Phosphoinositide-generated messengers in cardiac signal transduction

Microarray evaluation of bovine hepatic gene response to fescue toxicosis

‘Fescue toxicosis’ is a disease in livestock caused by ingestion of ergot alkaloids produced by the fungal endophyte Neotyphodium coenophialum in tall fescue; it is estimated to cost 1 billion USD in damages per year to the beef industry alone. Clinical signs include decreased reproductive fitness, necrosis of extremities, and reduced average daily gain and milk production. Little is known about the cellular mechanisms that mediate these toxic sequelae. We evaluated the effects of ergovaline-based fescue toxicosis on gene expression via oligonucleotide microarray. Liver biopsies were obtained from steers (n=4) pre- and post-exposure (0 and 29 days) to feed containing 579 ng/g ergovaline. Analyses were performed using both ANOVA with false discovery rate correction and Storey's optimal discovery procedure. Overall, down-regulation of gene expression was observed; heart contraction and cardiac development, apoptosis, cell cycle control, and RNA processing genes represented the bulk of differentially expressed transcripts. 2 CYPs (CYP2E1 and CYP4F6) were amongst the significantly upregulated results. Thus, exposure of cattle to toxic levels of ergovaline caused widespread changes in hepatic gene expression, which can both help explain macroscopic clinical signs observed in ruminant animals, and reinforce previous findings in monogastric models.

Modulation of enzymatic activities by n-3 polyunsaturated fatty acids to support cardiovascular health

Epidemiological evidence from Greenland Eskimos and Japanese fishing villages suggests that eating fish oil and marine animals can prevent coronary heart disease. Dietary studies from various laboratories have similarly indicated that regular fish oil intake affects several humoral and cellular factors involved in atherogenesis and may prevent atherosclerosis, arrhythmia, thrombosis, cardiac hypertrophy and sudden cardiac death. The beneficial effects of fish oil are attributed to their n-3 polyunsaturated fatty acid (PUFA; also known as omega-3 fatty acids) content, particularly eicosapentaenoic acid (EPA; 20:5, n-3) and docosahexaenoic acid (DHA; 22:6, n-3). Dietary supplementation of DHA and EPA influences the fatty acid composition of plasma phospholipids that, in turn, may affect cardiac cell functions in vivo. Recent studies have demonstrated that long-chain omega-3 fatty acids may exert beneficial effects by affecting a wide variety of cellular signaling mechanisms. Pathways involved in calcium homeostasis in the heart may be of particular importance. L-type calcium channels, the Na+-Ca2+ exchanger and mobilization of calcium from intracellular stores are the most obvious key signaling pathways affecting the cardiovascular system; however, recent studies now suggest that other signaling pathways involving activation of phospholipases, synthesis of eicosanoids, regulation of receptor-associated enzymes and protein kinases also play very important roles in mediating n-3 PUFA effects on cardiovascular health. This review is therefore focused on the molecular targets and signaling pathways that are regulated by n-3 PUFAs in relation to their cardioprotective effects.

Overexpression of diacylglycerol kinase zeta inhibits endothelin-1-induced decreases in Ca2+ transients and cell shortening in mouse ventricular myocytes

Endothelin-1 (ET-1) is released in various cardiovascular disorders including congestive heart failure, and may modulate significantly the disease process by its potent action on vascular and cardiac muscle cell function and gene regulation. In adult mouse ventricular cardiomyocytes loaded with indo-1, ET-1 induced a sustained negative inotropic effect (NIE) in association with decreases in Ca(2+) transients. The ET-1-induced effects on Ca(2+) transients and cell shortening were abolished in diacylglycerol (DAG) kinase zeta-overexpressing mouse ventricular myocytes. A nonselective protein kinase C (PKC) inhibitor, GF109203X, inhibited the ET-1-induced decreases in Ca(2+) transients and cell shortening in concentration-dependent manners, whereas a selective Ca(2+)-dependent PKC inhibitor, Gö6976, did not affect the ET-1-induced effects. A phospholipase Cbeta inhibitor, U73122, and an inhibitor of phospholipase D, C(2)-ceramide, partially, but significantly, attenuated the ET-1-induced effects. Derivatives of the respective inhibitors with no specific effects, U73343 and dihydro-C(2)-ceramide, did not affect the ET-1-induced effects. Taken together, these results indicate that activation of a Ca(2+)-independent PKC isozyme by 1,2-DAG, which is generated by phospholipase Cbeta and phospholipase D activation and inactivated by phosphorylation via DAG kinase, is responsible for the ET-1-induced decreases in Ca(2+) transients and cell shortening in mouse ventricular cardiomyocytes.

