Training-induced alterations in lactate dehydrogenase reaction kinetics in rats: a re-examination

The kinetics and the isozyme composition of lactate dehydrogenase (LDH) were measured in rat plantaris muscle during a 26 week endurance-training program. Alterations in the LDH isozyme pattern were detectable after 6 weeks as the percentage of the M4 isozyme was reduced from 89 to 76 % and the total percentage of M subunits compared with H subunits declined by 8 %. At 16 weeks, M4 accounted for only 62 % of the total. The replacement of M with H subunits continued when training was prolonged as M4 represented only 52 % of the total isozymes at 26 weeks. Conversely, training for 6 and 16 weeks produced no changes in either Vmax or Km. At 26 weeks, these values declined for both the forward (pyruvate to lactate) and backward reactions. The rate constants for both reactions were also reduced. These data suggest that changes in LDH isozyme pattern do not contribute significantly to the enhancement of lactate oxidation that may occur after training. They also suggest that the functional significance of alterations in LDH structure and/or function are best determined from analysis of the overall reaction kinetics as opposed to individual characteristics such as the isozyme pattern, Km or Vmax.

Glucose 6-phosphate alters rat skeletal muscle contractile apparatus and sarcoplasmic reticulum function

We investigated the effects of glucose 6-phosphate (G6P) on skeletal muscle contractile apparatus and sarcoplasmic reticulum (SR) function. Using rat extensor digitorum longus fibres, the presence of 5 mM G6P decreased the Ca2+ sensitivity of both force production and actomyosin ATPase (AM-ATPase) activity. Conversely, maximal Ca(2+)-activated force was unaffected while maximal AM-ATPase activity was increased by 37%. In SR vesicles isolated from rat gastrocnemius, G6P markedly altered Ca2+ handling. It increased Ca(2+)-stimulated Ca(2+)-ATPase activity but depressed the net rate of Ca2+ uptake. This latter effect appears to be due to G6P-stimulated Ca2+ release. When G6P was added to Ca(2+)-loaded vesicles, a small, transient release of Ca2+ was elicited. In addition, G6P lowered the threshold for Ca(2+)-induced Ca2+ release but depressed the net rates of both AgNO3- and caffeine-induced releases. It is possible that the accumulation of G6P during muscular activity may adversely affect muscle force production and contribute to the fatigue process via its action on the contractile apparatus and SR.

Calcium exchange hypothesis of skeletal muscle fatigue: a brief review

Skeletal muscle fatigue is often associated with diminished athletic performance and work productivity as well as increased susceptibility to injury. The exact cause of muscle fatigue probably involves a number of factors which influence force production in a manner dependent on muscle fiber type and activation pattern. However, a growing body of evidence implicates alterations in intracellular Ca2+ exchange as a major role in the fatigue process. These changes are thought to occur secondary to reductions in the rates of Ca2+ uptake and release by the sarcoplasmic reticulum (SR). This hypothesis is based on the finding that peak myoplasmic Ca2+ concentration ([Ca2+]i) is reduced as force declines during fatigue. In addition, direct measurements of Ca2+ uptake and release show that fatiguing activity causes intrinsic alterations in the functional properties of the SR. We also propose that fatigue-induced alterations in Ca2+ exchange may be beneficial, reducing the rate of energy utilization by the muscle fiber and preventing irreversible damage to the cell.

An instrument for the measurement of rapid reaction kinetics

Two versions of an inexpensive mixing device capable of measuring reaction kinetics with millisecond resolution are described. The instruments deliver reactants via microsyringes controlled by nitrogen pressure at time intervals that are either mechanically or software controlled. In addition, an optical system for calibration of the time required to deliver the reactants as well as the intervals between syringe firing is given. Data illustrating the effect of variations in volume and syringe size on the accuracy and reproducibility of these time measurements as well as the volume of fluid delivered are presented. Examples of kinetic measurements of several reactions illustrating the capabilities of the instrument are also presented.

Increased incidence of a resonance in the phosphodiester region of 31P nuclear magnetic resonance spectra in the skeletal muscle of fibromyalgia patients

OBJECTIVE

To determine if patients with fibromyalgia syndrome (FMS) are more susceptible to activity-induced muscle damage than are healthy subjects.

METHODS

Eleven FMS patients and 10 healthy subjects performed concentric and eccentric exercise with their dominant and nondominant forearms, respectively. 31P magnetic resonance spectroscopy (to assess inorganic phosphate [P(i)] and phosphocreatine [PCr]) and dolorimetry (to assess pain) were performed before and 20 minutes after exercise and at 4 subsequent 24-hour intervals.

