High fatty acid oxidation (FAO) rates contribute to ischemia-reperfusion injury of the myocardium. Because peroxisome proliferator-activated receptor (PPAR)alpha regulates transcription of several FAO enzymes in the heart, we examined the response of mice with cardiac-restricted overexpression of PPARalpha (MHC-PPARalpha) or whole body PPARalpha deletion including the heart (PPARalpha-/-) to myocardial ischemia-reperfusion injury. Isolated working hearts from MHC-PPARalpha and nontransgenic (NTG) littermates were subjected to no-flow global ischemia followed by reperfusion. MHC-PPARalpha hearts had significantly higher FAO rates during aerobic and postischemic reperfusion (aerobic 1,479 +/- 171 vs. 699 +/- 117, reperfusion 1,062 +/- 214 vs. 601 +/- 70 nmol x g dry wt(-1) x min(-1); P < 0.05) and significantly lower glucose oxidation rates compared with NTG hearts (aerobic 225 +/- 36 vs. 1,563 +/- 165, reperfusion 402 +/- 54 vs. 1,758 +/- 165 nmol x g dry wt(-1) x min(-1); P < 0.05). In hearts from PPARalpha-/- mice, FAO was significantly lower during aerobic and reperfusion (aerobic 235 +/- 36 vs. 442 +/- 75, reperfusion 205 +/- 25 vs. 346 +/- 38 nmol x g dry wt(-1) x min(-1); P < 0.05) whereas glucose oxidation was significantly higher compared with wild-type (WT) hearts (aerobic 2,491 +/- 631 vs. 901 +/- 119, reperfusion 2,690 +/- 562 vs. 1,315 +/- 172 nmol x g dry wt(-1) x min(-1); P < 0.05). Increased FAO rates in MHC-PPARalpha hearts were associated with a markedly lower recovery of cardiac power (45 +/- 9% vs. 71 +/- 6% of preischemic levels in NTG hearts; P < 0.05). In contrast, the percent recovery of cardiac power of PPARalpha-/- hearts was not significantly different from that of WT hearts (80 +/- 8% vs. 75 +/- 9%). This study demonstrates that chronic activation of PPARalpha is detrimental to the cardiac recovery during reperfusion after ischemia.

The mechanism of endothelin-1 (ET-1)-induced atrial natriuretic peptide (ANP) release was studied in neonatal rat ventricular cardiomyocytes. These cells expressed a single high-affinity class of ETAreceptor (dissociation constant = 54 ± 18 pM, n = 3), but no ETB receptors. Incubation of cardiomyocytes with ET-1 led to concentration-dependent ANP release and prostacyclin production. ET-1-induced ANP release was affected by neither protein kinase C (PKC) inhibition or downregulation nor by cyclooxygenase inhibition, indicating that ET-1-stimulated ANP secretion is not a PKC-mediated, prostaglandin-dependent process. Furthermore, ET-1 significantly stimulated adenosine 3′,5′-cyclic monophosphate (cAMP) production and increased cytosolic calcium concentration in these preparations. Both ET-1-induced calcium influx and ANP release were decreased by the cAMP antagonist Rp-cAMPS, the Rp diastereoisomer of cAMP. Moreover, ET-1-induced ANP secretion was strongly inhibited in the presence of nifedipine as well as in the absence of extracellular calcium. Thus our results suggest that ET-1 stimulates ANP release in ventricular cardiomyocytes via an ETAreceptor-mediated pathway involving cAMP formation and activation of a nifedipine-sensitive calcium channel.

In the genetically obese Zucker rat, the myocardial signal transduction cascade PKC-MAPK-ANP mRNA seems to be markedly impaired. It can be speculated that this abnormal cardiac cell signaling in obese rats reflects an early phase in the cardiac pathogenesis accompanying obesity.

Angiotensin II-induced activation of aldosterone secretion in adrenal glomerulosa cells is mediated by an increase of intracellular calcium. We describe here a new Ca 2؉ -regulatory pathway involving the inhibition by angiotensin II of calcium extrusion through the Na ؉ / Ca 2؉ exchanger. Caffeine reduced both the angiotensin II-induced calcium signal and aldosterone production in bovine glomerulosa cells. These effects were independent of cAMP or calcium release from intracellular stores. The calcium response to angiotensin II was more sensitive to caffeine than the response to potassium, suggesting that the drug interacts with a pathway specifically elicited by the hormone. In calcium-free medium, calcium returned more rapidly to basal levels after angiotensin II stimulation in the presence of caffeine. Thapsigargin had no effect on these kinetics, but diltiazem, which inhibits the Na ؉ /Ca 2؉ exchanger, markedly reduced the rate of calcium decrease and abolished caffeine action. The involvement of this exchanger was supported by the effect of cell depolarization and of a reduction of extracellular sodium on the rate of calcium extrusion. We also determined the mechanism of angiotensin II action on the exchanger. Phorbol esters reduced the rate of calcium extrusion, which was increased by baicalein, an inhibitor of lipoxygenases, and by SB 203580, an inhibitor of the p38 MAPK. Finally, we showed that angiotensin II acutely activates, in a caffeine-sensitive manner, p38 MAPK in glomerulosa cells. In conclusion, in bovine glomerulosa cells, the Na ؉ /Ca 2؉ exchanger plays a crucial role in extruding calcium, and, by reducing its activity, angiotensin II influences the amplitude of the calcium signal. The hormone exerts its action on the exchanger through a caffeine-sensitive pathway involving the p38 MAPK and lipoxygenase products.

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.