Chronic endothelin receptor blockade prevents both early hyperfiltration and late overt diabetic nephropathy in the rat

Diabetic nephropathy is associated with enhanced renal synthesis of endothelin (ET)-1. The goal of this study was to investigate the effects of dual ET receptor antagonism in the early phase (2 months) and in the late phase (5 months) of diabetic nephropathy in rats, and to compare this approach to angiotensin-converting enzyme inhibition. Four groups of uninephrectomized streptozotocin-induced diabetic rats were assigned to receive orally vehicle, bosentan, enalapril, or their combination. A fifth group consisted of nondiabetic, uninephrectomized rats. At 2 weeks, untreated diabetic rats exhibited increased glomerular filtration rate and renal plasma flow. Bosentan, enalapril, and the combination all prevented hyperfiltration and hyperperfusion. By 5 months, diabetic rats developed marked increases in mean arterial pressure and renal vascular resistance, progressive proteinuria, and renal structural damage with glomerular sclerosis and hypertrophy. Bosentan completely prevented the development of hypertension and renal vasoconstriction, and largely prevented the development of proteinuria and renal structural injury. The renal protective effect of bosentan was comparable to that of enalapril or the combination, although its anti-proteinuric effect was less. Clinical studies are warranted to assess whether ET receptor antagonism can have additive effects on top of ACE inhibition, the current treatment of choice in diabetic nephropathy.

Selective endothelin receptor blockade reverses mitochondrial dysfunction in canine heart failure

OBJECTIVE

Mitochondrial enzymatic activity reductions in both myocardial and skeletal muscle tissues have been reported in a canine model of pacing-induced congestive heart failure (CHF). Endothelin-1 (ET-1), a vasoconstrictor peptide with diverse biological properties, has been implicated in CHF pathogenesis, and ET-1 receptor blockade has been shown to attenuate CHF progression. We hypothesized that the beneficial effect of ET-1 receptor blockade may be mediated in part by improved mitochondrial function.

METHODS

Myocardium and skeletal muscle tissues were evaluated for respiratory complex I-V and citrate synthase activity levels in paced animals treated with and without LU 135252, a specific type A ET-1 receptor (ET(A)) antagonist.

RESULTS

Specific activity levels of complex V and III, which were 65% to 85% lower in both cardiac and skeletal muscle in paced compared to unpaced animals, were significantly increased in ET(A) antagonist-treated animals (50%-300% compared to untreated paced animals). Levels of other mitochondrial respiratory complex activities including complex I, II, and IV as well as citrate synthase were not significantly changed.

CONCLUSIONS

These findings suggest that endothelin activation may be involved in the myocardial dysfunction and mitochondrial enzyme deficiencies observed in pacing-induced CHF. Improvement of mitochondrial function may be a novel mechanism mediating the beneficial effect of ET(A) receptor blockade in CHF.

Modulation of cardiac PIP2 by cardioactive hormones and other physiologically relevant interventions

