Role of prostaglandin-mediated cyclic AMP formation in protein kinase C-dependent secretion of atrial natriuretic peptide in rat cardiomyocytes

Attenuation of renovascular hypertension by cyclooxygenase-2 inhibitor partly through ANP release

Angiotensin II (Ang II) is an important inflammatory mediator. Ang II induces cyclooxygenase-2 (COX-2) expression and prostaglandin F2α release followed by cardiac hypertrophy. Inhibition of COX-2 may modulate high blood pressure but controversy still exists. The aim of this study was to determine the role of COX-2 in the regulation of blood pressure and to define the mechanisms in two kidney one-clip hypertensive (2K1C) rats. Chronic treatment with nimesulide or NS-398 (5 mg/kg/day) for 3 weeks lowered high blood pressure and cardiac hypertrophy with decreased expression levels of cardiac hypertrophy markers [atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP)], Ang type 1 receptor, urotensin II, and urotensin II receptor in 2K1C rats. Plasma level of ANP was markedly increased and plasma levels of Ang II and aldosterone were decreased by treatment with nimesulide or NS-398. In both in vitro and in vivo experiments, nimesulide or NS-398 augmented ANP release in 2K1C rats. The inhibitory effect of NS-398 on blood pressure was attenuated by the pretreatment with natriuretic peptide receptor-A (NPR-A) antagonist (A71915, 30 μg/kg/day). These results suggest that chronic treatment with nimesulide or NS-398 attenuated hypertension and cardiac hypertrophy partly through ANP release in 2K1C rats.

Regulation of ANP secretion from isolated atria by prostaglandins and cyclooxygenase-2

Cyclooxygenase (COX) is a key enzyme regulating the production of various prostaglandins (PGs) from arachidonic acid. Angiotensin II has been reported to be an important inflammatory mediator, which increases COX-2. The aim of this study was to determine the role of various PGs and COX-2 in the regulation of atrial natriuretic peptide (ANP) secretion. PGF2alpha and PGD2 caused dose-dependent increases in ANP release and intra-atrial pressure. The potency for the stimulation of ANP secretion by PGF2alpha was higher than that by PGD2. In contrast, PGE2, PGI2, PGJ2, and thromboxane A2 did not show any significant effects. The increases in intra-atrial pressure and ANP secretion induced by PGF2alpha and PGD2 were significantly attenuated by the pretreatment with an inhibitor of PGF2alpha receptor. By the pretreatment with an inhibitor for phospholipase C (PLC), inositol 3-phosphate (IP3) receptor, protein kinase C (PKC), or myosin light chain kinase (MLCK), PGF2alpha-mediated increase in ANP secretion and positive inotropy were attenuated. Inhibitor for COX-1 or COX-2 did not cause any significant effects on atrial parameters. In hypertrophied rat atria, PGF2alpha-induced positive inotropy and ANP secretion were markedly attenuated whereas COX-2 inhibitor stimulated ANP secretion. The expression of COX-2 increased and the expression of PGF2alpha receptor mRNA decreased in hypertrophied rat atria. These results suggest that PGF2alpha increased the ANP secretion and positive inotropy through PLC-IP3-PKC-MLCK pathway, and the modulation of ANP secretion by COX-2 inhibitor and PGF2alpha may partly relate to the development of renal hypertension.

Effect of H-7 and Lat-B on retinal physiology

PURPOSE

To investigate the effects of H-7 and Latrunculin B (Lat-B) on retinal vascular permeability and electrophysiology at concentrations that increase outflow facility in monkeys.

METHODS

One eye of 1 rhesus and 22 cynomolgus monkeys received an intravitreal bolus injection of H-7 or Lat-B; the opposite eye received vehicle. Multifocal electroretinograms (mfERGs), and photopic and scotopic full-field electroretinograms (ffERGs, sERGs) were recorded in subsets of monkeys at baseline and at multiple time-points post-H-7 or Lat-B. Vitreous fluorophotometry (VF) and fluorescein angiography (FA) were also performed.

