Effect of insulin on intracellular pH in frog skeletal muscle fibers

Another Consequence of the Warburg Effect? Metabolic Regulation of Na+/H+ Exchangers May Link Aerobic Glycolysis to Cell Growth

To adjust cell growth and proliferation to changing environmental conditions or developmental requirements, cells have evolved a remarkable network of signaling cascades that integrates cues from cellular metabolism, growth factor availability and a large variety of stresses. In these networks, cellular information flow is mostly mediated by posttranslational modifications, most notably phosphorylation, or signaling molecules such as GTPases. Yet, a large body of evidence also implicates cytosolic pH (pHc) as a highly conserved cellular signal driving cell growth and proliferation, suggesting that pH-dependent protonation of specific proteins also regulates cellular signaling. In mammalian cells, pHc is regulated by growth factor derived signals and responds to metabolic cues in response to glucose stimulation. Importantly, high pHc has also been identified as a hall mark of cancer, but mechanisms of pH regulation in cancer are only poorly understood. Here, we discuss potential mechanisms of pH regulation with emphasis on metabolic signals regulating pHc by Na⁺/H⁺-exchangers. We hypothesize that elevated NHE activity and pHc in cancer are a direct consequence of the metabolic adaptations in tumor cells including enhanced aerobic glycolysis, generally referred to as the Warburg effect. This hypothesis not only provides an explanation for the growth advantage conferred by a switch to aerobic glycolysis beyond providing precursors for accumulation of biomass, but also suggests that treatments targeting pH regulation as a potential anti-cancer therapy may effectively target the result of altered tumor cell metabolism.

Acclimation temperature affects the metabolic response of amphibian skeletal muscle to insulin

Frog skeletal muscle mainly utilizes the substrates glucose and lactate for energy metabolism. The goal of this study was to determine the effect of insulin on the uptake and metabolic fate of lactate and glucose at rest in skeletal muscle of the American bullfrog, Lithobates catesbeiana, under varying temperature regimens. We hypothesize that lactate and glucose metabolic pathways will respond differently to the presence of insulin in cold versus warm acclimated frog tissues, suggesting an interaction between temperature and metabolism under varying environmental conditions. We employed radiolabeled tracer techniques to measure in vitro uptake, oxidation, and incorporation of glucose and lactate into glycogen by isolated muscles from bullfrogs acclimated to 5 °C (cold) or 25 °C (warm). Isolated bundles from Sartorius muscles were incubated at 5 °C, 15 °C, or 25 °C, and in the presence and absence of 0.05 IU/mL bovine insulin. Insulin treatment in the warm acclimated and incubated frogs resulted in an increase in glucose incorporation into glycogen, and an increase in intracellular [glucose] of 0.5 μmol/g (P<0.05). Under the same conditions lactate incorporation into glycogen was reduced (P<0.05) in insulin-treated muscle. When compared to the warm treatment group, cold acclimation and incubation resulted in increased rates of glucose oxidation and glycogen synthesis, and a reduction in free intracellular glucose levels (P<0.05). When muscles from either acclimation group were incubated at an intermediate temperature of 15 °C, insulin's effect on substrate metabolism was attenuated or even reversed. Therefore, a significant interaction between insulin and acclimation condition in controlling skeletal muscle metabolism appears to exist. Our findings further suggest that one of insulin's actions in frog muscle is to increase glucose incorporation into glycogen, and to reduce reliance on lactate as the primary metabolic fuel.

Role of the sodium-hydrogen exchanger in ischemia-reperfusion injury in diabetes

Ischemic heart disease is a significant problem in the diabetic population. Animal models of diabetes show a paradoxical resistance to ischemic challenge. The present treatise will discuss the mechanics involved and the central role that Na+-H+ exchanger plays in this response to ischemic-reperfusion injury.

