Pharmacology of Non-Psychoactive Phytocannabinoids and Their Potential for Treatment of Cardiometabolic Disease.

The use of Cannabis sativa by humans dates back to the third millennium BC, and it has been utilized in many forms for multiple purposes, including production of fibre and rope, as food and medicine, and (perhaps most notably) for its psychoactive properties for recreational use. The discovery of Δ9-tetrahydrocannabinol (Δ9-THC) as the main psychoactive phytocannabinoid contained in cannabis by Gaoni and Mechoulam in 1964 (J Am Chem Soc 86, 1646-1647), was the first major step in cannabis research; since then the identification of the chemicals (phytocannabinoids) present in cannabis, the classification of the pharmacological targets of these compounds and the discovery that the body has its own endocannabinoid system (ECS) have highlighted the potential value of cannabis-derived compounds in the treatment of many diseases, such as neurological disorders and cancers. Although the use of Δ9-THC as a therapeutic agent is constrained by its psychoactive properties, there is growing evidence that non-psychoactive phytocannabinoids, derived from both Cannabis sativa and other plant species, as well as non-cannabinoid compounds found in Cannabis sativa, have real potential as therapeutics. This chapter will focus on the possibilities for using these compounds in the prevention and treatment of cardiovascular disease and related metabolic disturbances.

Future directions for the discovery of natural product-derived immunomodulating drugs: an IUPHAR positional review.

Drug discovery from natural sources is going through a renaissance, having spent many decades in the shadow of synthetic molecule drug discovery, despite the fact that natural product-derived compounds occupy a much greater chemical space than those created through synthetic chemistry methods. With this new era comes new possibilities, not least the novel targets that have emerged in recent times and the development of state-of-the-art technologies that can be applied to drug discovery from natural sources. Although progress has been made with some immunomodulating drugs, there remains a pressing need for new agents that can be used to treat the wide variety of conditions that arise from disruption, or over-activation, of the immune system; natural products may therefore be key in filling this gap. Recognising that, at present, there is no authoritative article that details the current state-of-the-art of the immunomodulatory activity of natural products, this in-depth review has arisen from a joint effort between the International Union of Basic and Clinical Pharmacology (IUPHAR) Natural Products and Immunopharmacology Sections, with contributions from a number of world-leading researchers in the field of natural product drug discovery, to provide a "position statement" on what natural products has to offer in the search for new immunomodulatory argents. To this end, we provide a historical look at previous discoveries of naturally occurring immunomodulators, present a picture of the current status of the field and provide insight into the future opportunities and challenges for the discovery of new drugs to treat immune-related diseases.

GPR55 deficiency is associated with increased adiposity and impaired insulin signaling in peripheral metabolic tissues.

Emerging evidence indicates that G-protein coupled receptor 55 (GPR55), a nonclassic receptor of the endocannabinoid system that is activated by L-α-lysophosphatidylinositol and various cannabinoid ligands, may regulate endocrine function and energy metabolism. We examined how GPR55 deficiency and modulation affects insulin signaling in skeletal muscle, adipose tissue, and liver alongside expression analysis of proteins implicated in insulin action and energy metabolism. We show that GPR55-null mice display decreased insulin sensitivity in these tissues, as evidenced by reduced phosphorylation of PKB/Akt and its downstream targets, concomitant with increased adiposity and reduced physical activity relative to wild-type counterparts. Impaired tissue insulin sensitivity coincided with reduced insulin receptor substrate-1 abundance in skeletal muscle, whereas in liver and epididymal fat it was associated with increased expression of the 3-phosphoinoistide lipid phosphatase, phosphatase and tensin homolog. In contrast, GPR55 activation enhanced insulin signaling in cultured skeletal muscle cells, adipocytes, and hepatocytes; this response was negated by receptor antagonists and GPR55 gene silencing in L6 myotubes. Sustained GPR55 antagonism in 3T3-L1 adipocytes enhanced expression of proteins implicated in lipogenesis and promoted triglyceride accumulation. Our findings identify GPR55 as a positive regulator of insulin action and adipogenesis and as a potential therapeutic target for countering obesity-induced metabolic dysfunction and insulin resistance.-Lipina, C., Walsh, S. K., Mitchell, S. E., Speakman, J. R., Wainwright, C. L., Hundal, H. S. GPR55 deficiency is associated with increased adiposity and impaired insulin signaling in peripheral metabolic tissues.

