Effect of lithium salts on lactate dehydrogenase, adenylate kinase, and 1-phosphofructokinase activities.

Inhibitions of 30 nM rabbit muscle 1-phosphofructokinase (PFK-1) by lithium, potassium, and sodium salts showed inhibition or not depending upon the anion present. Generally, potassium salts were more potent inhibitors than sodium salts; the extent of inhibition by lithium salts also varied with the anion. Li(2)CO(3) was a relatively potent inhibitor of PFK-1 but LiCl and lithium acetate were not. Our results suggest that extents of inhibition by monovalent salts were due to both cations and anions, and the latter needs to be considered before inhibition can be credited to the cation. An explanation for monovalent salt inhibitions is proffered involving interactions of both cations and anions at negative and positive sites of PFK-1 that affect enzyme activity. Our studies suggest that lithium cations per se are not inhibitors: the inhibitors are the lithium salts, and we suggest that in vitro studies involving the effects of monovalent salts on enzymes should involve more than one anion.

Characteristics of rabbit muscle adenylate kinase inhibition by ascorbate.

Earlier studies [1-3] showed that of the glycolytic enzymes, the muscle isozymes PFK-1, LDH, and AK were inhibited by ascorbic acid. These studies on the characteristics of the inhibition of RMAK by ascorbate are part of a hypothesis [3] that ascorbate facilitates the storage of skeletal muscle glycogen by inhibiting glycolysis when the muscle is at rest. These studies examine conditions for RMAK inhibition, prevention of inhibition, and reversal of ascorbate inhibition. We found that the concentration of RMAK was an important condition for inhibition. Above 200 nM RMAK, inhibition by ascorbate could not be demonstrated and below that concentration RMAK became increasingly sensitive to ascorbate inhibition. Associated with increased sensitivity to inhibition by ascorbate is a deviation from a linear to a concave relationship between low RMAK concentrations and enzyme activity. At low RMAK concentrations, the concave relationship becomes convex in the presence of muscle aldolase. In addition, aldolase reverses inhibitions by ascorbate. A comparison of inhibition of RMAK byascorbate and inhibition of LDH-m4 [3] is discussed. Other proteins prevent RMAK inhibition but do not reverse inhibition by ascorbate. The role of RMAK as a factor in the control of the rate of glycolysis is presented as is the role of compartmentalization with respect to the proposed role for ascorbate inhibition.

The antidiabetic agent rosiglitazone upregulates SERCA2 and enhances TNF-alpha- and LPS-induced NF-kappaB-dependent transcription and TNF-alpha-induced IL-6 secretion in ventricular myocytes.

Positive hemodynamic effects of the antidiabetic agent rosiglitazone on perfused whole hearts have recently been described, but the mechanisms regulating these effects are not well understood. This study reports the effects of rosiglitazone on calcium regulation in isolated neonatal rat ventricular myocytes by measurement of Ca2+ transient decay rates and SERCA2 gene expression, and shows that rosiglitazone enhances known cardioprotective signaling pathways. Myocyte treatment with 10 micromol/L rosiglitazone accelerated Ca2+ transient decay rates by approximately 30%, enhanced SERCA2 mRNA levels by approximately 1.5-fold and SERCA2 production by approximately 3-fold. Rosiglitazone treatment (1, 5, and 10 micromol/L) also led to a dose-dependent increase (approximately 1.2-1.5-fold) in SERCA2 promoter activity. Comparable levels of cardiac SERCA overexpression have been associated with physiologically relevant and compensatory effects in vivo. These data link thiazolidinedione-induced improvement in cardiac myocyte function to an upregulation of SERCA2 gene expression. Since NF-kappaB-dependent pathways, including the upregulation of IL-6 secretion, were shown to protect neonatal rat ventricular myocytes from apoptosis upon TNFalpha stimulation, additional experiments were designed to determine whether rosiglitazone enhances TNFalpha-induced NF-kappaB-dependent transcription and IL-6 secretion. Because the endotoxin stress response in ventricular myocytes involves the upregulation of TNFalpha, and the activation of NF-kappaB, the effects of rosiglitazone on lipopolysaccharide-induced NF-kappaB-dependent transcription were also investigated. Treatment of neonatal rat ventricular myocytes with 10 micromol/L rosiglitazone enhanced TNF-alpha- and lipopolysaccharide-induced NF-kappaB-dependent transcription by approximately 1.8- and approximately 1.4-fold respectively, and TNF-alpha-induced IL-6 secretion by n1.5-fold. Rosiglitazone had no significant effects on basal levels of NF-kappaB-dependent transcription and IL-6 secretion. Thus, cardioprotective effects of rosiglitazone may be partly mediated by NF-kappaB.