Substrate Utilization in Perfused Skeletal Muscle

Abstract

D uring a fast, fatty acids and later ketone bodies readily replace glucose as a fuel for muscle. 1~4 This switch in muscle fuels is one of-the first steps man takes to restrict the unnecessary use of carbohydrates and thereby limit the need for gluconeogenesis from protein when he is deprived of food. Although diminished circulating levels of insulin and glucose undoubtedly contribute to these changes, other mechanisms such as diminished insulin sensitivity and inhibition of glucose metabolism by lipid substrates may also play a role. 5 " 7 In order to study this problem, we have evaluated some of the factors governing the metabolism of glucose in skeletal muscle using the isolated perfused rat hindquarter preparation. This report deals with the effects of starvation, moderately severe diabetes, and acetoacetate on glucose uptake and disposition in resting and exercising muscle. METHODSThe hindquarter preparation used has been described previously. 8 In essence, the tissue of the lower third of an anesthetized rat is perfused through a cannula inserted into the aorta, and perfusate leaving the preparation exits through a cannula in the vena cava. To make the hindquarter as much as possible a muscle preparation, skin and superficial adipose tissue are removed and abdominal branches of the great vessels that do not supply muscle are ligated.The perfusion medium is composed of Krebs-Henseleit solution and contains aged human red cells (7-8 g/100 ml), 4% albumin, 5 mM glucose, and lactate and pyruvate at physiologic concentrations. Throughout the perfusion, it is gassed with 95% O 2 -5% CO 2 ; flow to the preparation is maintained at a rate of 11 ml/min. We have replaced the bulb oxygenator utilized in early experiments with a silastic coil in order to eliminate the need for a second passage of medium through the pump and thereby to diminish hemolysis. Flow rate with this system is 12-13 ml/min. This has not appeared to alter the metabolism of the preparation. On the other hand, as hemolyzed red cells degrade insulin, 9 -93 it has made experiments at low concentrations of insulin more feasible (see below).The isolated perfused hindquarter offers several advantages over many earlier skeletal muscle preparations; the tissue receives exogenous substrates and O 2 through the usual vascular channels, normal anatomical relationships to bone and nerve are maintained, and the muscle can be made to exercise by sciatic nerve stimulation. A comparison of the metabolic characteristics of the perfused hindquarter and hindquarter muscle of an intact rat is shown in Table 1. RESULTS Glucose utilization.It is not clear whether during starvation the utilization of glucose by skeletal muscle is diminished by factors other than the decreases in plasma insulin and glucose. We have previously shown that the stimulation of glucose uptake by insulin is not depressed in hindquarters of starved rats; however, supraphysiologic concentrations of insulin were used. 11 To explore the possibility that glucose utilization might be diminished ...

Effects of non-esterified fatty acid availability on insulin stimulated glucose utilisation and tissue pyruvate dehydrogenase activity in the rat

“…In contrast to reports using isolated muscle preparations [10] or the perfused rat hindquarter [11][12][13], our findings support the idea that the "glucose-fatty acid cycle" is operative in resting skeletal muscle. Rennie and Holloszy [15] drew attention to the importance of adequate tissue oxygenation for demonstration of an effect of increased NEFA on glucose metabolism in muscle.…”

“…However, the effects of an acute increase in fatty acid availability on skeletal muscle glucose utilisation remain controversial. Schonfeld and Kipnis [10] using rat diaphragm, and others using the isolated perfused rat hindquarter [11][12][13] could not demonstrate inhibition of glucose metabolism by increased fatty acid availability. A sparing effect of fatty acids on muscle glucose metabolism has, however, been demonstrated in well oxygenated perfused skeletal muscle [14] and in the exercising rat [15].…”

Effect of Fasting and Streptozotocin Diabetes on Insulin Binding and Action in the Isolated Mouse Soleus Muscle

“…At the end of incubation, muscles were freeze-clamped, homogenized, and glycogen synthase activity was measured as described in Methods, in the absence (I form) or in the presence (I + D form) of 7.2 mM glucose-6-phosphate. The two muscles from one mouse were studied as pairs incubated with or without (basal) insulin (33 nM (33). Another striking feature of the present study is the observation that, at maximally effective doses, insulin elicited in muscles from 48-h-fasted and from streptozotocin-diabetic mice responses were identical to, or even greater than, the responses observed in control mice.…”

Effect of diabetes on the sugar nucleotides in several tissues of the rat

“…In contrast to skeletal muscle, the levels in heart were unchanged, indicating a dissimilarity between these two types of muscle. Contrasting responses of heart and skeletal muscle in regard to the levels of glucose 6-phosphate, citrate and glycogen have also been demonstrated under conditions of starvation [21].…”