Acute dietary zinc deficiency in rats exacerbates myocardial ischaemia-reperfusion injury through depletion of glutathione.

Zn plays an important role in maintaining the anti-oxidant status within the heart and helps to counter the acute redox stress that occurs during myocardial ischaemia and reperfusion. Individuals with low Zn levels are at greater risk of developing an acute myocardial infarction; however, the impact of this on the extent of myocardial injury is unknown. The present study aimed to compare the effects of dietary Zn depletion with in vitro removal of Zn (N,N,N',N'-tetrakis(2-pyridinylmethyl)-1,2-ethanediamine (TPEN)) on the outcome of acute myocardial infarction and vascular function. Male Sprague-Dawley rats were fed either a Zn-adequate (35 mg Zn/kg diet) or Zn-deficient (<1 mg Zn/kg diet) diet for 2 weeks before heart isolation. Perfused hearts were subjected to a 30 min ischaemia/2 h reperfusion (I/R) protocol, during which time ventricular arrhythmias were recorded and after which infarct size was measured, along with markers of anti-oxidant status. In separate experiments, hearts were challenged with the Zn chelator TPEN (10 µm) before ischaemia onset. Both dietary and TPEN-induced Zn depletion significantly extended infarct size; dietary Zn depletion was associated with reduced total cardiac glutathione (GSH) levels, while TPEN decreased cardiac superoxide dismutase 1 levels. TPEN, but not dietary Zn depletion, also suppressed ventricular arrhythmias and depressed vascular responses to nitric oxide. These findings demonstrate that both modes of Zn depletion worsen the outcome from I/R but through different mechanisms. Dietary Zn deficiency, resulting in reduced cardiac GSH, is the most appropriate model for determining the role of endogenous Zn in I/R injury.

GPR55 regulates the responsiveness to, but does not dimerise with, α1A-adrenoceptors.

Emerging evidence suggests that G protein coupled receptor 55 (GPR55) may influence adrenoceptor function/activity in the cardiovascular system. Whether this reflects direct interaction (dimerization) between receptors or signalling crosstalk has not been investigated. This study explored the interaction between GPR55 and the alpha 1A-adrenoceptor (α1A-AR) in the cardiovascular system and the potential to influence function/signalling activities. GPR55 and α1A-AR mediated changes in both cardiac and vascular function was assessed in male wild-type (WT) and GPR55 homozygous knockout (GPR55-/-) mice by pressure volume loop analysis and isolated vessel myography, respectively. Dimerization of GPR55 with the α1A-AR was examined in transfected Chinese hamster ovary-K1 (CHO-K1) cells via Bioluminescence Resonance Energy Transfer (BRET). GPR55 and α1A-AR mediated signalling (extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation) was investigated in neonatal rat ventricular cardiomyocytes using AlphaScreen proximity assays. GPR55-/- mice exhibited both enhanced pressor and inotropic responses to A61603 (α1A-AR agonist), while in isolated vessels, A61603 induced vasoconstriction was attenuated by a GPR55-dependent mechanism. Conversely, GPR55-mediated vasorelaxation was not altered by pharmacological blockade of α1A-ARs with tamsulosin. While cellular studies demonstrated that GPR55 and α1A-AR failed to dimerize, pharmacological blockade of GPR55 altered α1A-AR mediated signalling and reduced ERK1/2 phosphorylation. Taken together, this study provides evidence that GPR55 and α1A-AR do not dimerize to form heteromers, but do interact at the signalling level to modulate the function of α1A-AR in the cardiovascular system.

2-arachidonyl glycerol activates platelets via conversion to arachidonic acid and not by direct activation of cannabinoid receptors.