RESULTS

No differences between the H-7 or Lat-B treated and control eyes were found in ffERGs, mfERGs, sERGs, or in FAs in any monkey. No significant difference was found in vitreous fluorescein levels between H-7 treated or Lat-B treated vs. control eyes.

CONCLUSIONS

No effect on retinal vascular permeability or retinal electrophysiology was apparent after intravitreal administration of H-7 or Lat-B at doses that increase outflow facility and lower IOP when given intracamerally.

Bremazocine increases C-type natriuretic peptide levels in aqueous humor and enhances outflow facility

A relatively selective agonist of kappa opioid receptors (KOR), bremazocine (BRE), lowers intraocular pressure in rabbits, in part, by increasing natriuretic peptide levels in aqueous humor and by enhancing total outflow facility (TOF). Natriuretic peptide (NP) levels [atrial NP (ANP), brain NP (BNP), and C-type NP (CNP)] were measured in aqueous humor of rabbits either by radioimmunoassay or enzyme immunoassay. TOF was determined in rabbits by two-level constant pressure perfusion of the anterior chamber. Experimental regimens included topical treatment with BRE in the presence or absence of KOR antagonist (norbinaltorphimine), protein kinase C inhibitor (chelerythrine), and natriuretic peptide receptor antagonist (isatin). The rank order of basal NP levels in aqueous humor of rabbits was BNP CNP > ANP. Topical administration of BRE (1-100 microg) caused dose-related elevations of CNP levels in aqueous humor that were inhibited by topical pretreatment with either norbinaltorphimine (100 microg, bilaterally) or chelerythrine (10 microg, bilaterally). Topically administered BRE (100 microg) also elevated levels of ANP and BNP in aqueous humor and evoked an 80% increase in TOF. The increase in TOF was antagonized by topical pretreatment with either norbinaltorphimine (100 microg, bilaterally) or isatin (100 microg, bilaterally). Bremazocine induced an increase in NP (ANP, BNP, and CNP) levels and TOF in rabbits by activating KOR. The increase in CNP levels elicited by BRE was inhibited by norbinaltorphimine and chelerythrine; therefore, this event is most likely mediated by a KOR-linked activation of protein kinase C. These data provide evidence that the increase in TOF elicited by BRE was mediated by a KOR-activated paracrine effect of NPs on tissues within ocular outflow tract(s).

Promoter regulatory elements and DNase I-hypersensitive sites involved in serglycin proteoglycan gene expression in human erythroleukemia, CHRF 288-11, and HL-60 cells

We have compared regulation of the serglycin gene in human erythroleukemia (HEL) and CHRF 288-11 cells, which have megakaryocytic characteristics, with promyelocytic HL-60 cells. Deletion constructs were prepared from the region -1123/+42 to -20/+42, and putative regulatory sites were mutated. In all three cell lines, the two major regulatory elements for constitutive expression were the (-80)ets site and the cyclic AMP response element (CRE) half-site at -70. A protein from HEL and CHRF, but not HL60, nuclear extracts bound to the (-80)ets site. Another protein from all three cell lines bound to the (-70)CRE. Phorbol 12-myristate 13-acetate (PMA) and dibutyryl cyclic AMP (dbcAMP) increased expression of the reporter in HEL cells 2.5-3- and 4.5-fold, respectively, from all constructs except those with (-70)CRE mutations. PMA virtually eliminated expression of serglycin mRNA and promoter constructs, but dbcAMP increased expression in HL-60 cells. The effects of PMA and dbcAMP on promoter expression correlated with mRNA expression. The strengths of two DNase I-hypersensitive sites in the 5'-flanking region and the first intron in all three cells correlated with relative endogenous serglycin mRNA expression. An additional DNase I-hypersensitive site in HL60 DNA in the first intron may be related to the high serglycin expression in HL60 relative to HEL or CHRF cells.

