phosphocreatine and acadesine

The AMP-activated protein kinase (AMPK) is activated by a fall in the ATP:AMP ratio within the cell in response to metabolic stresses. Once activated, it phosphorylates and inhibits key enzymes in energy-consuming biosynthetic pathways, thereby conserving cellular ATP. The creatine kinase-phosphocreatine system plays a key role in the control of ATP levels in tissues that have a high and rapidly fluctuating energy requirement. In this study, we provide direct evidence that these two energy-regulating systems are linked in skeletal muscle. We show that the AMPK inhibits creatine kinase by phosphorylation in vitro and in differentiated muscle cells. AMPK is itself regulated by a novel mechanism involving phosphocreatine, creatine and pH. Our findings provide an explanation for the high expression, yet apparently low activity, of AMPK in skeletal muscle, and reveal a potential mechanism for the co-ordinated regulation of energy metabolism in this tissue. Previous evidence suggests that AMPK activates fatty acid oxidation, which provides a source of ATP, following continued muscle contraction. The novel regulation of AMPK described here provides a mechanism by which energy supply can meet energy demand following the utilization of the immediate energy reserve provided by the creatine kinase-phosphocreatine system.

Topics: Amino Acid Sequence; Aminoimidazole Carboxamide; AMP-Activated Protein Kinases; Animals; Clone Cells; Creatine; Creatine Kinase; Enzyme Inhibitors; Hydrogen-Ion Concentration; Liver; Models, Chemical; Molecular Sequence Data; Multienzyme Complexes; Muscle Fibers, Skeletal; Muscle, Skeletal; Phosphocreatine; Phosphorylation; Protein Kinases; Protein Serine-Threonine Kinases; Rabbits; Rats; Ribonucleosides

Since abnormalities in regional myocardial function and nucleotide metabolism persist for a prolonged period after a brief coronary occlusion the temporal relation between the resolution of myocardial dysfunction and repletion of nucleotide pools in postischaemic myocardium was studied in conscious mildly sedated animals. In a second experiment 5-amino-4-imidazolecarboxamide riboside (AICAriboside) was infused in an attempt to influence myocardial function by altering the rate of adenine nucleotide synthesis. Conscious dogs mildly sedated with morphine underwent coronary occlusion for 15 min followed by reperfusion for 30 min or 12 h, at which time a myocardial sample was obtained for nucleotide analysis. Segment shortening averaged 62% of control values at 15 min of reperfusion and increased to 81% of control by 12 h of reperfusion (p less than 0.05). Adenine nucleotide content was 75(5)% of control after 30 min of reperfusion and did not change significantly over the next 12 h of reperfusion. Thus the early return of systolic function was not accompanied by a detectable increase in total adenine nucleotide content. In the second experiment a pronounced stimulation of the proximal purine nucleotide synthetic pathway occurred as evidenced by a 13-fold to 25-fold increase in inosine monophosphate content. One branch of the distal purine pathway was also stimulated as evidenced by complete repletion of guanine nucleotide pools, but the product of the other branch (adenine nucleotides) did not increase significantly. These results indicate a selective limitation of the distal adenine nucleotide synthetic pathway in postischaemic myocardium.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Aminoimidazole Carboxamide; Animals; Coronary Disease; Dogs; Heart; Hemodynamics; Myocardium; Nucleotides; Phosphocreatine; Purine Nucleotides; Ribonucleosides

Controversy exists as to whether the purine nucleotide cycle is important in normal skeletal muscle function. Patients with disruption of the cycle from a deficiency of AMP deaminase exhibit variable degrees of muscle dysfunction. An animal model was used to examine the effect of inhibition of the purine nucleotide cycle on muscle function. When the compound 5-amino-4-imidazolecarboxamide riboside (AICAriboside) is phosphorylated to the riboside monophosphate in the myocyte it is an inhibitor of adenylosuccinate lyase, one of the enzymes of the purine nucleotide cycle. AICAriboside was infused in 28 mice, and 22 mice received saline. Gastrocnemius muscle function was assessed in situ by recording isometric tension developed during stimulation. The purine nucleotide content of the muscle was measured before and after stimulation. Disruption of the purine nucleotide cycle during muscle stimulation was evidenced by a greater accumulation of adenylosuccinate, the substrate for adenylosuccinate lyase, in the animals receiving AICAriboside (0.60 +/- 0.10 vs. 0.05 +/- 0.01 nmol/mumol total creatine, P less than 0.0001). There was also a larger accumulation of inosine monophosphate in the AICAriboside vs. saline-treated animals at end stimulation (73 +/- 6 vs. 56 +/- 5 nmol/mumol total creatine, P less than 0.03). Inhibition of flux through the cycle was accompanied by muscle dysfunction during stimulation. Total developed tension in the AICAriboside group was 40% less than in the saline group (3,023 +/- 1,170 vs. 5,090 +/- 450 g . s, P less than 0.002). An index of energy production can be obtained by comparing the change in total phosphagen content per unit of developed tension in the two groups. This index indicates that less high energy phosphate compounds were generated in the AICAriboside group, suggesting that interruption of the purine nucleotide cycle interfered with energy production in the muscle. We conclude from these studies that defective energy generation is one mechanism whereby disruption of the purine nucleotide cycle produces muscle dysfunction.

Topics: Adenylosuccinate Lyase; Aminoimidazole Carboxamide; Animals; Lyases; Mice; Mice, Inbred C57BL; Muscle Contraction; Muscles; Phosphocreatine; Purine Nucleotides; Ribonucleosides; Ribonucleotides