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT: There are conflicting views in the literature as to whether cannabinoids have an impact on platelet activity and to what extent cannabinoid receptors are involved. This is an important issue to resolve because platelet effects of putative therapeutic cannabinoid inhibitors and stimulators will have an impact on their potential benefits and safety. WHAT THIS PAPER ADDS: The data presented in this manuscript clearly show that the endocannabinoid 2-arrachidonyl glycerol can activate platelet activity, but that the effects are mediated through an aspirin-sensitive pathway that is not affected by cannabinoid receptor antagonists or FAAH inhibition, but is abolished by MAGL inhibition. The findings question the role of cannabinoid receptors in platelet function and suggest that platelet function is unlikely to be directly affected by cannabinoid receptor antagonists, at least in the acute phase. AIMS: Cannabinoid receptor-1 (CB(1)) antagonists suppress appetite and induce weight loss. Direct antagonism of CB(1) receptors on platelets might be an additional benefit for CB(1) antagonists, but the role of CB(1) receptors in platelets is controversial. We tested the hypothesis that the endocannabinoid, 2-arachidonyl glycerol (2-AG), induces platelet aggregation by a COX-mediated mechanism rather than through CB(1) receptor activation, in blood obtained from healthy volunteers and patients with coronary artery disease receiving low dose aspirin. METHODS: Aggregatory responses to the cannabinoids 2-AG and Delta(9)-THC were examined in blood sampled from healthy volunteers (n= 8) and patients (n= 12) with coronary artery disease receiving aspirin using whole blood aggregometry. The effects of CB(1) (AM251) and CB(2) (AM630) antagonists, as well as fatty acid amide hydrolase (FAAH) and monoacyl glycerol lipase (MAGL) inhibitors and aspirin on 2-AG-induced aggregation were also assessed. RESULTS: AM251 (100 nm-30 microm) had no effect on platelet aggregation induced by either ADP (P= 0.90) or thrombin (P= 0.86). 2-AG, but not Delta(9)-THC, induced aggregation. 2-AG-induced aggregation was unaffected by AM251 and AM630 but was abolished by aspirin (P < 0.001) and by the MAGL inhibitor, URB602 (P < 0.001). Moreover, the aggregatory response to 2-AG was depressed (by >75%, P < 0.001) in blood from patients with coronary artery disease receiving aspirin compared with that from healthy volunteers. CONCLUSIONS: 2-AG-mediated activation of platelets is via metabolism to arachidonic acid by MAGL, and not through direct action on CB(1) or CB(2) receptors, at least in the acute phase.

Seaweed-derived bioactives as potential energy regulators in obesity and type 2 diabetes.

There is epidemiological evidence that dietary intake of seaweeds is associated with a lower prevalence of chronic diseases. While seaweeds are of high nutritious value, due to their high content of fiber, polyunsaturated fatty acids and minerals, they also contain an abundance of bioactive compounds. There is a growing body of scientific data that these bioactive moieties exert effects that could correct the metabolic dysregulation that is present in obesity and Type 2 diabetes (T2D). In this review we describe how the molecular mechanisms, specific to different tissues, that underly obesity and T2D are influenced by both seaweed extracts and seaweed-derived bioactive molecules. In obesity, modulation of antioxidant capacity and reduction of intracellular ROS levels within tissues, and regulation of signaling pathways involved in enhancing browning of white adipose tissue, have been highlighted as key mechanism and identified as a potential target for optimal energy metabolism. In T2D, management of post-prandial blood glucose by modulating α-glucosidase or α-amylase activities, modulation of the AMPK signaling pathway, and similarly to obesity, reduction of ROS and NO production with subsequent increased expression of antioxidant enzymes have been shown to play a key role in glucose metabolism and insulin signaling. Future studies aimed at discovering new therapeutic drugs from marine natural products should, therefore, focus on bioactive compounds from seaweed that exert antioxidant activity and regulate the expression of key signaling pathways involved in glucose homeostasis, mechanisms that are common to both obesity and T2D management. In addition, more data is required to provide evidence of clinical benefit.

l-α-Lysophosphatidylinositol (LPI) aggravates myocardial ischemia/reperfusion injury via a GPR55/ROCK-dependent pathway.

