R. A. John Challiss

An investigation of the β-adrenoceptor that mediates metabolic responses to the novel agonist BRL28410 in rat soleus muscle

The beta-adrenoceptor agonist BRL26830A selectively stimulates metabolic rate in the rat and this thermic effect is resistant to blockade by propranolol. These effects of BRL26830A are partly due to selective stimulation by its metabolite BRL28410, of brown adipocyte beta-adrenoceptors, these receptors being resistant to propranolol. To investigate whether the metabolic effects of BRL28410 in skeletal muscle are also mediated by atypical beta-adrenoceptors, the potencies of BRL28410 and isoprenaline were compared for beta-adrenoceptor mediated responses in rat stripped soleus muscle. In addition, pA2 values for antagonism by propranolol of these responses were determined. Isoprenaline had similar EC50 values for stimulation of lactate formation (4.3 X 10(-9) M) and inhibition of glycogen synthesis (3.4 X 10(-9) M) and these values are similar to its reported EC50 values for stimulation of atrial rate (beta 1-adrenoceptor-mediated) and relaxation of the uterus (beta 2-adrenoceptor-mediated). BRL28410 had similar EC50 values for stimulation of lactate formation (3.7 X 10(-6) M) and inhibition of glycogen synthesis (3.8 X 10(-6) M). These values are only about two-fold less than reported values for relaxation of the uterus, but ten-fold less than reported values for stimulation of atrial rate. The pA2 value of dl-propranolol for antagonism of the effect of isoprenaline on glycogen synthesis (8.38 +/- 0.13) was in the range expected for beta 1- or beta 2-adrenoceptors, but with BRL28410 as agonist the pA2 value was about one unit lower (7.39 +/- 0.11). The beta-adrenoceptors that mediate the metabolic effects of BRL28410 in soleus muscle therefore differ from those that mediate atrial rat, uterine relaxation and adipocyte lipolysis. In addition, the low pA2 value of dl-propranolol versus BRL28410 in rat soleus muscle, which contrasts with the normal pA2 value previously reported for guinea-pig trachea, suggests that beta 2-adrenoceptors in these two tissues can be differentiated with suitable pharmacological agents.

Effect of adenosine deaminase and an adenosine analogue on insulin sensitivity in soleus muscle of the rat

The concentration of insulin that produces half‐maximal stimulation of glycolysis in stripped soleus muscle preparations is decreased from ∼100−10 μunits/ml by the presence of adenosine deaminase in the incubation medium. This suggests that adenosine decreases insulin sensitivity. The effect of the deaminase is abolished by addition of the adenosine analogue, N 6‐phenylisopropyladenosine which is not metabolised by the deaminase. The effect of the deaminase in isolated soleus muscle is similar to that of a period of physical training of the rat.

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.

Maximal activities of hexokinase, 6-phosphofructokinase, oxoglutarate dehydrogenase, and carnitine palmitoyltransferase in rat and avian muscles

The maximum activities of 6-phosphofructokinase and oxoglutarate dehydrogenase in muscle provide quantitative indices of the maximum capacities of anaerobic glycolysis and the Krebs cycle (i.e. the aerobic capacity) respectively. These activities were measured in red, white, and cardiac muscle of birds and the rat. The activities in the white pectoral muscle of the domestic fowl suggest that the Krebs cycle plus electron transfer could provide only about 1% of the rate of ATP production provided by anaerobic glycolysis whereas in pigeon pectoral muscle the predicted maximal rates from the two processes are similar. In contrast to domestic-fowl pectoral muscle, the white rat muscle, epitrochlearis, contains a significant activity of oxoglutarate dehydrogenase, which indicates that the Krebs cycle could provide about 12% of the maximum rate of ATP formation. This may be explained by a higher proportion of type-I and -IIA fibres in the rat muscle compared to the avian muscle. In the aerobic muscles of the rat the maximum activities of carnitine palmitoyl transferase indicate that fatty-acid oxidation could provide a high rate of ATP formation.

Effect of a novel thermogenic β-adrenoceptor agonist (BRL 26830) on insulin resistance in soleus muscle from obese Zucker rats

Young lean (Fa/?) and obese (fa/fa) rats were treated with the thermogenic beta-adrenoceptor agonist, BRL 26830, for 3 weeks. In lean rats this treatment had no effect on body weight but there was a marked increase in the insulin sensitivity of soleus muscle strips with respect to glycolytic rate. Treatment of obese rats with BRL 26830 produced a small but not significant decrease in body weight but the sensitivity of both glycolysis and glycogen synthesis to insulin was increased so that muscles of treated obese rats showed similar insulin sensitivity to untreated lean rats. It is suggested that such changes are unlikely to be merely a secondary consequence of an anti-obesity action.

One-dimensional rotating-frame imaging of phosphorus metabolites in vivo

Abstract Rotating-frame imaging has been applied to the study of metabolic heterogeneity of animal tissue. An orthogonal two-coil system has been used for transmitting and receiving signal; this was designed to ensure accurate spatial assignment (1 mm resolution) and optimal signal sensitivity. The latter was achieved using a Helmholtz receiver coil. Otherwise identical data sets, collected at rest and in steady-state working muscle may be directly compared to gauge the effect of the stimulation. We have demonstrated the applicability of this method as an internally controlled tool to investigate heterogeneity in animal tissue metabolism under various physiological conditions.

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.