Phosphatidylinositol 4,5-bisphosphate (PIP2) affects profoundly several cardiac ion channels and transporters, and studies of PIP2-sensitive currents in excised patches suggest that PIP2 can be synthesized and broken down within 30 s. To test when, and if, total phosphatidylinositol 4-phosphate (PIP) and PIP(2) levels actually change in intact heart, we used a new, nonradioactive HPLC method to quantify anionic phospholipids. Total PIP and PIP2 levels (10-30 micromol/kg wet weight) do not change, or even increase, with activation of Galpha(q)/phospholipase C (PLC)-dependent pathways by carbachol (50 microM), phenylephrine (50 microM), and endothelin-1 (0.3 microM). Adenosine (0.2 mM) and phorbol 12-myristate 13-acetate (1microM) both cause 30% reduction of PIP2 in ventricles, suggesting that diacylglycerol (DAG)-dependent mechanisms negatively regulate cardiac PIP2. PIP2, but not PIP, increases reversibly by 30% during electrical stimulation (2 Hz for 5 min) in guinea pig left atria; the increase is blocked by nickel (2 mM). Both PIP and PIP2 increase within 3 min in hypertonic solutions, roughly in proportion to osmolarity, and similar effects occur in multiple cell lines. Inhibitors of several volume-sensitive signaling mechanisms do not affect these responses, suggesting that PIP2 metabolism might be sensitive to membrane tension, per se.

Endothelin mediates the altered renal hemodynamics associated with experimental congestive heart failure

Congestive heart failure (CHF) is commonly associated with renal dysfunction. The goal of the current study was to evaluate the role of endothelin in the renal dysfunction of experimental CHF by using tezosentan, a potent dual endothelin receptor antagonist. Rats were subjected to coronary artery ligation. Cardiac and renal hemodynamics were assessed after 3-5 weeks, when CHF had developed. Compared with control rats, CHF rats had significantly higher left ventricular end-diastolic pressure (LVEDP), lower mean arterial pressure, and reduced dP/dt(max). CHF rats had severe renal vasoconstriction, as assessed by increased renal vascular resistance (RVR, p < 0.001), decreased renal plasma flow (RPF, p < 0.001), and glomerular filtration rate (GFR, p < 0.001). Filtration fraction rose (p < 0.001). Urine flow rate and sodium excretion were markedly lower. Acute administration of tezosentan induced a marked decrease in LVEDP without change of dP/dt(max) and heart rate. Tezosentan decreased RVR (-43%, p < 0.001) and increased RPF and GFR. Filtration fraction decreased slightly. Tezosentan also increased urine flow rate and sodium excretion. These findings demonstrate that endothelin at least partly mediates the altered renal hemodynamics associated with experimental CHF. Dual endothelin receptor blockade could be useful for the improvement of both cardiac and renal function in CHF.

Endothelin mediates some of the renal actions of acutely administered angiotensin II

Recent studies suggest that endogenous endothelin mediates much of the vasoconstrictor activity and vascular fibrotic damage caused by chronic administration of angiotensin II. The present study uses the mixed endothelin-A and endothelin-B receptor antagonist bosentan and the endothelin-A-selective blocker BQ-123 to study the contribution of endogenous endothelin to the pressor and renal action of acutely administered angiotensin II in conscious, chronically catheterized rats. Exposure to angiotensin II at 0.48 pmol 0.5 ng/100 g body weight per min IV (low dose) and 1.91 pmol 2.0 ng/100 g body weight per min IV (high dose) raised mean arterial blood pressure (18+/-4 mm Hg, P<0.01, and 39+/-4 mm Hg, P<0.005, respectively) while also increasing renal vascular resistance (4.3+/-1 mm Hg/mL per min, P<0.001, and 10+/-1 mm Hg/mL per min, P<0.001, respectively). In the presence of bosentan, pressor and renal vasoconstrictor responses to low-dose angiotensin II were blunted (P<0.02 and P<0.01, respectively), and the results with BQ-123 were similar. In contrast, these parameters were unaffected during high-dose angiotensin II infusion+bosentan, although BQ-123 did selectively reduce the rise in renal vascular resistance, possibly via an endothelin B-mediated nitric oxide effect. In contrast, high-dose angiotensin II caused natriuretic and diuretic effects that were completely prevented by bosentan. These results show that endothelin (via endothelin A) contributes to the pressor and renal vasoconstrictor actions of acutely administered low-dose angiotensin II. Furthermore, our data suggest that the previously described angiotensin II-induced natriuresis and diuresis observed with a high pressor dose of angiotensin II is mediated by endothelin.

