channels in smooth muscle cells from guinea pig basilar artery

We studied single Ca2+ channels in smooth muscle cells from the basilar artery of the guinea pig using conventional patch-clamp techniques. With 40 mM or 90 mM Ba2+ as the charge carrier, a 23-pS inward current channel was observed in 46/187 cell-attached patches studied without the dihydropyridine, BAY K8644, in the pipette solution. At 0 mV, this channel exhibited short and long openings with time constants of 1.03 and 3.65 ms, respectively. The probability of channel opening was voltage dependent with half-activation occurring at +9.9 mV. In 14/26 patches tested, addition of 8-bromo-cyclic adenosine monophosphate (8-Br-cAMP) to the bath increased the probability of opening at -10 mV by a factor of 2.6, from 0.0272 +/- 0.0429 to 0.0695 +/- 0.0788 (P < 0.01, paired t-test). Mean data from five patches fit to a Boltzmann function indicated that at positive potentials, the probability of opening increased by a factor of 1.7, from 0.352 to 0.600, whereas the voltage dependence, the number of channels, the number of open states, the time constants of the open states, and the proportion of time spent in each open state were unchanged. When BAY K8644 was added to the pipette solution, the 23-pS channel was observed in nearly all patches (62/66), but the voltage dependence of activation was shifted -15.3 mV compared to control. In some patches studied with 90 mM Ba2+, a 9-pS inward current channel also was observed and its activity also was increased significantly by 8-Br-cAMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein kinase-C mediates dual modulation of L-type Ca2+ channels in human vascular smooth muscle

The role of protein kinase C (PKC) in cellular regulation of L-type Ca2+ channels was investigated in human umbilical vein smooth muscle. Activation of PKC, by low concentrations (< 30 nM) of 12-O-tetradecanoyl-phorbol-13-acetate (TPA) caused inhibition of Ca2+ channels, while higher concentrations of TPA (> 100 nM) elicited a transient rise, followed by sustained inhibition of Ca2+ channel activity in cell-attached patches. Low TPA concentrations predominantly reduced channel availability, while high concentrations of TPA (100 nM) transiently increased channel availability and, in addition, prolonged mean open time. The inactive 4-alpha-phorbol-12,13- didecanoate failed to affect channel activity, and pretreatment of the cells with PKC inhibitors (H-7, chelerythrine) antagonized inhibitory and stimulatory effects of TPA. Our results provide evidence for two distinct PKC-dependent mechanisms of L-type Ca2+ channel regulation in smooth muscle.

Nitric oxide decreases [Ca2+]i in vascular smooth muscle by inhibition of the calcium current

Endothelium derived relaxing factor (nitric oxide, or NO) activates cytoplasmic guanylate cyclase in vascular smooth muscle and decreases vascular tone through cGMP-dependent mechanisms that are not yet understood fully. In cultured vascular smooth muscle cells (A7r5 cell line) sodium nitroprusside (NP), a vasodilator that decomposes into nitric oxide, lowered [Ca2+]i in cells in which [Ca2+]i was elevated after depolarization. NP decreased current through voltage-gated calcium channels, but did not affect release of calcium from intracellular stores. Hemoglobin, a scavenger of NO, reversed the effect of NP on [Ca2+]i and 8-Br-cGMP, a membrane permeant form of cGMP, mimicked the effect of NP on [Ca2+]i and on calcium currents. Thus, the signal transduction mechanism of endothelium dependent relaxation of vascular smooth muscle involves a decrease in [Ca2+]i by inhibition of Ca2+ entry. Relaxation or vasodilation would then result from decreased activity of myosin light chain kinase, in addition to myosin light chain dephosphorylation.