The phospholipid l-α-lysophosphatidylinositol (LPI), an endogenous ligand for GPR55, is elevated in patients with acute coronary syndrome, and a GPR55 antagonist cannabidiol (CBD) reduces experimental ischemia/reperfusion (I/R) injury. While LPI activates multiple signaling pathways, little is known about which ones are important in cardiomyocytes. In this study we explored whether activation of the Rho kinase/ROCK/p38 MAPK pathway is responsible for LPI-induced extension of I/R injury. Using a high-throughput screening method (dynamic mass redistribution; DMR), mouse- and human-induced pluripotent stem cell (iPSC) cardiomyocytes exposed to LPI were shown to exhibit a rapid, sustained, and concentration-dependent (1 nmol L-1-30 µmol L-1) cellular response. Y-27632 (ROCK inhibitor; 10 & 50 µmol L-1) and CBD (1 µmol L-1) both abolished the DMR response to LPI (10 µmol L-1). In murine iPSC cardiomyocytes, LPI-induced ROCK and p38 MAPK phosphorylation, both of which were prevented by Y-27632 and CBD, but did not induce JNK activation or cleavage of caspase-3. In hearts isolated from wild type (WT) mice subjected to 30 minutes global I/R, LPI (10 µmol L-1) administered via the coronary circulation increased infarct size when applied prior to ischemia onset, but not when given at the time of reperfusion. The exacerbation of tissue injury by LPI was not seen in hearts from GPR55-/- mice or in the presence of Y-27632, confirming that injury is mediated via the GPR55/ROCK/p38 MAPK pathway. These findings suggest that raised levels of LPI in the vicinity of a developing infarct may worsen the outcome of AMI.

Phospholipid chlorohydrin induces leukocyte adhesion to ApoE-/- mouse arteries via upregulation of P-selectin.

HOCl-modified low-density lipoprotein (LDL) has proinflammatory effects, including induction of inflammatory cytokine production, leukocyte adhesion, and ROS generation, but the components responsible for these effects are not completely understood. HOCl and the myeloperoxidase-H(2)O(2)-halide system can modify both protein and lipid moieties of LDL and react with unsaturated phospholipids to form chlorohydrins. We investigated the proinflammatory effects of 1-stearoyl-2-oleoyl-sn-3-glycerophosphocholine (SOPC) chlorohydrin on artery segments and spleen-derived leukocytes from ApoE(-/-) and C57 Bl/6 mice. Treatment of ApoE(-/-) artery segments with SOPC chlorohydrin, but not unmodified SOPC, caused increased leukocyte-arterial adhesion in a time- and concentration-dependent manner. This could be prevented by pretreatment of the artery with P-selectin or ICAM-1-blocking antibodies, but not anti-VCAM-1 antibody, and immunohistochemistry showed that P-selectin expression was upregulated. However, chlorohydrin treatment of leukocytes did not increase expression of adhesion molecules LFA-1 or PSGL-1, but caused increased release of ROS from PMA-stimulated leukocytes by a CD36-dependent mechanism. The SOPC chlorohydrin-induced adhesion and ROS generation could be abrogated by pretreatment of the ApoE(-/-) mice with pravastatin or a nitrated derivative, NCX 6550. These findings suggest that phospholipid chlorohydrins formed in HOCl-treated LDL could contribute to the proinflammatory effects observed for this modified lipoprotein in vitro.

Textual synthesis of policies and guidance statements for remote healthcare practitioners on managing medical emergencies in the oil and gas industry: a systematic review protocol.

REVIEW QUESTION/OBJECTIVE: The objective of this review is to collate, synthesize and present the available evidence on the policies and guidance statements for remote healthcare practitioners on managing medical emergencies in the offshore oil and gas industry.More specifically, the review seeks to answer the following questions.

Adrenomedullin acts via nitric oxide and peroxynitrite to protect against myocardial ischaemia-induced arrhythmias in anaesthetized rats.