Inhibition of endosomal acidification in normal cells mimics the derangements of cellular insulin and insulin-receptor metabolism observed in non-insulin-dependent diabetes mellitus

Dissociation of the insulin-insulin receptor complex plays a crucial role in the processing of both insulin and the insulin receptor, and the acidification of endocytic vesicles may be the mechanism by which internalized insulin is dissociated from its receptor and properly sorted and processed. Internalized insulin-insulin receptor complexes are abnormally processed in cells from patients with non-insulin-dependent diabetes mellitus (NIDDM). Accordingly, to further investigate the mechanisms of the derangements observed in NIDDM cells, we examined the effects of the ionophore monensin, which inhibits endosomal acidification, on the cellular processing of insulin and insulin receptor in monocytes from control subjects (n = 12) and NIDDM patients (n = 14). This study confirms that monocytes from NIDDM patients, compared with cells from normal controls, had reduced binding (P < .01), internalization (P < .01), and degradation (P < .01) of insulin. In addition, the release of intracellular radioactivity was slower (P < .01), and recycling of the insulin receptor was inhibited (P < .01). Moreover, these defects were associated with a significant (P < .01) decrease of dissociation of the internalized insulin-insulin receptor complex. In cells from normal controls, incubation with monensin decreased insulin binding (P < .01), but not insulin internalization. High-performance liquid chromatography (HPLC) analysis of intracellular radioactivity showed that after monensin intracellular intact insulin significantly increased (P < .01), thus suggesting a decrease of intracellular insulin degradation. Moreover, insulin receptor recycling was completely disrupted. All of these derangements were associated with a significant decrease (P < .01) of dissociation of insulin-insulin receptor complexes. On the contrary, in diabetic monocytes, monensin had no significant additional effect on NIDDM-linked alterations. Comparison of the results obtained in cells from NIDDM patients to those found in monensin-treated normal cells demonstrates that NIDDM and monensin gave rise to a superimposable impairment of dissociation of the intracellular insulin-insulin receptor complex, associated with similar abnormal sorting and processing of insulin and its receptor. The only defect present in NIDDM cells but not in monensin-treated cells is the decrease of insulin internalization, which thus seems independent of the action of monensin on the processing of internalized insulin-insulin receptor complex. These results suggest that the impairment of dissociation of the insulin-insulin receptor complex may play a crucial role in the subsequent altered processing of insulin and insulin receptor. Moreover, they raise the question as to a possible similar alteration of the same intracellular mechanism by NIDDM and monensin, and point out that the derangements found in cells from NIDDM patients could be localized within the endosomal apparatus and consist mainly of a defective acidification of its interior.

Insulin resistance in systemic hypertension: pharmacotherapeutic implications

Systemic hypertension, a vascular disease with multiple origins, now is being linked to subtle abnormalities in glucose metabolism, which include insulin resistance and hyperinsulinemia. These conditions often occur together in patients with obesity, noninsulin-dependent diabetes, or both. Hyperinsulinemia and insulin resistance may cause systemic hypertension through multiple mechanisms. Insulin has a salt-retaining effect on the kidney. Also, insulin can augment catecholamine release, increase vascular sensitivity to vasoconstrictor substances, and decrease vascular sensitivity to vasodilator substances. In addition, insulin can increase production of tissue growth factors and help retain sodium and calcium in cells. Insulin resistance in patients can be treated with regular aerobic exercise, weight reduction, and a high-fiber diet. Pharmacologic approaches include hypoglycemic drugs, weight-reducing agents, and certain antihypertensive drugs that may have a favorable impact on both blood pressure and insulin resistance.

Plasma triglycerides and 24 hour urinary sodium excretion in elderly hypertensives. A pathogenetic connection?

Hypertension among the elderly generally represents a salt- sensitive state. However this salt-sensitivity does not appear to result from age-related increase in either sodium or salt intake. Since 20 years new trends seem to relate the role of sodium in the genesis of hypertension to a primary abnormality of electrolyte transport of cell membrane. Lipid abnormalities have also been described in untreated patients with high blood pressure. Plasma triglycerides were considerably higher (p < 0.01) in the hypertensives than in the controls. 24 hour sodium excretion was significantly lower (p < 0.0001) in hypertensives than in the controls. We have found a strong correlation among reduced sodium excretion, higher triglycerides and elevated blood pressure in the elderly. The blood pressure correlated negatively with 24 hour sodium excretion (p < 0.0001 for systolic and p < 0.002 for diastolic) and positively with plasma triglycerides (p < 0.0001 for systolic and p < 0.001 for diastolic). The poor literature regarding an association of these two alterations in human hypertensives makes our results provocative. We speculated that these alterations may be a facet of the insulin resistance commonly detectable in human hypertensives. However, further investigations are required to answer to this intriguing hypothesis.