Mechanism of the contractile response to platelet-activating factor (PAF) of the rat stomach fundus. II. PAF-induced phosphatidylinositol turnover and desensitization

1. Accumulation of [3H]-inositol phosphates (IPs) was slightly enhanced by PAF in a concentration-dependent manner, but the accumulation was very small as compared with that induced by carbachol. 2. The levels of [32P]-phosphatidic acid which is transformed from diacylglycerol (DAG) were increased by treatment with PAF or with carbachol. 3. PAF-induced contraction was significantly reduced by treatment with phorbol 12-myristate 13-acetate (PMA). 4. These results suggest that while PAF slightly stimulates the turnover of phosphatidylinositol (PI) in the rat stomach fundus, this response may not be responsible for the PAF-induced contractile response, and that the desensitization induced by repeated application of PAF may be due to the activation of protein kinase C.

Modulation of protein kinase C alters hemodynamics and metabolism in the isolated liver in fed and fasted rats

The activation of protein kinase C (PKC) has been implicated in the pathogenesis of gram-negative sepsis. The effects of PKC modulation on hepatic flow and metabolism were studied using isolated liver perfusion. The liver was isolated from well-fed or overnight-fasted, male Sprague-Dawley rats weighing 250-310 g, and perfused at a constant pressure of 12 cmH2O using a recirculating system. Phorbol 12-myristate 13-acetate (PMA), a potent activator of PKC, decreased hepatic flow and oxygen consumption, and increased net lactate production. It enhanced net glucose production in fed animals. Neither 4 alpha-phorbol didecanoate, an inactive phorbol ester for PKC nor 4 alpha-phorbol, an inactive phorbol had any significant effect. The effects of PMA were augmented by increasing calcium concentration in the medium. PMA at an initial concentration of 4 x 10(-8) M stimulated net lactate and/or glucose production more than a reduction of perfusion pressure from 12 to 6 cmH2O. Staurosporine, a potent PKC inhibitor, significantly attenuated the PMA-induced alterations of hepatic flow and oxygen consumption. These results indicate that modulation of PKC exerts significant effects on hepatic flow and metabolism, which are dependent on extracellular calcium concentrations and feeding conditions, and that the effect of PMA on carbohydrate metabolism is not merely attributed to decreases in hepatic flow and oxygen consumption. It is suggested that PKC activation may be involved in the alterations of hepatic flow and metabolism during severe sepsis.

Effects of dexamethasone on L-type calcium currents in the A7r5 smooth muscle-derived cell line

Patch clamp experiments were used to characterize the effect of dexamethasone on calcium currents in A7r5 cells. Pretreatment for 48 h with 200 nM dexamethasone did not affect the single channel conductance, the voltage dependence of channel opening, or the voltage-dependent inactivation of L-type channels. However, dexamethasone caused an approximately 2-fold increase in the amplitude of L-type calcium currents in 5 out of 9 experiments, suggesting an increase in the number of active channels. The effect of dexamethasone appeared to be greatest on batches of cells with low control current density. The amplitude of T-type calcium current was not affected by dexamethasone.

ATP-induced entry of calcium in thyroid FRTL-5 cells. Studies with phorbol myristate acetate and thapsigargin

Receptor-mediated Ca2+ entry was investigated in fura-2-loaded thyroid FRTL-5 cells. Activation of protein kinase C (PKC) by phorbol myristate acetate (PMA) attenuated the ATP-induced increase in intracellular free Ca2+ ([Ca2+]i). In PKC down-regulated cells, the ATP-induced increase in [Ca2+]i was increased compared with control cells. This enhanced increase in [Ca2+]i was apparently dependent on extracellular Ca2+, as no difference was observed between control cells and PKC down-regulated cells in Ca(2+)-free buffer. Addition of Ca2+ to cells stimulated with ATP in Ca(2+)-free buffer rapidly increased [Ca2+]i. The increase was blocked by PMA. However, PKC down-regulation had no effect on the [Ca2+]i response. Stimulating FRTL-5 cells with thapsigargin increased [Ca2+]i. Addition of ATP after thapsigargin had almost no effect on [Ca2+]i. In PKC down-regulated cells, addition of ATP after thapsigargin evoked a substantial increase in [Ca2+]i which was dependent on extracellular Ca2+. The results indicate that PKC has a modulatory effect on the ATP-induced entry of Ca2+ in FRTL-5 cells.