1. The overall aim of this study was to determine if adrenomedullin (AM) protects against myocardial ischaemia (MI)-induced arrhythmias via nitric oxide (NO) and peroxynitrite. 2. In sham-operated rats, the effects of in vivo administration of a bolus dose of AM (1 nmol kg-1) was assessed on arterial blood pressure (BP), ex vivo leukocyte reactive oxygen species generation and nitrotyrosine deposition (a marker for peroxynitrite formation) in the coronary endothelium. 3. In pentobarbitone-anaesthetized rats subjected to ligation of the left main coronary artery for 30 min, the effects of a bolus dose of AM (1 nmol kg-1, i.v.; n=19) or saline (n=18) given 5 min pre-occlusion were assessed on the number and incidence of cardiac arrhythmias. In a further series of experiments, some animals received infusions of the NO synthase inhibitor N(G)-nitro-L-arginine (LNNA) (0.5 mg kg-1 min-1) or the peroxynitrite scavenger N-mercaptopropionyl-glycine (MPG) (20 mg kg-1 h-1) before AM. 4. AM treatment significantly reduced mean arterial blood pressure (MABP) and increased ex vivo chemiluminescence (CL) generation from leukocytes in sham-operated animals. AM also enhanced the staining for nitrotyrosine in the endothelium of coronary arteries. 5. AM significantly reduced the number of total ventricular ectopic beats that occurred during ischaemia (from 1185+/-101 to 520+/-74; P<0.05) and the incidences of ventricular fibrillation (from 61 to 26%; P<0.05). AM also induced a significant fall in MABP prior to occlusion. AM-induced cardioprotection was abrogated in animals treated with the NO synthase inhibitor LNNA and the peroxynitrite scavenger MPG. 6. This study has shown that AM exhibits an antiarrhythmic effect through a mechanism that may involve generation of NO and peroxynitrite.

Fatty acid and phospholipid chlorohydrins cause cell stress and endothelial adhesion.

The oxidation of low-density lipoprotein (LDL) is thought to contribute to atherogenesis, which is an inflammatory disease involving activation of phagocytic cells. Myeloperoxidase, an enzyme which is able to produce hypochlorous acid (HOCl), is released from these phagocytic cells, and has been found in an active form in atherosclerotic plaques. HOCl can oxidize both the lipid and protein moiety of LDL, and HOCl-modified LDL has been found to be pro-inflammatory, although it is not known which component is responsible for this effect. As HOCl can oxidize lipids to give chlorohydrins, we hypothesized that phospholipid chlorohydrins might have toxic and pro-inflammatory effects. We have formed chlorohydrins from fatty acids (oleic, linoleic and arachidonic acids) and from phospholipids (stearoyl-oleoyl phosphatidylcholine, stearoyl-linoleoyl phosphatidylcholine and stearoyl-arachidonoyl phosphatidylcholine), and investigated various biological effects of these oxidation products. Fatty acid and phospholipid chlorohydrins were found to deplete ATP levels in U937 cells in a concentration-dependent manner, with significant effects observed at concentrations of 25 microM and above. Low concentrations (25 microM) of stearoyl-oleoyl phosphatidylcholine and stearoyl-arachidonoyl phosphatidylcholine chlorohydrins were also found to increase caspase-3 activity. Finally, stearoyl-oleoyl phosphatidylcholine chlorohydrin increased leukocyte adhesion to artery segments isolated from C57Bl/6 mice. These results demonstrate potentially harmful effects of lipid chlorohydrins, and suggest that they may contribute to some of the pro-inflammatory effects that HOCl-modified low density lipoprotein has been found to induce.

Locally administered antiproliferative drugs inhibit hypercontractility to serotonin in balloon-injured pig coronary artery.

Although drugs such as sirolimus and paclitaxel are effective in reducing restenosis, their effects on vascular function are often overlooked. In this study, we have examined the effects of local delivery of several anti-restenotic drugs given in vivo after balloon injury on in vitro vascular contraction and relaxation 28 days after injury. Paclitaxel (50 microM), the farnesyl protein transferase inhibitor L744 (25 microM), sirolimus (25 microM) and Van 10/4 (decahydro-1,1,4,7-tetramethyl-1H-cycloprop[e]azulen-4-o-[2-(3-methylpent-2-enoyl)-fucopyranoside]; 25 microM) were delivered to porcine coronary arteries in vivo and the arteries removed 28 days later. Contractions to KCl and 5-hydroxytryptamine (5-HT) and relaxations to calcimycin and 3-morpholinosydnonimine (SIN-1) were measured in control (LCx) and balloon-injured (LAD) rings. In vehicle-infused coronary arteries, contraction to KCl and 5-HT was significantly enhanced 28 days after balloon injury, while the response to calcimycin had recovered fully, indicating endothelial regrowth. The response to SIN-1 was unchanged. None of the four drugs tested had any effect on the enhanced response to KCl 28 days after injury or on recovery of the calcimycin response. The hyper-responsiveness to 5-HT was eliminated by sirolimus, Van 10/4 and L744, but not paclitaxel. This study demonstrates that local drug infusion with structurally different antiproliferative drugs at the time of balloon angioplasty does not affect endothelial recovery and may in some cases prevent hyper-responsiveness to constrictor agents.