Forearm resistance vessel abnormalities and insulin resistance in obese adolescents

To determine if structural changes in forearm resistance vessels are associated with insulin resistance, we evaluated the relation between minimum forearm vascular resistance and insulin resistance in 95 obese adolescents before and after weight loss. Insulin resistance was assessed by fasting insulin levels and sum of insulin values after an oral glucose tolerance test in all 95 subjects and whole body glucose uptake during euglycemic hyperinsulinemia in 35 of 95 subjects. Structural changes in forearm vessels were assessed by measurement of minimum forearm vascular resistance during 10 minutes of ischemic exercise. As compared with our normal values, obese adolescents had a significantly (p less than 0.01) decreased maximal forearm blood flow (41.6 +/- 1.4 versus 67.1 +/- 2.4 ml/min/100 ml) and increased minimum forearm vascular resistance (2.9 +/- 0.4 versus 1.6 +/- 0.7 mm Hg/ml/min/100 ml). There was a significant relation (p less than 0.01) between minimum forearm vascular resistance and fasting insulin, sum of insulins, and whole body glucose uptake. After a 20-week weight-loss program, minimum forearm vascular resistance decreased (3.0 +/- 0.3 versus 2.0 +/- 0.2, p less than 0.01), maximal forearm blood flow increased (41 +/- 2.3 versus 57.4 +/- 3.9, p less than 0.01), and forearm volume remained unchanged. We also observed a significant (p less than 0.01) relation between the decrease in minimum forearm vascular resistance and the decrease in fasting insulin (r = 0.29), decrease in sum of insulins (r = 0.42), and increase in whole body glucose uptake (r = 0.63).(ABSTRACT TRUNCATED AT 250 WORDS)

Diabetes mellitus and hypertension

Diabetes mellitus and hypertension are common diseases that coexist at a greater frequency than chance alone would predict. Hypertension in the diabetic individual markedly increases the risk and accelerates the course of cardiac disease, peripheral vascular disease, stroke, retinopathy, and nephropathy. Our understanding of the factors that markedly increase the frequency of hypertension in the diabetic individual remains incomplete. Diabetic nephropathy is an important factor involved in the development of hypertension in diabetics, particularly type I patients. However, the etiology of hypertension in the majority of diabetic patients cannot be explained by underlying renal disease and remains "essential" in nature. The hallmark of hypertension in type I and type II diabetics appears to be increased peripheral vascular resistance. Increased exchangeable sodium may also play a role in the pathogenesis of blood pressure in diabetics. There is increasing evidence that insulin resistance/hyperinsulinemia may play a key role in the pathogenesis of hypertension in both subtle and overt abnormalities of carbohydrate metabolism. Population studies suggest that elevated insulin levels, which often occurs in type II diabetes mellitus, is an independent risk factor for cardiovascular disease. Other cardiovascular risk factors in diabetic individuals include abnormalities of lipid metabolism, platelet function, and clotting factors. The goal of antihypertensive therapy in the patient with coexistent diabetes is to reduce the inordinate cardiovascular risk as well as lowering blood pressure.

Effects of increasing extracellular pH on protein synthesis and protein degradation in the perfused working rat heart

Increasing the extracellular pH over the range pH 7.4-8.9 stimulated protein synthesis by about 60% in the rat heart preparation anterogradely perfused in vitro. Protein degradation was inhibited by this pH increase. The magnitudes of the effects at pH 8.9 on protein synthesis and degradation were similar to those of high concentrations of insulin. Cardiac outputs were increased, as were cardiac phosphocreatine contents, indicating that the alterations in extracellular pH did not adversely affect the physiological viability of the preparation. ATP contents were unaltered. The creatine kinase equilibrium was used to assess the magnitude of the change in intracellular pH induced by these treatments. The increase in intracellular pH was about 0.2 for a 1-unit increase in extracellular pH. Thus small changes in intracellular pH have dramatic effects on cardiac protein turnover.