Phosphatidylinositol metabolism in hypertrophic rat heart

The accumulation of inositol 1,4,5-trisphosphate (IP3) after hormonal stimulation has a physiological role, possibly by alteration of Ca2+ levels in cardiac myocyte. However, this accumulation has not been studied under pathophysiological conditions. In this report, we examine phosphatidylinositol metabolism during cellular response to norepinephrine in pressure-overloaded hypertrophic rat heart. After stimulation with norepinephrine, the accumulations of IP3 and diacylglyceride significantly increased in isolated myocytes from stroke-prone spontaneously hypertensive rat (SHRSP) heart, indicating phosphatidylinositol-specific phospholipase C activity increased in SHRSP heart cells. Protein kinase C activity was also enhanced in SHRSP, with a marked increase in particulate activity. We determined the intracellular calcium concentration and found it to be higher in SHRSP than in Wistar-Kyoto (WKY) rats at 30-40 weeks of age. Ca2+ influx was also elevated in SHRSP stimulated by norepinephrine. In SHRSP heart, cytosolic Ca2+ concentration may rise quickly in response to some stimuli, such as alpha 1-adrenergic stimulation, which is shown to be one of the pathways that increases cytosolic Ca2+ levels in hypertrophied rat heart. These data suggest that a part of the phosphatidylinositol-turnover pathway, such as the phosphatidylinositol 4,5-bisphosphate-IP3-Ca2+ pathway or the diacylglyceride-protein kinase C pathway, may play an important role in the development of hypertrophy in SHRSP heart.

The actions of phorbol esters upon isolated calcium currents of Helix aspersa neurones

1. Voltage-clamp recordings have been made from identified neurones in the suboesophageal ganglia of Helix aspersa. 2. Calcium currents were isolated pharmacologically and studied under two-electrode voltage-clamp. 3. PdBu but not PMA caused a transient enhancement followed by an irreversible inhibition of the calcium current. 4. It is concluded that activation of protein kinase C within these neurones may show selectivity with respect to the exogenous activator used.

Protein kinase C of smooth muscle

The primary mechanism of regulation of smooth muscle contraction involves the phosphorylation of myosin catalyzed by Ca2+/calmodulin-dependent myosin light chain kinase. However, additional mechanisms, both Ca(2+)-dependent and Ca(2+)-independent, can modulate the contractile state of smooth muscle. Protein kinase C was first implicated in the regulation of smooth muscle contraction with the observation that phorbol esters induce slowly developing, sustained contractions. Protein kinase C occurs in at least four Ca(2+)-dependent (alpha, beta I, beta II, and gamma) and four Ca(2+)-independent (delta, epsilon, zeta, and eta) isoenzymes. Only the alpha, beta, epsilon, and zeta isoenzymes have been identified in smooth muscle. Both classes of isoenzymes have been implicated in the regulation of smooth muscle contraction. However, the physiologically important protein substrates of protein kinase C have not yet been identified. Specific isoenzymes may be activated by different contractile agonists, and individual isoenzymes exhibit some degree of substrate specificity. Prolonged activation of protein kinase C can result in its proteolysis to the constitutively active catalytic fragment protein kinase M, which would dissociate from the sarcolemma and phosphorylate proteins such as myosin that are inaccessible to membrane-bound protein kinase C. Protein kinase M induces relaxation of demembranated smooth muscle fibers contracted at submaximal Ca2+ concentrations. We suggest that protein kinase C plays two distinct roles in regulating smooth muscle contractility. Stimuli triggering phosphoinositide turnover or phosphatidylcholine hydrolysis induce translocation of protein kinase C (probably specific isoenzymes) to the sarcolemma, phosphorylation of protein, and a slow contraction. Prolonged association of the kinase with the membrane may lead to proteolysis and release into the cytosol of protein kinase M, resulting in myosin phosphorylation and relaxation.