PDGF-induced signaling in proliferating and differentiated vascular smooth muscle: effects of altered intracellular Ca2+ regulation.

OBJECTIVE: Platelet-derived growth factor-BB (PDGF)-induced intracellular signaling is involved in phenotypic modulation of vascular smooth muscle (VSM). This study has examined the PDGF-induced Ca2+ increase and the resultant effect on signaling pathways in proliferative compared with fully differentiated VSM. METHODS: PDGF-induced changes in Ca2+ were measured in portal vein (PV) myocytes from 2-4-day-old (proliferating), compared to 6-week-old (differentiated), Sprague Dawley rats. Phospholipase C (PLC)gamma expression and activation of extracellular signal-regulated kinase (ERK) 1/2 was determined by immunoblotting or confocal immunolabelling. Activation of the Ca(2+)-dependent transcription factor, nuclear factor of activated T-cells (NFATc), was assessed by electromobility shift assay. RESULTS: PDGF increased the intracellular Ca2+ concentration in differentiated, but not in proliferating, PV myocytes. This is probably due to very low expression of PLC(gamma) in proliferating PV. In 6-week-old PV, PDGF stimulation induced nuclear translocation and activation of NFATc. PDGF did not induce NFATc activation in neonatal PV. PDGF-induced ERK1/2 activation was observed in both 2-4-day-old and 6-week-old PV. In 6-week-old PV, ERK1/2 activation was Ca(2+)-dependent and protein kinase C-dependent. However in 2-4-day-old PV, PDGF-induced ERK1/2 activation was via a Ca(2+)-independent, atypical protein kinase C. PLC(gamma) expression was also decreased in the neointima, compared to media, of balloon-injured rabbit subclavian arteries. CONCLUSIONS: The regulation of PDGF-induced Ca2+ increases by PLC(gamma) expression in VSM may provide a mechanism for coordinating different signaling pathways leading to activation of specific transcription factors. This may play an important role in the phenotypic modulation of VSM.

New insights into the yin and yang of the endocannabinoid system in health and disease.

LINKED ARTICLES: This article is part of a themed section on Endocannabinoids. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.7/issuetoc.

TNFalpha increases the inflammatory response to vascular balloon injury without accelerating neointimal formation.

There is now clear evidence for a contributory role of inflammatory processes to restenosis following vascular balloon injury and stent implantation. The aim of the present study was to study the effects of TNFalpha, administered locally in vivo immediately following balloon angioplasty, on the leukocyte adhesive response and extent of neointimal formation in a rabbit model of subclavian artery injury. Initial in vitro studies were performed with normal isolated artery rings to assess the vascular adhesive response to TNFalpha or IL-1beta. Pre-incubation with either cytokine prior to addition of (51)Cr-labelled leukocytes enhanced the adhesion of leukocytes to the artery in both a time- and concentration-dependent manner. Although both cytokines induced an increase in the expression of the adhesion molecules ICAM-1 and VCAM-1, only antibodies to ICAM-1 blocked the enhanced adhesion induced by the cytokines. In artery segments retrieved from rabbits that had previously undergone subclavian artery angioplasty either 24 h or 8 days previously, there was an injury-induced increase in adhesion of leukocytes assessed ex vivo. In segments obtained from rabbits that received a 15 min local infusion of TNFalpha (2 ng/min) to the injured artery immediately after the angioplasty procedure, leukocyte adhesion assessed ex vivo was further significantly enhanced. The pro-adhesive effect of TNFalpha was associated with an increased expression of both ICAM-1 and VCAM-1. However, TNFalpha administration did not alter the extent of neointimal formation observed 8 days after injury. These findings suggest that while TNFalpha may play a role following vascular injury, it does not act alone to induce neointimal formation. Thus anti-inflammatory strategies targeted at multiple cytokines may be more appropriate than targeting a single cytokine to reduce the response to vascular injury.