31P NMR study of insulin effects on the isolated perfused rabbit urinary bladder

Insulin stimulates hexose transport, intermediary metabolism, and cell growth and development. These effects are well-documented in skeletal but not smooth muscle. 31P NMR spectroscopy was performed on rabbit urinary bladders (n = 4) to characterize insulin's actions on smooth muscle. The bladder and its vasculature were surgically isolated from the animal and perfused with a PSS/red blood cell perfusate. After a control steady state was achieved (approx 1-2 h), insulin (0.100 mU/ml) was added to the perfusate. Relative levels of intracellular phosphorylated compounds, pH, and free Mg2+ were measured and compared to control values. Also, extracellular pH and fractional volume were assessed using phenylphosphonate, a 31P NMR extracellular pH and volume indicator. Insulin induced significant increases in PCr (16 +/- 9%) at the expense of Pi, intracellular pH (delta pH 0.24 +/- 0.07), and fractional extracellular volume (49 +/- 1%). Intracellular free Mg2+ and extracellular pH did not change. These results indicate that in situ smooth muscle is sensitive to physiological levels of insulin. In fact, insulin improves the energy state of smooth muscle cells and the overall tissue perfusion.

Intracellular pH regulation in papillary muscle cells from streptozotocin diabetic rats: an ion-sensitive microelectrode study

Intracellular pH regulation was studied in papillary muscle from STZ-induced diabetic rat hearts. In control bicarbonate solution there was no difference between the steady-state pHi values recorded from diabetic or normal papillary muscle. The addition of insulin had no effect on the pHi of either group. The amplitude of NH4+-induced alkalinization and the time course of recovery from alkalinization were similar in both normal and diabetic muscles. In both preparations, the recovery from alkalinization was similarly delayed by the disulfonic stilbene DIDS. This suggests the participation of a Cl-/HCO3- exchange in the recovery from alkalosis in rat myocardial cells that is not changed by diabetes. On the other hand, the amplitude of the acidification induced by the withdrawal of NH4+ was markedly increased in diabetic papillary muscles as compared to normal muscles. Moreover, there was a marked slowing down of the recovery from acidosis in the diabetics. The amplitude of NH+4 withdrawal-induced acidification was increased equally by amiloride in both normal and diabetic muscles. These findings suggest that diabetes is associated with a change in the activity of the amiloride-sensitive Na+/H+ exchange.

The regulation of the intracellular pH in cells from vertebrates

Eukaryotic cells control their intracellular pH using ion-transporting systems that are situated in the plasma membrane. This paper describes the different mechanisms that are involved and how their activity is regulated.

Relationship between ionic surroundings and insulin actions on glucose transport and Na,K-pump in muscles

1. It is well known that insulin has various effects on glucose transport and the Na,K-pump in muscles. It is also known to have some effects on the membrane potential--in general, insulin induces a hyperpolarization of the membrane in muscles. Furthermore, it is suggested that the actions of insulin are modified by changes in ionic surroundings. 2. In this review article, the actions of ionic surroundings and insulin on glucose transport in muscles are discussed; in particular, the effects of changes in extracellular and/or intracellular concentrations of Na, K, Ca and H ions will be mentioned. 3. The actions of ionic surroundings and insulin on the Na,K-pump in muscles are discussed; in particular, the effects of changes in extracellular an/or intracellular concentrations of Na, K, Ca and H ions will be examined. 4. The relationship between the actions of ionic surroundings and insulin are discussed. 5. In particular, the effects of changes in ionic surroundings on the insulin-induced hyperpolarization of the membrane are discussed by relating it to the Na,K-pump function. The relationship between the insulin-induced change in membrane potential and glucose transport will be also mentioned.

Aldosterone activates Na+/H+ exchange and raises cytoplasmic pH in target cells of the amphibian kidney

The hypothesis was tested if the mineralocorticoid hormone aldosterone stimulates Na+/H+ exchange in "giant cells" fused from individual target cells of the distal nephron of the frog kidney. By means of microelectrodes, steady-state intracellular pH (pHi) and pHi recovery from an acid load were recorded continuously while the fused cells were exposed to aldosterone. Twenty minutes after addition of the hormone, pHi started to rise and reached a new steady state after about 60 min (delta pHi = 0.28 +/- 0.01). After hormone treatment, pHi recovered significantly faster in response to an intracellular acid load. The diuretic drug amiloride blocked pHi recovery. Experiments in intact tubules showed that aldosterone induces H+ and K+ secretion. Thus, intracellular alkalosis, mediated by Na+/H+ exchange, could serve as a signal that activates pH-sensitive K+ channels of the luminal cell membrane.