RESULTS

Neither group exhibited increased P(i)/PCr ratios or reduced dolorimetry scores following the exercise protocols. FMS patients did display a phosphodiester resonance at a higher rate than healthy subjects (37% versus 12%), but this was not related to the exercise.

CONCLUSION

Unchanged P(i)/PCr ratios and dolorimetry scores following acute exercise provide evidence against the hypothesis that FMS patients are more susceptible to activity-induced muscle damage than are healthy subjects, although P(i)/Pcr and pain may not adequately document such damage. The frequent occurrence of phosphodiester in the spectra of FMS patients may indicate a sarcolemmal abnormality in these subjects.

Fatigue-induced alterations in Ca2+ and caffeine sensitivities of skinned muscle fibers

The purpose of this investigation was to examine the Ca2+ and caffeine sensitivities of skinned skeletal muscle fibers after fatigue. Single frog semitendinosus fibers were chemically skinned in either a rested state or after tetanic contractions (80 Hz, 100 ms) evoked at 2 s-1 for 5 min. This protocol reduced tetanic force to 1.8 +/- 0.2% of control. Maximal Ca(2+)-activated force (F0, 20 degrees C) was not significantly different between rested and fatigued fibers. However, the concentration of Ca2+ required to evoke 50% of F0 was significantly lower in the fatigued fibers (1.80 +/- 0.18 vs. 1.33 +/- 0.16 microM; P < 0.05), an effect that persisted as the skinned fiber was allowed to incubate in the relaxing solution for > 90 min. The addition of caffeine (25 mM) after Ca2+ loading of the sarcoplasmic reticulum (SR) for periods of 5-30 s (0.25 microM free [Ca2+]) evoked smaller contractures in fatigued fibers than in rested fibers. However, when the loading period was prolonged (60-240 s), force developed after caffeine application was not significantly different between conditions. This suggests that the rate, but not the maximal capacity of Ca2+ loading by the SR, is reduced by fatigue. After Ca2+ loading (120 s), the minimal caffeine concentration required to evoke a contracture in fatigued fibers (5.7 +/- 0.3 mM) was significantly greater than that of control fibers (3.1 +/- 0.4 mM), an effect that persisted with prolonged incubation of the skinned fibers. In addition, the rate of force increase in response to 8 mM caffeine was reduced in fatigued fibers by 41%.(ABSTRACT TRUNCATED AT 250 WORDS)

Prolonged exercise reduces Ca2+ release in rat skeletal muscle sarcoplasmic reticulum

Prolonged exercise decreased the rate of Ca+ release in sarcoplasmic reticulum (SR) vesicles isolated from rat muscle by 20-30% when release was initiated by 5, 10, and 20 microM AgNO3 [3H]Ryanodine binding was also depressed by 20% in SR vesicles isolated from the exercised animals. In contrast, the maximum amount of Ca2+ release in the presence of ruthenium red, a known inhibitor of the Ca2+ release mechanism, was not affected by prolonged exercise. These results suggest that exercise depressed Ca2+ release from SR by directly modifying the Ca2+ release channel.

Chronic low frequency stimulation reduces myosin phosphorylation in rabbit fast twitch muscle

The effect of 1-12 days of electrical stimulation (10 Hz) on the ability to phosphorylate the P-light chain of myosin was studied in rabbit tibialis anterior muscle. Myosin phosphorylation was induced by exposure of the stimulated muscle and that of the contralateral leg to a single conditioning stimulus train (5 Hz) for 25 s via the motor nerve. Isometric tension was measured as were the myosin light chain composition and the activities of the enzymes responsible for phosphorylation and dephosphorylation. A computer simulation of the potential effect of a stimulation-induced disruption of Ca2+ metabolism on phosphorylation was also performed. Chronic stimulation for as little as 1 day eliminated light chain phosphorylation and reduced the myosin light chain kinase activity by approximately 36%. Conversely, phosphatase activity and light chain composition were unaffected. The model demonstrated that a slight depression in the magnitude of the Ca2+ transient could potentially attenuate phosphorylation. The data suggest that phosphorylation of myosin is extremely sensitive to prolonged muscle activity. Furthermore, it appears more likely that this sensitivity is related to regulation of intracellular free Ca2+ than to the other elements of the calmodulin-dependent system for myosin phosphorylation examined.

