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Tolbutamide-mediated enhancement of anaerobic metabolism
BOTH NEUTRAL AND ACID TRIGLYCERIDE LIPASES ARE PRESENT IN RAT HEART MYOCYTES. A. Kryski and D.L. Severson. University of Calgary, Calgary, Alberta. Canada. Preparations from rat heart have been characterized as having triacylglycerol ester hydrolase (TGL) activity with both an acid (pH 4.5-5) and neutral (pH 7.5-8) pH optimum, in addition to lipoprotein lipase (LPL) activity. This study was done in order to determine if these enzymes were present Calcium tolerant in isolated cardiac muscle cells (myocytes). myocytes isolated from rat heart excluded 0.4% Trypan Blue and had rod-shaped morphology. Lipase activity at pH 8.2 was increased with the addition of serum (1.6 fold) indicating the presence of LPL in myocytes. Specific activity of a neutral TGL, determined in myocyte homogenates at pH 7.5, was 10.5 + 1.0 nmoles/hr/mg protein compared to 11.4 k 1.2 nmoles/hr/mg in a low speed supernatant (S.4) from heart homogenates. Acid TGL activity (pH 5.0) was 10.6 i 0.4 in the myocytes and 10.0 _C 0.1 in the whole heart. Neutral TGL activity measured in the presence of CaCl (5mM), MgC12 (5mM) and NaCl (1OOmM) was inhibited by 34%, 24% and 70% respectively. In contrast, the same concentrations of CaCl M&l and NaCl stimulated neutral TGL activity by 2.6, 3.z'and 2!5-fold respectively in the S.4 fraction from the whole heart. Acid TGL activity was inhibited by metal ions and NaCl by 49-65% and 34-36% in preparations from whole hearts and myocytes. This data suggests a fundamental difference in the properties of the neutral TGL in myocytes compared to the whole heart. (Supported by the Medical Research Council of Canada)
TOLBUTAMIDE-MEDIATED ENHANCEMENT OF ANAEROBIC METABOLISM W.G. Lampson, J.H. Kramer and S.W. Schaffer. Dept. Pharmacol. Univ. So. Ala., Mobile, AL. 36688 Previous studies have shown that tolbutamide, over a concentkation range of 0.2-0.7mM, provides protection to the ischemic myocardium (J. Molec. Cell. Cardiol., 1981, 13, 341345). This protection appears to be due, in part, to tolbutamide-mediated enhancement of anaerobic metabolism. Rat hearts perfused with 5mM glucose and 5mM acetate exhibited a 50% increase in glucose utilization following exposure to 0.6mM tolbutamide. Analysis of tissue glycolytic intermidiates showed that this increase resulted from an activation of phosphofructokinase. The combined effects of an increase in AMP and inorganic phosphate levels, a reduction of the ATP/ADP ratio, and a direct effect of tolbutamide on the enzyme appears to account for the observed activation. In addition, tolbutamide doubles phosphorylase a content leading to an increase in the rate of glycogenolysis. No elevation in tissue CAMP levels was observed following exposure to tolbutamide suggesting that activa$$on of phosphorylase a is associated with an increase in Ca rather than CAMP. It was also found that tolbutamide stimulates oxygen consumption of the heart approximately 15% and increases acetate utilization by 10%. Thus, it appears that tolbutamide enhances anaerobic metabolism of the heart, partially at the expense of acetate metabolism. Supported by grant HL28445.
TOLBUTAMIDE-MEDIATED ENHANCEMENT OF ANAEROBIC METABOLISM W.G. Lampson, J.H. Kramer and S.W. Schaffer. Dept. Pharmacol. Univ. So. Ala., Mobile, AL. 36688 Previous studies have shown that tolbutamide, over a concentkation range of 0.2-0.7mM, provides protection to the ischemic myocardium (J. Molec. Cell. Cardiol., 1981, 13, 341345). This protection appears to be due, in part, to tolbutamide-mediated enhancement of anaerobic metabolism. Rat hearts perfused with 5mM glucose and 5mM acetate exhibited a 50% increase in glucose utilization following exposure to 0.6mM tolbutamide. Analysis of tissue glycolytic intermidiates showed that this increase resulted from an activation of phosphofructokinase. The combined effects of an increase in AMP and inorganic phosphate levels, a reduction of the ATP/ADP ratio, and a direct effect of tolbutamide on the enzyme appears to account for the observed activation. In addition, tolbutamide doubles phosphorylase a content leading to an increase in the rate of glycogenolysis. No elevation in tissue CAMP levels was observed following exposure to tolbutamide suggesting that activa$$on of phosphorylase a is associated with an increase in Ca rather than CAMP. It was also found that tolbutamide stimulates oxygen consumption of the heart approximately 15% and increases acetate utilization by 10%. Thus, it appears that tolbutamide enhances anaerobic metabolism of the heart, partially at the expense of acetate metabolism. Supported by grant HL28445.