Role of p38 mitogen-activated protein kinase in cardiac myocyte secretion of the inflammatory cytokine TNF-alpha.

This study examined the hypothesis that burn trauma promotes cardiac myocyte secretion of inflammatory cytokines such as tumor necrosis factor (TNF)-alpha and produces cardiac contractile dysfunction via the p38 mitogen-activated protein kinase (MAPK) pathway. Sprague-Dawley rats were divided into four groups: 1) sham burn rats given anesthesia alone, 2) sham burn rats given the p38 MAPK inhibitor SB203580 (6 mg/kg po, 15 min; 6- and 22-h postburn), 3) rats given third-degree burns over 40% total body surface area and treated with vehicle (1 ml of saline) plus lactated Ringer solution for resuscitation (4 ml x kg(-1). percent burn(-1)), and 4) burn rats given injury and fluid resuscitation plus SB203580. Rats from each group were killed at several times postburn to examine p38 MAPK activity (by Western blot analysis or in vitro kinase assay); myocardial function and myocyte secretion of TNF-alpha were examined at 24-h postburn. These studies showed significant activation of p38 MAPK at 1-, 2-, and 4-h postburn compared with time-matched shams. Burn trauma impaired cardiac mechanical performance and promoted myocyte secretion of TNF-alpha. SB203580 inhibited p38 MAPK activity, reduced myocyte secretion of TNF-alpha, and prevented burn-mediated cardiac deficits. These data suggest p38 MAPK activation is one aspect of the signaling cascade that culminates in postburn secretion of TNF-alpha and contributes to postburn cardiac dysfunction.

Effect of taurine depletion on angiotensin II-mediated modulation of myocardial function.

Taurine depletion was induced by either incubation of isolated myocytes with 5 mM beta-alanine or feeding rats with water containing 3% beta-alanine. Hearts of taurine depleted rats exhibited an impairment in myocardial relaxation, associated with a decrease in Na(+)-Ca2+ exchanger activity. Exposure of the heart to angiotensin II, an activator of the Na(+)-Ca2+ exchanger, eliminated the relaxation defect. In agreement with the contractile results, taurine depletion prolonged the calcium transient, an effect which was partially eliminated by exposure to angiotensin II. Although peak systolic [Ca2+]i was modestly depressed in the taurine depleted myocyte, peak ventricular pressure was normal. This may be related to an elevation in pHi induced by taurine depletion. Angiotensin II had little effect on contractility of the taurine depleted heart, presumably because of two opposing effects, a reduction in pHi and an increase in [Ca2+]i. Thus, taurine depletion alters contractile function and ion transport and both of these effects are modulated by exposure of the heart to angiotensin II.

Shape and size changes induced by taurine depletion in neonatal cardiomyocytes.

Taurine is a very important organic osmolyte in most adult cells. Because of this property it has been proposed that large changes in the intracellular content of taurine can osmotically stress the cell, causing changes in its size and shape. This hypothesis was examined by measuring cell dimensions of taurine deficient cardiomyocytes using confocal microscopy. Incubation of isolated neonatal rat myocytes with medium containing 5 mM beta-alanine led to a 55% decrease in intracellular taurine content. Associated with the loss of taurine was a reduction in cell size. Two factors contributed to the change in cell size. First, there was a shift in cell shape, favoring the smaller of the two cellular configurations commonly found in the myocyte cell culture. Second, the size of the polyhedral configuration was reduced after beta-alanine treatment. These same two events also contributed to size reduction in cardiomyocytes incubated with medium containing 30 mM mannitol. Nonetheless, some qualitative differences exist between cells osmotically stressed by increasing the osmolality of the incubation medium and decreasing intracellular osmolality. The results support a role for taurine in the regulation of osmotic balance in the neonatal cardiomyocyte.

Mechanisms underlying depressed Na+/Ca2+ exchanger activity in the diabetic heart.

OBJECTIVES: Depression in Na+/Ca2+ exchanger activity is an important factor in the development of the diabetic cardiomyopathy. Since the mechanism underlying this depression remains unknown, the aim of this study was to determine the contribution of hyperglycemia and insulinopenia towards the observed impairment in Na+/Ca2+ exchanger activity. METHODS: Non-insulin-dependent diabetes was induced in neonatal Wistar rats by injection of 90 mg/kg streptozotocin. Na+/Ca2+ exchange in sarcolemmal vesicles and isolated cardiomyocytes was determined by Na(+)-dependent 45Ca2+ transport. To assess the role of insulin deficiency and hyperglycemia on Na+/Ca2+ exchanger activity, neonatal cardiomyocytes were incubated for 3 days in media containing either 5 mM glucose and 56 U/l insulin (Control), 30 mM glucose and 56 U/l insulin (High glucose) or 5 mM glucose and 0 insulin (Insulin deficiency). Since hyperglycemia has been shown to affect protein kinase C activity, Ca(2+)-dependent isoforms of protein kinase C were examined in non-diabetic and diabetic heart using hydroxylapatite chromatography. Also examined was Na+/Ca2+ exchanger mRNA levels in diabetic and non-diabetic hearts using Northern slot blot analysis. RESULTS: Acute insulin produced a dose-dependent increase in Na+/Ca2+ exchanger activity, which was dramatically attenuated in diabetic membrane. Myocytes incubated in media containing 30 mM glucose exhibited a 33% reduction in Na+/Ca2+ exchanger activity, while insulinopenia reduced activity by 63%. Exchanger mRNA levels of the diabetic heart were normal; however, diabetes was associated with major changes in protein kinase C activity. CONCLUSIONS: Reduced Na+/Ca2+ exchanger activity resulting from diabetes, hyperglycemia or insulinopenia may be related to changes in protein kinase C activity, but is not caused by altered expression of the transporter.