Distributional patterns of phospholipase C isozymes in rat pancreas

Phospholipase C (PLC) isozymes are believed to play a role in regulating pancreatic exocrine and endocrine secretion. In an attempt to investigate the role of PLC, we examined the distribution patterns of PLC isozymes in the normal rat pancreas by Western blot analysis and immunohistochemistry. Western blot analysis was performed on pancreatic acinar tissues and the islet of Langerhans, which were separated from each other. PLC-beta isozymes (beta1, beta2, beta3, and beta4), delta1, and delta2 were detected in both acinar and islet cells, whereas PLC-gamma1 and gamma2 were observed only in acinar tissues. On immunohistochemistry, the immunoreactivities of PLC isozymes except for PLC-gamma1 were observed as follows: PLC-beta1, in both the exocrine and endocrine tissues; PLC-beta2, mainly in the periphery of the islet and acinar cells; PLC-beta3, in the periphery of the islet and in some ductal epithelium; PLC-beta4, through the islet of Langerhans and ductal epithelium; PLC-gamma1, not detected in pancreatic tissue; PLC-gamma2, mainly in acinar cells; PLC-delta1 and delta2, in the islet and in ductal epithelium. These results suggest that the intrapancreatic site-specific existence of PLC isozymes may modulate pancreatic exocrine and endocrine functions through a PLC-mediated signal transduction.

Extracellular regulated kinase, but not protein kinase C, is an antiapoptotic signal of insulin-like growth factor-1 on cultured cardiac myocytes

This study aims to elucidate the signaling pathway for insulin-like growth factor-1 (IGF-1) in cultured neonatal rat cardiomyocytes and particularly the role of IGF-1 in cardiac apoptosis. IGF-1 stimulated polyphosphoinositide turnover, translocation of protein kinase C (PKC) isoforms (alpha, epsilon, and delta) from the soluble to the particulate fraction, activation of phospholipid-dependent and Ca(2+)-, phospholipid-dependent PKC, and activation of the extracellular-regulated kinase (ERK). IGF-1 attenuated sorbitol-induced cardiomyocyte viability and nuclear DNA fragmentation. These antiapoptotic effects of IGF-1 were blocked by PD-098059 (an MEK inhibitor) but not by bisindolylmaleimide I (BIM, a specific PKC inhibitor). The ERK pathway may therefore be an important component in the mechanism whereby IGF-1 exerts its antiapoptotic effect on the cardiomyocyte.

Endothelin-1 and photoreleased diacylglycerol increase L-type Ca2+ current by activation of protein kinase C in rat ventricular myocytes

The amphotericin B-perforated whole-cell patch clamp technique was used to determine the modulation of L-type Ca2+ channels by protein kinase C (PKC)-mediated pathways in adult rat ventricular myocytes. Application of 10 nM endothelin-1 (ET-1) increased peak Ca2+ current (ICa) by 28.2 +/- 2.5 % (n = 13) and slowed current decay. These effects were prevented by the endothelin receptor antagonist PD145065 (10 microM) and by the PKC inhibitor chelerythrine (8 microM). To establish if direct activation of PKC mimicked the ET-1 effect, the active and inactive phorbol esters (phorbol-12-myristate-13-acetate and 4alpha-phorbol-12, 13-didecanoate) were tested. Both phorbol esters (100 nM) resulted in a small (approximately 10%) increase in ICa, suggesting PKC-independent effects. Bath application of dioctanoylglycerol (diC8), a diacylglycerol (DAG) analogue which is capable of directly activating PKC, caused a gradual decline in peak ICa (50.4 +/- 6.2 %, n = 5) and increased the rate of current decay. These effects were unaffected by the PKC inhibitor chelerythrine (8 microM). Intracellular photorelease of caged diC8 with 3 or 10 s exposure to UV light produced a concentration-dependent increase in peak ICa (20. 7 +/- 8.5 % (n = 8) for 3 s UV and 60.8 +/- 11.4 % (n = 13) for 10 s UV), which could be inhibited by chelerythrine. Our results demonstrate that both ET-1 and intracellularly photoreleased diC8 increase ICa by a PKC-mediated pathway, which is in direct contrast to the PKC-independent inhibition of ICa produced by bath-applied diC8. We conclude that specific cellular pools of DAG are crucially important in the regulation of ICa by PKC.