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.

Desensitization of histamine H1 receptor-mediated cyclic GMP production in guinea-pig lung

Histamine H1 receptor-mediated production of cGMP in guinea-pig lung tissue becomes rapidly desensitized after previous exposure to histamine. This desensitization is clearly concentration dependent and appears to be homologous. Responses to histamine are also inhibited by previous treatment with phorbol 12,13-dibutyrate. Yet, the time course of the inhibition is considerably slower and the maximal inhibition is significantly less compared to receptor desensitization. Moreover, the effects of the phorbol ester are not confined to H1 receptor responses. Since the effects of receptor desensitization are also not prevented by several protein kinase C inhibitors, the development of homologous H1 receptor desensitization is not dependent upon protein kinase C activation, but is caused by a yet unidentified mechanism.

Calcium channels and nifedipine inhibition of serotonin-induced [3H]thymidine incorporation in cultured cerebral smooth muscle cells

Cultures of smooth muscle cells were prepared from the basilar artery of adult guinea pigs. Passaged cultures (10-30 passages) that expressed serotonin receptors were studied using [3H]thymidine incorporation. When tested in quiescent medium, serotonin potently stimulated [3H]thymidine incorporation (EC50 of 31 nM) by as much as 400% at 24 h. The number of cells was not significantly increased at 24 or 48 h. At concentrations of 10(-8)-10(-5) M 5-HT, [3H]thymidine uptake was reduced 40-50% by the dihydropyridine Ca2+ channel blocker, nifedipine (1 microM). To demonstrate a possible mechanism for the sensitivity to nifedipine, Ca2+ currents were measured using the whole cell patch clamp technique. The cells expressed dihydropyridine-sensitive L-type Ca2+ channels, but not other subtypes of Ca2+ channels, as indicated by the kinetic and voltage-dependent characteristics of the current and by the stimulatory effect of Bay K 8644. The magnitude of the Ca2+ currents was related exponentially to the membrane surface area, measured as cell capacitance. These data support the association of dihydropyridine-sensitive Ca2+ channels with mitogenesis in vascular smooth muscle, and suggest an alternate mechanism of action for the beneficial effect of dihydropyridines in prophylaxis of cerebral vasospasm.

Phorbol esters induce oscillatory contractions of intestinal smooth muscles

The actions of tumor-promoting phorbol esters in smooth muscle excitation-contraction coupling were studied in isolated guinea pig ileum in the presence of various contractile agents. Muscarinic agonists, histamine and bradykinin elicited an initial transient phasic contraction and a subsequent sustained tonic contraction in guinea pig ileum. The Ca2+ channel antagonist nifedipine selectively inhibited the tonic contraction. Phorbol esters, protein kinase C activators, induced immediate muscle relaxation followed by oscillatory contractions when added during the tonic phase of contraction. Phorbol esters, when added in advance, slightly altered the ligand-induced phasic contraction but converted tonic contractions into oscillatory spikes. The amplitude, frequency and shape of the oscillation induced by phorbol esters were dependent upon the dose of phorbol ester: amplitude was increased and frequency was decreased by increasing the doses of phorbol ester. In contrast, the phorbol ester potentiated the tonic contraction induced by high potassium chloride with little effect on the phasic component. It also sensitized the muscles to Bay K 8644. Bay K 8644, which was ineffective in stimulating muscle contraction at 1 nM, became a very effective stimulator in the presence of the phorbol ester. All of these phorbol ester-induced potentiations and oscillations were sensitive to inhibition by staurosporine or nifedipine. These data suggest that in guinea pig ileum, protein kinase C plays a positive regulatory role in Ca2+ channel activation and promotes a complex regulatory effect on Ca(2+)-mobilizing ligand-stimulated Ca2+ channel activity, which results in oscillatory contractile responses to carbachol, methacholine, histamine and bradykinin.