Urocortin, a member of the corticotropin-releasing factor family, in normal and diseased heart

In the present study we investigated the form of expression, action, second messenger, and the cellular location of urocortin, a member of the corticotropin-releasing factor (CRF) family, in the heart. Urocortin mRNA, as shown by quantitative RT-PCR analysis, is expressed in the cultured rat cardiac nonmyocytes (NMC) as well as myocytes (MC) in the heart, whereas CRF receptor type 2beta (CRF-R2beta), presumed urocortin receptor mRNA, is predominantly expressed in MC compared with NMC. Urocortin mRNA expression is higher in left ventricular (LV) hypertrophy than in normal LV, whereas CRF-R2beta mRNA expression is markedly depressed in LV hypertrophy compared with normal LV. Urocortin more potently increased the cAMP levels in both MC and NMC than did CRF, and its effect was more potent in MC than in NMC. Urocortin significantly increased protein synthesis by [(14)C]Phe incorporations and atrial natriuretic peptide secretion in MC and collagen and increased DNA synthesis by [(3)H]prolin and [(3)H]Thy incorporations in NMC. An immunohistochemical study revealed that urocortin immunoreactivity was observed in MC in the normal human heart and that it was more intense in the MC of the human failing heart than in MC of the normal heart. These results, together with the recent evidence of urocortin for positive inotropic action, suggest that increased urocortin in the diseased heart may modulate the pathophysiology of cardiac hypertrophy or failing heart, at least in part, via cAMP signaling pathway.

Role of cell contractions in cAMP-induced cardiomyocyte atrial natriuretic peptide release

Incubation of spontaneously beating ventricular cardiomyocytes from neonatal rats with prostaglandin E(2) (0.1 microM) or forskolin (0.1 microM) simultaneously increased the rate of cellular contraction and atrial natriuretic peptide (ANP) secretion. Both responses were maximal within 10-20 min of application and were accompanied by three- to fourfold increases in cAMP formation. By contrast, a higher regimen of forskolin (10 microM) promoted a 20- to 30-fold increase in basal cAMP production, which was accompanied by the abolition of contractile activity and ANP release. Low regimens of forskolin (0.1 microM) doubled the occurrence of cytosolic Ca(2+) transients associated with monolayer contraction, whereas higher regimens of forskolin (10 microM) completely suppressed Ca(2+) transients. Moreover, in quiescent cultures that were pretreated with ryanodine, tetrodotoxin, nifedipine, or butanedione monoxime, prostaglandin E(2) (0.1 microM) and forskolin (0.1 microM) failed to elicit significant ANP secretion, suggesting that cAMP-elevating agents promote ANP secretion to a great extent via an increase in cellular contraction frequency in ventricular cardiomyocytes.

Effects of adrenomedullin on cultured rat cardiac myocytes and fibroblasts

The direct effects of adrenomedullin, a novel vasorelaxant peptide, on protein synthesis and atrial natriuretic peptide release in myocytes and on DNA and collagen syntheses in fibroblasts were examined using cultured ventricular cardiocytes. The protein synthesis of cardiac myocytes was not affected by adrenomedullin under non-stimulated conditions. Endothelin-1-induced protein synthesis in myocytes was slightly but significantly elevated by adrenomedullin. Likewise, the secretion of atrial natriuretic peptide from myocytes stimulated by endothelin-1 was increased by adrenomedullin. In cardiac fibroblasts, adrenomedullin clearly inhibited DNA synthesis and collagen production in a dose-dependent manner under both basal and angiotensin II-stimulated conditions. DNA and collagen syntheses by cardiac fibroblasts were suppressed by both 8-bromo cAMP and forskolin. Furthermore, a cAMP-specific phosphodiesterase inhibitor decreased DNA and collagen syntheses in fibroblasts and enhanced the inhibitory effects of adrenomedullin on these syntheses. Our observations suggest that adrenomedullin has opposite effects on cultured cardiac myocytes and fibroblasts and that the effects of adrenomedullin at least on fibroblasts are probably mediated through a cAMP-dependent pathway. As adrenomedullin is produced and secreted from both types of cardiac cells, adrenomedullin may play a role as an autocrine/paracrine modulator in the process of cardiac remodeling, mainly by suppressing mitogenesis and collagen synthesis in fibroblasts.