Intracellular pH in sheep Purkinje fibres and ferret papillary muscles during hypoxia and recovery

1. The changes of intracellular pH (pHi) of papillary muscles from ferret and Purkinje fibres from sheep heart during hypoxia and recovery from hypoxia were recorded with pH-sensitive micro-electrodes filled with neutral H+ carrier. 2. Hypoxia was produced by replacement of O2 with N2 in the superfusing solutions. When oxidative phosphorylation was prevented, developed tension fell within 20 min to about 16 and 21% of its control value for papillary muscle and Purkinje fibres respectively. On restoration of O2, recovery of developed tension in ferret papillary muscle is preceded by a transient additional decrease. 3. In ferret papillary muscle, the pHi first increased by a mean value of 0.11 pH units after 3 min hypoxia, then decreased by about 0.24 pH units after 20 min. In sheep Purkinje fibres, the initial alkalosis was small or absent, and after 5-9 min, the pHi started to fall reaching 0.17 pH units after 20 min of hypoxia. On return to oxygenated solution, a transient additional intracellular acidification occurred. This acidification reached its peak of 0.31 pH units in papillary muscle and of 0.13 pH units in Purkinje fibres. In both preparations hypoxia was accompanied by a depolarization of a few millivolts. 4. The presence of cyanide (1-2 mM) or fluorodinitrobenzene (20-40 microM) prevented the additional intracellular acidification occurring on return to oxygenated solution. Removal of cyanide itself produced a transient but smaller and slower acidification. 5. On both preparations, exposure to a Tyrode solution containing 10 mM-L-lactate produced a transient intracellular acidification. After recovery from this acidification the acidification produced by hypoxia was increased without affecting the extra acidification on reintroduction of O2. 6. After reduction of the rate of glycolysis by removal of glucose and application of 2-deoxyglucose, the transient intracellular acidification, occurring on return to oxygenated solution after hypoxia, was inhibited in both preparations. In ferret papillary muscle, insulin (100 mU/ml) potentiated the changes of pHi occurring during hypoxia. 7. Using Na+-sensitive glass micro-electrodes it was found that the intracellular Na+ activity rose slightly during the later stages of hypoxia and rose transiently on readdition of O2. These results are consistent with a Na+-H+ exchange being stimulated by acidosis. 8. The origins of the pH changes during and after hypoxia are discussed as are the differences between the responses of sheep Purkinje fibres and ferret papillary muscle.

Effect of calcium and other divalent cations on intracellular pH regulation of frog skeletal muscle

1. We examined, in frog semitendinosus muscle, the effect of calcium release, induced by depolarization or caffeine, on intracellular pH (pHi) recovery from an acid load applied at least 40 min later. We also studied the effect of external Ca and other divalent cations on recovery. We used pH-sensitive micro-electrodes; the external pH (pHo) was always 7.35. 2. In fibres depolarized by 50 mM-K, constant [K] X [Cl] in the presence of 1 mM-tetracaine (which blocks Ca release), the rate of pHi recovery from 5% CO2-induced acidification was 0.15 +/- 0.02 delta pHi h-1 (n = 7), whereas in depolarized fibres that had never been exposed to the drug, the rate of recovery was 0.27 +/- 0.01 delta pHi h-1 (n = 5). Yet, when Ca release was not blocked and the depolarized fibres were exposed to tetracaine shortly before CO2 exposure, a similar slow rate of 0.14 +/- 0.03 delta pHi h-1 (n = 7) was observed. When Ca release was blocked by tetracaine, but the drug washed out before recovery, the rate was again 0.27 +/- 0.02 delta pHi h-1 (n = 6). 3. In fibres first depolarized to about -23 mV in 50 mM-K, constant [K] X [Cl] (recovery of 0.23 +/- 0.03 delta pHi h-1, n = 6), and then repolarized to -79 mV in 2.5 mM-K, the slow rate of recovery was the same (0.03 +/- 0.02 delta pHi h-1) as that in fibres without a history of depolarization and thus of Ca release. 4. In fibres depolarized to -50 mV (15 mM-K, constant Cl) and then exposed to caffeine (4 mM) which releases Ca from intracellular stores, the recovery was the same (0.07 +/- 0.03 delta pHi h-1, n = 5) as in depolarized fibres not exposed to caffeine (0.09 +/- 0.01 delta pHi h-1, n = 5). 5. We conclude that in frog muscle transient Ca release induced by either depolarization or caffeine does not affect the rate of subsequent pHi recovery. Tetracaine reversibly inhibits pHi recovery, but this inhibition is not due to its blocking of Ca release. 6. Recovery from CO2-induced acidification of fibres depolarized to -21 mV in 50 mM-K, constant Cl was halved, from 0.31 +/- 0.04 delta pHi h-1 (n = 10) to 0.15 +/- 0.01 delta pHi h-1 (n = 13), when external Ca was raised from 4 to 10 mM.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties of the intracellular pH-regulating systems of frog skeletal muscle

