Exaggerated atrial arachidonate metabolism in rabbit left ventricular myocardial infarction

Effect of isoproterenol on lipid metabolism and prostaglandin production in cultures of newborn rat heart cells, under normoxic and hypoxic conditions

Catecholamines are known to exert deleterious effects on heart cells and to provoke biochemical alterations similar to those observed during myocardial infarction. In order to investigate the mechanisms of these effects, we have studied in cultures of muscle (M) and fibroblast-like (F) cells derived from newborn rat hearts, the action of isoproterenol on membrane lipid metabolism and on prostaglandin production. We showed in F cells that beta-agonist stimulation produced a striking loss of membrane phospholipids and a moderate hydrolysis of cell triglycerides. In addition, isoproterenol treatment induced a significant stimulation of the secretion of prostacyclin but not of prostaglandin E2 by F cells. None of these effects were potentiated by oxygen deprivation. In contrast, M cells, which are sensitive to ischemia, failed to respond to isoproterenol treatment. These results suggest that catecholamines and hypoxia may exert combined deleterious effects on heart tissue by acting separately on the different target cells in vivo.

Prostaglandins regulate the synthesis and secretion of the atrial natriuretic peptide

We have examined the effects of several PGs on the synthesis and release of the atrial natriuretic peptide (ANP) in vivo and in vitro. PGF2 alpha infusion in anesthetized rats resulted in a significant increase in plasma immunoreactive (ir) ANP levels in vivo despite effecting only modest changes in hemodynamics. The PGs were also effective at promoting irANP secretion in primary cultures of neonatal rat atrial and ventricular cardiocytes. PGF2 alpha increased irANP release with half-maximal induction seen at approximately 10(-8) M; PGE2 was somewhat less effective and prostacyclin (PGI2) was without effect. The PGs also increased ANP mRNA levels in these cells, suggesting that these agents exert a major effect on the synthesis as well as the secretion of the prohormone. Transient expression analysis of atrial cells transfected with 2,500 bp of human (h) ANP 5' flanking sequence linked to a chloramphenicol acetyltransferase (CAT) reporter demonstrated that PGF2 alpha (10(-5) M) increased hANP promoter activity approximately twofold relative to the control. PGF2 alpha had no effect on the promoterless control (pSV0-lamin CAT). Treatment of cultured atriocytes with high concentrations of a cyclooxygenase inhibitor resulted in a significant suppression of ANP secretion in vitro and a truncation of the plasma ANP response to volume infusion in vivo. Taken together these studies support a role for PGs as regulators of cardiac ANP synthesis and secretion, and suggest an additional mechanism whereby eicosanoids may act to control cardiovascular and renal homeostasis.

Essential fatty acid deficiency inhibits early but not late leukocyte infiltration in rabbit myocardial infarcts

Essential fatty acid (EFA) deficiency, induced by elimination of the dietary (n-6) fatty acids, has been shown to limit inflammatory cell influx and consequent enhanced eicosanoid production in experimental glomerulonephritis and hydronephrosis. To determine whether EFA-deficiency exerts anti-inflammatory effects following left ventricular myocardial infarction (LVMI), male weanling rabbits were fed EFA-deficient diet for 3 months prior to 60 minutes of distal left circumflex coronary artery occlusion followed by reperfusion. One and 4 days later, corresponding to infiltration of cardiac tissue with polymorphonuclear (PMN) and mononuclear leukocytes respectively, infarcted hearts were buffer perfused and stimulated to produce eicosanoids with f-met-leu-phe or bradykinin. One day following LVMI, the hearts of EFA-deficient rabbits demonstrated a marked suppression of PMN infiltration and eicosanoid production relative to controls. Four days following myocardial infarction, no differences were observed in mononuclear cell invasion, collagen deposition, or eicosanoid production between EFA-deficient and normal hearts. Our data show that EFA-deficiency inhibits PMN influx and consequent enhanced eicosanoid production without affecting the later appearance of mononuclear cells, collagen deposition, or eicosanoid production. Recent studies have shown that suppression of PMN invasion limits the extent of tissue damage following LVMI. Selective inhibition of PMN infiltration is possible and may be useful in the management of acute myocardial infarction.