Activation of visceral afferents by bradykinin and ischemia: independent roles of PKC and prostaglandins

We have shown that the cyclooxygenase (COX) and protein kinase C (PKC) systems both contribute to afferent activation in response to bradykinin (BK) and abdominal ischemia. Because the contribution from PKC to C fiber activation may depend, in part, on prostaglandin production, we hypothesized that an intact COX system is required for PKC-induced activation of ischemically sensitive abdominal visceral afferents by BK and abdominal ischemia. Single-unit activity of abdominal visceral C fibers was recorded from the right thoracic sympathetic chain of anesthetized cats. Three repeated injections of BK (1-2 micrograms/kg ia) produced similar increases in afferent activity from the baseline of 1.32 +/- 0.24, 1.37 +/- 0.32, and 1.41 +/- 0.24 impulses/s (n = 5). In another group of animals (n = 5), the second and third BK injections were performed after COX inhibition (indomethacin; 5 mg/kg iv) and then combined COX + PKC inhibition [PKC-(19-36), 20 micrograms/kg iv], respectively. Inhibition of COX reduced (P < 0.05) the afferent response to BK (0.59 +/- 0.12 impulses/s) compared with the unblocked condition (1.14 +/- 0.27 impulses/s), whereas combined COX + PKC inhibition further attenuated the increase from baseline (0.18 +/- 0.09 impulses/s; P < 0.05). Similar results were obtained in a third group of cats when the antagonists were administered in reverse order (n = 7). In a fourth group of cats (n = 9) that were pretreated with indomethacin, ischemia increased afferent activity (0.78 +/- 0.17 impulses/s). However, neural activity was attenuated (0.51 +/- 0.14 impulses/s; P < 0.05) during a second bout of ischemia in the presence of indomethacin + PKC-(19-36). These results suggest that the contribution from PKC to the activation of ischemically sensitive C fibers, particularly by BK, does not require an intact cyclooxygenase system.

Regulation of natriuretic peptide secretion by the heart

Secreted by the heart, more specifically by atrial cardiomyocytes under normal conditions but also by ventricular myocytes during cardiac hypertrophy, natriuretic peptides are now considered important hormones in the control of blood pressure and salt and water excretion. Studies on natriuretic peptide secretagogues and their mechanisms of action have been complicated by hemodynamic changes and contractions to which the atria are constantly subjected. It now appears that atrial stretch through mechano-sensitive ion channels, adrenergic stimulation via alpha 1A-adrenergic receptors, and endothelin via its ETA receptor subtype are major triggering agents of natriuretic peptide release. With several other stimuli, such as angiotensin II and beta-adrenergic agents, modulation of natriuretic peptide release appears to be linked to local generation of prostaglandins. In all cases, intracellular calcium homeostasis, controlled by several ion channels, is considered a key element in the regulation of natriuretic peptide secretion.

Oxidative damage of cardiomyocytes is limited by extracellular regulated kinases 1/2-mediated induction of cyclooxygenase-2