1. The properties of the systems that regulate intracellular pH (pHi) in frog muscle (Rana pipiens) were studied in semitendinosus fibres using pH-sensitive micro-electrodes. All experiments were done at 22 degrees C and at external pH (pHo) 7.35. 2. Normally polarized fibres acidified to pHi approximately 6.8 by an NH4Cl pre-pulse (nominal absence of CO2) recovered at a rate of 0.26 +/- 0.04 delta pHi h-1 (n = 10). This corresponds to a net equivalent H ion efflux, JH, of 5.0 pmol cm-2 s-1. This rate was not affected by depolarizing the fibres to -20 mV in 50 mM-K, constant Cl (0.29 +/- 0.03 delta pHi h-1, JH = 4.9 pmol cm-2 s-1, n = 13). Amiloride (1 mM) reduced recovery by almost 90%, while 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid (SITS, 0.1 mM) reduced recovery by only 18%. Removal of external Na (substitution by N-methyl-D-glucammonium) abolished recovery. Thus, Na-H exchange is responsible for most of the recovery from acidification induced by an NH4Cl pre-pulse. 3. The rate of recovery after an NH4Cl pulse increased linearly as pHi was reduced from 7.25 to 6.55. The dependence of this recovery upon external Na (at pHi 6.90) can be described by Michaelis-Menten kinetics; the apparent Michaelis constant (Km) is 12 +/- 3 mM. 4. Recovery of normally polarized fibres from acidification induced by 5% CO2 is very slow (about 0.03 delta pHi h-1). This recovery could be converted into an acidification of 0.06-0.07 delta pHi h-1 either by removal of Na (as previously described) or by amiloride. We ascribe this acidification of the polarized fibres to HCO3- efflux. 5. In fibres depolarized in 50 mM-K, at constant external Cl concentration, recovery from CO2 acidification was brisk (0.28 +/- 0.01 delta pHi h-1, JH = 9.4 pmol cm-2 s-1, n = 66). It was reduced by about 50% with either SITS or amiloride, and abolished by removal of Na. In the absence of Cl (substituted by gluconate), recovery was also reduced by about 50% and was unaffected by SITS, but nearly abolished by amiloride. Thus, in depolarized fibres, in addition to Na-H exchange, there is an active, SITS-sensitive component of recovery that requires Na, Cl and HCO3.(ABSTRACT TRUNCATED AT 400 WORDS)

Application of high-field 1H-NMR spectroscopy for the study of perifused amphibian and excised mammalian muscles

Frog sartorius and gastrocnemius muscles were perifused at 20 degrees C, the intracellular pH (pHi) and the concentration of phosphocreatine were determined in the resting muscle by 1H-NMR spectroscopy at 470 MHz; values of pHi = 7.31 +/- 0.05 (n = 7) and concentration of phosphocreatine = 20.4 +/- 1.1 mumol/g wet wt. (n = 6) were found. The hydrolysis of phosphocreatine and the simultaneous increase in lactate upon perifusion with 10 mM caffeine (in Ringer's solution) was followed with a time resolution of 1 min. Lactate increased at a rate of 1.0 mumol/g per min, but no pHi change was recorded during the time monitored. The lower limit for the buffering capacity of the muscle cytosol was estimated to be 16.7 mumol/g muscle per pH unit from the uncertainty in pHi determination (+/- 0.03 pH units) and from the amount of lactate produced and phosphocreatine hydrolyzed. Changes in pHi, lactate concentration and fatty acyl chain intensity were monitored by 1H-NMR spectroscopy at 361 MHz in ischemic rat skeletal muscle, excised and stored at 20 degrees C. The resonances in the 1H-NMR spectrum of a human skeletal muscle perchloric acid extract are reported and tentatively assigned.