Fred J. Lozeman

Effects of adenosine deaminase on the sensitivity of glucose transport, glycolysis and glycogen systhesis to insulin in muscles of the rat

1. Soleus, extensor digitorum longus (EDL) or hemi-diaphragm muscles of the rat were incubated in the presence of insulin and rates of the processes of glycolysis and glycogen synthesis were measured. 2. The concentrations of insulin required to cause half-maximal stimulation of glycolysis in both soleus and EDL preparations were significantly decreased by the presence of adenosine deaminase in the medium. 3. Adenosine deaminase increased the sensitivity of the process of hexose transport to insulin (in an identical manner to the change in sensitivity of glycolysis) in the EDL preparation. 4. None of the adenosine mediated effects on insulin-stimulated rates of glycolysis were observed in the hemi-diaphragm preparation or on the rates of glycogen synthesis in any of the three muscle preparations. 5. Therefore, changes in the adenosine system in skeletal muscle influence insulin sensitivity regardless of fibre type composition of the muscle.

Increased insulin sensitivity in soleus muscle from cold-exposed rats: reversal by an adenosine-receptor agonist

The effect of 0.5,2,7 and 14 days cold exposure at 4°C on insulin sensitivity was investigated in the stripped soleus muscle preparation incubated in vitro. Cold‐exposure for 2 or 7 days increased the sensitivity of glycolysis, but did not affect the sensitivity of glycogen synthesis to insulin. Cold‐exposure for 0.5 or 14 days had no effect on the sensitivity of either process to insulin. The increased sensitivity to insulin after exposure of animals to the cold for 2 days was completely reversed by addition of the adenosine receptor agonist, 2‐chloroadenosine, to the incubation medium. This suggests that cold exposure may increase insulin sensitivity in the muscle, either by a decrease in the concentration of adenosine in the muscle, or by a decrease in the number or affinity of the adenosine receptors.

Effects of dipyridamole on adenosine concentration, insulin sensitivity and glucose utilisation in soleus muscle of the rat

Adenosine has been shown to modulate the sensitivity of skeletal muscle to insulin (Budohoski et al. 1984). In an attempt to further characterize the modulatory action of adenosine on insulin sensitivity inskeletal muscle we have investigated the effect of the nucleoside transport inhibitor dipyridamole in isolated incubated soleus muscle strips. At a concentration of 50 μM, dipyridamole increased the concentration of adenosine in the soleus muscle by 36% and in the incubation medium by 32%. At this concentration of dipyridamole, the basal rates (in the presence of 1 μunit of insulin/ml) of lactate formation, 2-deoxy [2,6-3H]glucose phosphorylation and glucose oxidation were decreased by 48%, 43% and 47% respectively, whilst the rate of glycogen synthesis was unaffected. Insulin-stimulated rates (in the presence of 10000 μunit of insulin/ml) of lactate formation, 2-deoxy [2,6-3H] glucose phosphorylation, glycogen synthesis and glucose oxidation were decreased by 70%, 30%, 26% and 20% respectively in the presence of 50 μM dipyridamole. Although 50 μM dipyridamole was required to exert a significant effect on medium and soleus muscle adenosine concentrations, statistically significant effects on glycolytic rate were observed at concentrations as low as 2 μM dipyridamole.It is concluded that the results are not consistent with dipyridamole exerting an effect on skeletal muscle carbohydrate metabolism solely through elevation of the intracellular or interstial adenosine concentration, but strongly suggest that dipyridamole inhibits glucose transport and/or phosphorylation in skeletal muscle.

The Hormone-Modulatory Effects of Adenosine in Skeletal Muscle

The interactive effects of insulin and the local hormone adenosine have been investigated in vitro in a number of skeletal muscle preparations. Insulin affects carbohydrate metabolism in these preparations by stimulating the rate of glucose transport and increasing the rate of glyco-gen synthesis. The concentration of insulin necessary to stimulate the rates of both these processes half-maximally is 100 μU/ml. Removal of endogenously produced adenosine by addition of adenosine deaminase increased specifically the sensitivity of the rate of glucose transport to insulin, whilst exerting no effect on the rate of glycogen synthesis. This suggests that the molecular basis of such a modulatory effect is at the “post-receptor” level. This was observed in soleus muscle (90% type I fibres) and extensor digitorum longus (90% type II fibres), but not in hemi-diaphragm. The insulin-modulatory effect of endogenous adenosine could be mimicked in a predictable manner by addition of adenosine-receptor agonists or antagonists. The possibility that adenosine was exerting this novel action on glucose metabolism in skeletal muscle by changes in intracellular cyclic AMP concentration has been studied. Our results suggest that this is unlikely; adenosine-receptor agonists have a weak stimulatory effect on adenylate cyclase and, in addition, elevation of intracellular cyclic AMP concentrations either by addition of dibutyryl cyclic AMP or β-adrenoceptor agonists do not directly affect the sensitivity of the rate of glucose transport to insulin.