Oxidative stress causes cardiac damage following ischemia/reperfusion and in response to anthracyclines. Extracellular signal-regulated kinases (ERK) 1/2 are activated by oxidative stress in cardiac myocytes and protect cardiac myocytes from apoptosis. Prostaglandins (PG) also protect cells from injury in a number of tissues, including the cardiomyocyte. Cyclooxygenase (COX) the rate-limiting enzyme in PG biosynthesis has two isoforms, the constitutive COX-1 and an inducible COX-2. Here, we examined the effects of two oxidative stresses, hydrogen peroxide (H2O2) and the anthracycline doxorubicin on the activity of ERK1/2 and the expression of COX isoforms and PG formation in neonatal rat primary cardiomyocytes. These cells expressed COX-1 at rest and both COX isoforms on treatment with phorbol 12-myristate 13-acetate. Exposure to 50 microM H2O2 for 10 min or doxorubicin at 10 and 100 micrograms/ml caused expression of COX-2 that was prevented by free radical scavengers. COX-2 induction was associated with activation of ERK1/2 and the specific ERK-inhibitor PD098059 abolished COX-2 expression. Treatment of cells with decoy oligonucleotides corresponding to COX-2 promoter elements implicated the AP-1 and NF-kappaB-2 but not the NF-kappaB-1 in the transcription of COX-2. Induction of COX-2 mRNA and protein was accompanied by increased prostacyclin formation, which was abolished by the selective COX-2 inhibitor, NS-398, and PD098059. H2O2 and doxorubicin enhanced the release of lactate dehydrogenase and free radical scavengers prevented this. NS-398 enhanced the release of lactate dehydrogenase in response to H2O2 and doxorubicin, whereas the injury was prevented by iloprost, a stable prostacyclin analogue. In cardiomyocytes cell injury by H2O2 and doxorubicin is limited by an increase in prostacyclin formation that reflects induction of COX-2 mediated by ERK1/2 activation.

Modulation of the effects of extracellular ATP on [Ca2+]i in rat brain microvacular endothelial cells

This study examined the intracellular regulation of signal transduction initiated by activation of the P2Y2 purinoceptor in a cultured rat brain microvascular endothelial cell line (RBE4). Intracellular free Ca2+ ([Ca2+]i) was monitored in single cells, using FURA-2 fluorimetry. As previously described [Nobles, M., Revest, P.A., Couraud, P.-O., Abbott, N.J., 1995. Characteristics of nucleotide receptors that cause elevation of cytoplasmic calcium in immortalized rat brain endothelial cells, RBE4, and in primary cultures. Br. J. Pharmacol., 115, 1245-1252], extracellular ATP (100 microM, 20 s) evoked a transient increase in intracellular free calcium concentration ([Ca2+]i). The amplitude of the Ca2+ transient evoked by ATP decreased with successive applications (desensitisation), as expected for a P2 purinoceptor. The modulation of the Ca2+ signal downstream to the activation of the ATP receptor was investigated, using agents selected for their ability to interfere with the intracellular pathways activated by ATP. The amplitude of the Ca2+ transient observed on the second application of ATP was compared in the presence and absence of these agents. The Ca2+ transient triggered by ATP was decreased by the inhibitor of nitric oxide synthesis, N-omega-nitro-L-arginine methyl ester (L-NOARG). The inhibition induced by 100 microM L-NOARG was reversed by coapplication of the permeant cGMP analogue 8-brcGMP (100 microM). 8-BrcGMP caused a transient increase in [Ca2+]i when applied alone, and a dose-dependent inhibition of the increase in [Ca2+]i elicited by ATP. Indomethacin, an inhibitor of prostaglandin synthesis, inhibited the response to ATP. The inhibition caused by 10 microM indomethacin was reversed by coapplication of the permeant analogue of cAMP, 8-brcAMP (100 microM). 8-BrcAMP caused a transient rise in [Ca2+]i when applied alone, and a dose-dependent inhibition of the Ca2+ response evoked by ATP. The non-permeant cyclic nucleotides cAMP and cGMP did not affect the desensitising response to ATP, nor did they reverse the inhibitory actions of L-NOARG or indomethacin. It is concluded that cyclic nucleotides, nitric oxide, and prostaglandin synthesis pathways are able to interact with the Ca2+ second messenger pathway in rat brain endothelial cells activated by extracellular ATP.

