THE EFFECT OF CALCIUM IONS ON KETONE-BODY PRODUCTION BY RAT-LIVER SLICES

THE EFFECT OF CALCIUM IONS ON THE LEAKAGE OF PROTEIN AND ENZYMES FROM RAT-LIVER SLICES

1. The effects of Ca(2+) ions and the nutritional state of the animals on the distribution of soluble protein between the medium, cytoplasmic and particulate fractions after incubation of rat-liver slices have been studied. 2. The Ca(2+)ions decreased the leakage of cytoplasmic protein into the medium and increased the loss of protein from the particles; the effects were more marked with livers from starved animals and animals fed on a low-carbohydrate diet. 3. The results obtained for protein were confirmed by measurements of the changes in distribution of compartment-specific enzymes, and these indicated that both the cytoplasmic and particulate fractions contributed to the protein found in the medium. 4. EDTA decreased the loss of protein and enzymes from both the cytoplasmic and particulate fractions; this effect was independent of the Ca(2+) ion concentration. 5. The inclusion of pyruvate, succinate or citrate in the medium had no marked effects on the leakage of protein.

Effects of 8-N,N-diethylamino-octyl-3,4,5-trimethoxybenzoate (TMB-8) HCl and verapamil on the metabolism of free fatty acid by hepatocytes

The influence of calcium antagonists on hepatic lipid metabolism was investigated in freshly dispersed rat hepatocytes incubated with [1-14C]oleate and verapamil or 8-N,N-diethylamino-octyl-3,4,5-trimethoxybenzoate (TMB-8). Synthesis of triglyceride was calculated from the specific radioactivity of [1-14C]oleate in extracted total lipid, after separation of each lipid class by thin-layer chromatography. Ketogenesis was measured enzymatically or as the amount of radioactivity incorporated into neutralized acid-soluble extracts. Neither verapamil nor TMB-8 affected triglyceride synthesis. In contrast, TMB-8 and verapamil exerted a concentration-dependent inhibition of ketogenesis, with TMB-8 being more potent than verapamil (inhibition by 50 microM TMB-8 was 73 +/- 9% versus 38 +/- 2% inhibition by 50 microM verapamil). Increasing the concentrations of calcium (0 to 4.2 mM) or oleate (0 to 2.0 mM) increased the rate of ketogenesis but did not alter the antiketogenic potency of TMB-8 or verapamil, indicating that inhibition of ketogenesis by these drugs was not calcium dependent. Since the calcium antagonists did not affect ketogenesis from octanoic acid, and since carnitine stimulated ketogenesis from [1-14C]oleate by 25% and reversed the antiketogenic effects of TMB-8 and verapamil, it appeared that the two calcium antagonists inhibited ketogenesis by interfering with the activity of the outer mitochondrial carnitine palmitoyltransferase. In assays of the outer carnitine palmitoyltransferase in isolated mitochondria, both TMB-8 and verapamil were inhibitory. TMB-8 was the more potent inhibitor of this enzyme, and carnitine was able to overcome inhibition by each of the inhibitors. These results suggest that verapamil and TMB-8 may inhibit ketogenesis by mechanisms independent of their well known effects on cellular calcium concentrations, and that inhibition may be competitive with respect to carnitine concentration. However, direct inhibition of carnitine palmitoyltransferase may not explain completely the inhibition of ketogenesis by these drugs, since concentrations required for enzyme inhibition were greater than those required for inhibition of ketogenesis in isolated hepatocytes.