Electrophysiological and haemodynamic effects of endothelin ETA and ETB receptors in normal and ischaemic working rabbit hearts.

The aims of this study were to determine if endothelin-1 (ET-1) under normal and ischaemic conditions exhibits a direct arrhythmogenic effect that is independent of its ability to cause coronary vasoconstriction, and to determine the contribution of the ET(A) and ET(B) receptor subtype. ET(A/B) (with ET-1) and ET(A) (ET-1 in the presence of BQ-788) receptor activation resulted in a significant reduction in both epi- and endocardial monophasic action potential duration (MAPD(90)). ET(A) receptor activation reduced both epi- and endocardial effective refractory period (ERP). This MAPD(90) and ERP shortening were associated with a reduction in coronary flow, myocardial contractility and induction of ventricular fibrillation (VF) during ERP measurement. The ET(B) agonist sarafotoxin (S6c) had no marked, or concentration-dependent, effect on MAPD(90), ERP, myocardial contractility or induction of arrhythmias. Neither ET-1 nor S6c, given prior to coronary artery occlusion, significantly changed the ischaemia-induced dispersion of MAPD(90), ERP or the % incidence of VF. In conclusion, neither ET(A) nor ET(B) receptor stimulation has a direct arrhythmogenic effect in isolated rabbit hearts under normal or ischaemic conditions. The ET-1-induced arrhythmogenic effect observed in nonischaemic hearts is likely to be the result of the associated coronary vasoconstriction caused by ET(A) receptor stimulation resulting in myocardial ischaemia.

Endothelin and the ischaemic heart.

Soon after its identification as a powerful vasoconstrictor peptide, endothelin (ET-1) was implicated as a detrimental agent involved in determining the outcome of myocardial ischaemia and reperfusion. Early experimental studies demonstrated that ET(A) selective and mixed ET(A)/ET(B) receptor antagonists can reduce infarct size and prevent ischaemia-induced ventricular arrhythmias in models of ischaemia/reperfusion, implying that ET-1 acts through the ET(A) receptor to contribute to injury and arrhythmogenesis. However, as our understanding of the physiology of ET-1 has expanded, the role of ET-1 in the ischaemic heart appears ever more complex. Recent evidence suggests that ET-1 exerts actions on the heart that are not only detrimental (vasoconstriction, inhibition of NO production, activation of inflammatory cells), but which may also contribute to tissue repair, such as inhibition of cardiomyocyte apoptosis. In addition, ET-1-induced mast cell degranulation has been linked to a homeostatic mechanism that controls endogenous ET-1 levels, which may have important implications for the ischaemic heart. Furthermore the mechanism by which ET-1 promotes arrhythmogenesis remains controversial. Some studies imply a direct electrophysiological effect of ET-1, via ET(A) receptors, to increase monophasic action potential duration (MAPD) and induce early after-depolarisations (EADs), while other studies support the view that coronary constriction resulting in ischaemia is the basis for the generation of arrhythmias. Moreover, ET-1 can induce cardioprotection (precondition) against infarct size and ventricular arrhythmias, through as yet incompletely understood mechanisms. To enable us to identify the most appropriate means of targeting this system in a therapeutically meaningful way we need to continue to explore the physiology of ET-1, both in the normal and the ischaemic heart.

Matrix metalloproteinases, oxidative stress and the acute response to acute myocardial ischaemia and reperfusion.

Matrix metalloproteases (MMPs) are responsible for the breakdown of extracellular matrix materials, including collagen and elastin. There is substantial evidence that, although the activity of MMPs in normal tissue is low, there is an increase in activity under a range of disease states that contributes to the chronic pathology of the disease. In cardiovascular disease, MMPs have been implicated in the development of ventricular remodelling post-infarction, and also in the degradation of the fibrous cap of atherosclerotic plaques, thereby contributing to plaque rupture. Recent attention has turned to using the presence of circulating MMPs in patients with recent acute myocardial infarction or unstable angina as a prognostic indicator for eventual chronic outcome. In addition, an emerging role for MMPs in contributing to the early consequences of acute myocardial infarction, such as cardiac dysfunction, has been identified.