Modulation of beta-adrenergic receptor-stimulated lipolysis in the heart by prostaglandins

The purpose of the present study was to investigate the contribution of prostaglandins to lipolysis elicited by beta-adrenergic receptor activation in the heart. We have studied the effect of prostaglandin E2 (PGE2), prostaglandin I2 (PGI2), and their precursor arachidonic acid (AA) in the presence and absence of a cyclooxygenase inhibitor, sodium meclofenamate, on glycerol output elicited by stimulation of beta-adrenergic receptors in the isolated rabbit heart with isoproterenol (ISOP). Bolus injections of ISOP (475 pmol) produced a constant increase in glycerol and 6-ketoprostaglandin F1 alpha (6-keto-PGF1 alpha) output. Infusion of sodium meclofenamate (16 microM) reduced basal and attenuated ISOP-induced 6-keto-PGF1 alpha output and enhanced glycerol output. During inhibition of endogenous prostaglandin synthesis with meclofenamate, infusion of PGI2 or PGE2 (0.1-1 microM) inhibited ISOP-induced glycerol output. Infusion of AA (0.1-1 microM) increased 6-keto-PGF1 alpha and reduced glycerol output. Infusion of sodium meclofenamate abolished the effect of AA to increase 6-keto-PGF1 alpha and to decrease glycerol output. These data suggest that prostaglandins synthesized in the heart act as an inhibitory modulator of beta-adrenergic receptor-stimulated cardiac lipolysis.

In rats, atrial natriuretic peptide secretion is regulated differently in the right and left atria

Recent studies have suggested the secretion of Atrial Natriuretic Peptide (ANP) is regulated by receptor mediated activation of protein kinase C, which causes the autocrine release of prostaglandins. Prostaglandins stimulate ANP secretion via the adenylate cyclase second messenger system. This report examined the response of right and left atrial ANP secreting cells to the three endothelin isopeptides and to cyclooxygenase inhibition. Our results show that right atrial ANP secretion is stimulated by endothelin 1 and 2 but not 3. In addition, right atrial ANP secretion is reduced by inhibition of cyclooxygenase. In contrast, left atrial ANP secretion is stimulated by endothelin 2 and 3 but not 1. Inhibition of cyclooxygenase did not affect left atrial ANP secretion. These results show the regulation of ANP secretion is different between the two atrial chambers. Right atrial cells appear to contain the prostaglandin-mediated response to protein-kinase C activation, whereas left atrial cells regulate ANP secretion differently.

Requirement for prostaglandin synthesis in secretion of atrial natriuretic factor from isolated rat heart

Release of atrial natriuretic factor (ANF) from the heart is primarily affected by myocyte stretch. In addition, ANF release can be modulated by a variety of hormones and neurotransmitters, but the mechanisms involved in such modulation are not completely understood. In the present study, we investigated the effect of inhibition of cyclooxygenase activity on release of ANF from the isolated, spontaneously beating rat heart: (1) during basal conditions; and (2) in response to arginine vasopressin (AVP), acetylcholine (ACh) and angiotensin II (ANG II), in order to determine if cardiac prostaglandin synthesis is involved in modulation of basal and hormone-mediated ANF secretion. Basal secretion in the time controls remained stable for the duration of the experiment. AVP, ACh and ANG II reduced basal secretion significantly by 58 +/- 4%, 51 +/- 6% and 26 +/- 8%, respectively, independently of concomitant changes in coronary flow and heart rate. Inhibition of cyclooxygenase with indomethacin (1 x 10(-5) M) decreased basal ANF release by 38 +/- 6%, indicating that basal secretion requires prostaglandin production. The effects of AVP, ACh and ANG II were maintained during perfusion with indomethacin, suggesting a common mechanism of action which operates via inhibition of cyclooxygenase. Based on our previous findings that the effects of indomethacin, AVP and ACh are overcome by inhibition of NO/EDRF synthesis, we suggest a common mechanism of action by means of which NO/EDRF mediates the effects of these agents by inhibiting cyclooxygenase activity.