Endothelin and angiotensin mediate most glomerular responses to nitric oxide inhibition

BACKGROUND

Endothelin (ET) and angiotensin mediate glomerular responses to systemic nitric oxide (NO) inhibition. Acute systemic NO synthase (NOS) inhibition in the rat causes marked increases in both preglomerular (RA) and efferent arteriolar (RE) resistances and a fall in the glomerular capillary ultrafiltration coefficient (Kf). In contrast, local intrarenal NOS inhibition increases RA, but has no effect on RE while producing a similar Kf lowering effect as seen with systemic NOS inhibition. These studies were designed to assess whether the increase in RE during systemic NOS inhibition is mediated by endogenous ET and whether angiotensin II (Ang II) also contributes.

METHODS

Micropuncture measurements were made before and during acute systemic NOS inhibition with N-monomethyl L-arginine (NMA) alone, NMA + the nonpeptide ETA and ETB receptor antagonist, bosentan, NMA + the Ang II type 1 receptor blocker, losartan, and NMA during combined bosentan and losartan.

RESULTS

The falls in single nephron glomerular filtration rate (SNGFR) and glomerular plasma flow seen with systemic NOS inhibition were prevented by concomitant administration of bosentan and losartan alone and in combination. The increases in systemic blood pressure (BP), glomerular BP (PGC), RA, and RE and the reduction in Kf seen with systemic NOS inhibition were attenuated by either bosentan or losartan. An attenuation in the elevation in total renal vascular resistance seen with systemic NOS inhibition was also observed with bosentan. Combined ET and Ang II type 1 blockade completely prevented the increase in systemic BP, PGC, and RE and the fall in Kf with systemic NOS inhibition, leaving only a very attenuated rise in RA.

CONCLUSIONS

These findings suggest that endogenous ET and Ang II partially mediate the glomerular hemodynamic responses (including the increased RE) to acute systemic NOS inhibition. The actions of ET and Ang II are mainly additive, and almost all of the vasoconstrictor responses to acute NOS inhibition are prevented when both vasoconstrictor systems are blocked.

Acute interactions between endothelin and nitric oxide in the control of renal haemodynamics

1. Endogenous endothelin (ET) does contribute to control of renal vascular tone via nitric oxide (NO)-dependent vasodilation in the rat. 2. Endothelin mediates some of the renal vascular responses to acute nitric oxide synthase (NOS) inhibition, being particularly important when a rise in renal perfusion pressure occurs. 3. Tonically produced NO blunts the renal vasoconstrictor responses to acutely administered ET. 4. The similarity between the renal vascular responses to ET administration and NOS inhibition is not fortuitous but, in part, reflects important interactions between these vasoactive agents.

KATP channels are common mediators of ischemic and calcium preconditioning in rabbits

Calcium preconditioning (CPC), like ischemic preconditioning (IPC), reduces myocardial infarct size in dogs and rats. ATP-sensitive potassium (KATP) channels induce cardioprotection of IPC in these animals. To determine whether KATP channels mediate both IPC and CPC, pentobarbital sodium-anesthetized rabbits received 30 min of coronary artery occlusion followed by 180 min of reperfusion. IPC was elicited by 5 min of occlusion and 10 min of reperfusion, and CPC was elicited by two cycles of 5 min of calcium infusion with an interval period of 15 min. Infarct size expressed as a percentage of the area at risk was 38 +/- 3% (mean +/- SE) in controls. IPC, CPC, and pretreatment with a KATP channel opener, cromakalim, all reduced infarct size to 13 +/- 2, 17 +/- 2, and 12 +/- 3%, respectively (P < 0.01 vs. controls). Glibenclamide, a KATP channel blocker administered 45 min (but not 20 min) before sustained ischemia, attenuated the effects of IPC and CPC (31 +/- 4 and 41 +/- 6%, respectively). Thus KATP channel activation appears to contribute to these two types of cardioprotection in rabbits.