Metabolism of 125I-labeled lipoproteins by the isolated rat lung

The capacity of the isolated perfused rat lung to metabolize the protein moieties of serum lipoproteins was assessed using homologous (rat) and heterologous (human) plasma lipoproteins. The protein and lipid moieties of the plasma lipoproteins were labeled in vivo with Na[125I]. In selected cases the lipoprotein peptides were labeled in vivo with 14C- or 3H-labeled amino acids. Uptake of lipoprotein label during perfusion was monitored by measure of losses in perfusate label and by rises in pulmonary tissue labeling as shown by radioassay and by light and electron microscope radioautography. Lipoprotein degradation was assessed by fractionation of perfusate and lung tissue radioactive material into trichloroacetic acid (TCA)-isoluble, TCA-soluble, and ether-ethanol-soluble fractions. When heparin was included in the perfusion medium, there was selective degradation of the protein portion of very low density lipoprotein (VLDL) in the perfusate and concomitant uptake of radioactive label by the lungs. Low density lipoprotein (LDL)) was neither taken up nor catabolized by the isolated rat lung in the absence or presence of heparin. By light and electron microscopy, the label was localized over the interalveolar septa, predominantly the capillary endothelium. Disappearance of TCA-insoluble radioactivity from the perfusate was associated with the generation of both TCA-soluble iodide and noniodide radioactivity. Greater than 50% of the radioactive label taken up by the lungs was found in the delipidated TCA-insoluble fraction. This study provides in vitro evidence for pulmonary catabolism of VLDL apolipoproteins and uptake of peptide catabolic products of VLDL by the lung.

The effect of calcium ions on the glycolytic activity of Ehrlich ascites-tumour cells

1. Added Ca(2+) inhibited lactate formation from sugar phosphates by intact Ehrlich ascites-tumour cells. Lactate formation from glucose by these cells was unaffected by added Ca(2+). 2. The Ca(2+) inhibition of lactate formation by intact cells occurred in the extracellular medium. 3. Intact ascites-tumour cells did not take up Ca(2+)in vitro. 4. Glycolysis of sugar phosphates by cell extracts as well as pyruvate formation from 3-phosphoglycerate and phosphoenolpyruvate was inhibited by Ca(2+). 5. It was concluded that Ca(2+) inhibited the pyruvate-kinase (EC 2.7.1.40) reaction. Further, Ca(2+) inhibition of pyruvate kinase could be correlated with the overall inhibition of glycolysis. 6. Concentrations of Ca(2+) usually present in Krebs-Ringer buffers, inhibited glycolysis and pyruvate-kinase activity by approx. 50%. 7. The inhibition of glycolysis by added Ca(2+) could be partially reversed by K(+) and completely reversed by Mg(2+) or by stoicheiometric amounts of EDTA. 8. The hypothesis is advanced that the inability of tumour cells to take up Ca(2+) is a factor contributing towards their high rate of glycolysis.

The cellular transport of calcium in rat liver

The bidirectional transport of calcium in rat liver was studied using slices labeled with Ca(47) in a closed two compartment system. Steady-state conditions were observed with influx and efflux transfer coefficients of 0.070 and 0.018 per minute, respectively. The rapidly exchanging cell fraction of calcium existed at a concentration three times higher than the average cell concentration of calcium and occupied cell loci comprising less than 25% of the cell mass, suggesting that calcium associated with the cell membranes, nuclei, and mitochondria participated in the rapidly exchanging fraction. At pH 7.4 and 377deg;C, the influx transfer coefficient was 25% above the steady-state condition and accumulation of calcium by the slices occurred. Studies of the effects of varied physical and chemical conditions revealed that the influx transfer coefficient was increased by elevated pH, strontium, certain metabolic inhibitors, and 2 mM concentrations of cyclic adenosinemonophosphate and adenosinetriphosphate. The influx transfer coefficient was decreased by reduced temperature, decreased pH, magnesium, and 10 mM adenosinetriphosphate. The efflux transfer coefficient was increased by elevated pH, strontium, iodoacetate, and adenosinetriphosphate, and was decreased by reduced temperature and by N-ethylmaleimide. These data support the thesis that cell transport of calcium is accomplished by the attachment of calcium atoms to the cell surface and transport through the plasma membrane bound to either specific carriers or to membrane constituents. Conditions which change the affinities, capacities, and mobilities of plasma membrane ligands that bind calcium or cause extracellular chelation of calcium are capable of altering the rate of calcium transport.