fMet-Leu-Phe receptor expression by an interstitial cell in rabbit right atrium following left ventricular myocardial infarction

Previous studies have shown that rabbit hearts subjected to in vivo left ventricular myocardial infarction and subsequent ex vivo perfusion respond to N-formylmethionyl-leucyl-phenylalanine (fMLP) with enhanced eicosanoid synthesis. This synthetic response occurs primarily in the right cardiac atrium, a site distant from the injury, and is not the result of increased enzymatic capacity for arachidonate metabolism. To further investigate the mechanism of this enhanced synthetic response, [3H]fMLP binding was characterized and binding sites were localized autoradiographically in intact tissue sections prepared from control hearts and hearts subjected to left ventricular myocardial infarction (1, 2, and 4 days postinfarction). Analysis of binding isotherms revealed a saturable high affinity (KD approximately 1 nM) fMLP binding site in the right cardiac atrium. After myocardial infarction specific binding in right atria (2 day) increased 12-fold (Bmax = 14.8 +/- 2.4 fmol/cm2) compared with normal controls (Bmax = 1.2 +/- 0.1 fmol/cm2). Specific binding of fMLP also increased in the infarcted zone of left ventricle, but Bmax was only 30-50% that of right atria. Light microscopic autoradiography studies revealed that atrial fMLP binding sites were highly concentrated in small morphologically undifferentiated cells located in interstitial and perivascular spaces. These results demonstrate the existence of a formylated peptide receptor on a nonleukocytic cell and illustrate its regulation following left ventricular injury.

Role of invading leukocytes in enhanced atrial eicosanoid production following rabbit left ventricular myocardial infarction

The isolated perfused hearts of rabbits previously subjected to in vivo left ventricular myocardial infarction (LVMI) show a 5-10-fold increase in f-Met-Leu-Phe (FMLP) and bradykinin (BK)-stimulated eicosanoid metabolite production relative to noninfarcted hearts. This exaggerated arachidonate metabolism has been shown to occur primarily in the cardiac atria, a site remote from the zone of injury and to be associated with a 10-15-fold increase in atrial FMLP receptor number in the absence of atrial inflammation. All of these changes were temporally related to leukocyte infiltration into the infarct zone. To determine whether invading leukocytes mediate these responses, acute inflammatory cell influx was suppressed either by inducing leukopenia with nitrogen mustard or by administration of BW-755C, a mixed cyclooxygenase-lipoxygenase inhibitor. Both pharmacological manipulations resulted in a decrease in inflammatory cells in the infarct zone and a marked suppression (50-70%) of ex vivo agonist-stimulated eicosanoid metabolite production from perfused hearts and isolated atria. These manipulations also resulted in reversal of ex vivo FMLP-induced coronary vasoconstriction as well as augmentation of BK-induced coronary vasodilation. Further studies in nitrogen mustard-treated animals revealed a suppression of the LVMI-stimulated increase in atrial FMLP receptor number. These data show that suppression of leukocyte invasion after LVMI attenuates enhanced cardiac and atrial eicosanoid metabolite production, and results in marked changes in coronary vascular reactivity. An additional finding was that basal and stimulated LTB4 production was markedly increased in infarcted hearts. In vivo suppression of the increase in LTB4 production by BW-755C was associated with inhibition of inflammatory cell influx into the infarct zone. It therefore appears that LTB4 may be an important proinflammatory mediator of leukocyte invasion after LVMI.

Activated neutrophils release mediators that may contribute to myocardial injury and dysfunction associated with ischemia and reperfusion

Neutrophils accumulate in the ischemic myocardium and exacerbate postischemic cardiac dysfunction and injury. The formation of lipoxygenase metabolites of AA, derived either directly from the neutrophils or by interactions with other blood elements or cells, may promote neutrophil-mediated injury. Recognition of the roles played by neutrophils and AA metabolites in reperfusion injury may lead to the development of new therapies that can be used in conjunction with thrombolytic drugs to reduce the complications associated with restoring blood flow to the ischemic heart.