Prolonged exercise induces structural changes in SR Ca(2+)-ATPase of rat muscle

Sarcoplasmic reticulum (SR) isolated from the deep red portion of the gastrocnemius muscle of Sprague-Dawley rats after a single bout of prolonged exercise was shown to have depressed Ca(2+)-stimulated Mg(2+)-dependent ATPase activity over a temperature range of 15 to 42.5 degrees C when compared to SR obtained from control muscle. Inclusion of the calcium ionophore, A23187, failed to restore the depressed ATPase activity from SR of exercised muscle to control values, but it did normalize the stimulatory effect of temperature on ATPase activity. This depression was also manifested as an increased activation energy when the data were converted to an Arrhenius plot. SR vesicles from both groups showed no differences or discontinuities in plots of steady-state fluorescence anisotropy. When the binding characteristics of the fluorescent probe, fluorescein isothiocyanate (FITC), were analyzed, SR vesicles prepared from exercised muscle displayed a 40% reduction in binding capacity with no apparent change in Kd. These findings support the conclusion that a single bout of exercise induces a structural change in the Ca(2+)-ATPase protein of rat red gastrocnemius muscle that is not a direct result of gross lipid alterations or increased muscle temperature.

Muscle fatigue: conduction or mechanical failure?

It is well documented that repeated voluntary activity or electrical stimulation of skeletal muscle results in a decline in force production or power output. However, the precise physiological causes of "muscle fatigue" are not yet well understood. It is conceivable that the mechanism(s) may lie either in the conduction of action potentials in the central and peripheral nervous systems or in the transformation of the electrical event into mechanical force production by the muscle itself. In fact, none of the components of the electrical pathway from generation of impulses in the brain to their conduction over the neuron and the excitable membranes of the muscle can as yet be ruled out as potential contributors to the fatigue process. Relative to that on conduction failure, more information exists concerning the possibility that a defect in the excitation contraction coupling process in skeletal muscle, e.g., intracellular acidosis, inadequate supply of energy for contraction, or a disruption in Ca2+ homeostasis may also be significant in compromising force production following sustained activity. Despite this, the amount of conflicting data derived from these experiments has hindered the resolution of this question. In the future more attention must be given to such issues as the type of activity used to elicit fatigue and the fiber composition of the muscles studied. This is imperative as these factors clearly impact the nature of correlations between the biochemical and physiological events in muscle that are required to support prospective fatigue mechanisms.

The relationship between plasma free fatty acids and liver mitochondrial function in vivo

P/O ratio, state 3 and 4 respiration rates, and acceptor control index (ACI) were assessed in rat liver mitochondria following an overnight fast and single bout of treadmill exercise of 30-180 min. P/O was unaffected by fasting and 30 min of exercise; however, ACI was reduced because of an increase in state 4 respiration. Fasting, followed by running for 1 h or more decreased P/O approx. 40% and ACI by 50%, an effect that could be attributed to a reduction in state 3 respiration. The decrease in P/O was reversed 15 min after the cessation of exercise, whereas ACI remained depressed. All these functional alterations were mimicked by incubation of isolated mitochondria with palmitate and reversed by washing them with albumin. No direct correlation between plasma free fatty acids and the alterations in mitochondrial respiration was apparent. These data demonstrate that the decrease in the normal coupling of oxidation and phosphorylation in liver mitochondria produced by fasting/exercise is reversed rapidly in vivo. Furthermore, it is apparent that, if fatty acids act as a regulatory agent under these conditions, they do not do so solely on the basis of their plasma concentration.

Factors influencing the development and maintenance of aerobic fitness: lessons applicable to the fibrositis syndrome

Exercise may have therapeutic benefits in the treatment of fibrositis/fibromyalgia. However, little is known concerning what type of exercise best serves patients with this condition, their capacity for exercise or the identity of the physiological or psychological alterations elicited by increased activity that might be responsible for the positive effects. We discuss these issues from the perspective of the classical responses reported in healthy subjects. In addition, since the benefits of exercise programs are dependent upon continual participation. strategies for initiating activity and maximizing compliance are discussed.

Effects of exercise of varying duration on sarcoplasmic reticulum function

Sarcoplasmic reticulum (SR) Ca2+ uptake and Ca2+-Mg2+-ATPase activity were examined in muscle homogenates and the purified SR fraction of the superficial and deep fibers of the gastrocnemius and vastus muscles of the rat after treadmill runs of 20 or 45 min or to exhaustion (avg time to exhaustion 140 min). Vesicle intactness and cross-contamination of isolated SR were estimated using a calcium ionophore and mitochondrial and sarcolemmal marker enzymes, respectively. Present findings confirm previously reported fiber-type specific depression in the initial rate and maximum capacity of Ca2+ uptake and altered ATPase activity after exercise. Depression of the Ca2+-stimulated ATPase activity of the enzyme was evident after greater than or equal to 20 min of exercise in SR isolated from the deep fibers of these muscles. The lowered ATPase activity was followed by a depression in the initial rate of Ca2+ uptake in both muscle homogenates and isolated SR fractions after greater than or equal to 45 min of exercise. Maximum Ca2+ uptake capacity was lower in isolated SR only after exhaustive exercise. Ca2+ uptake and Ca2+-sensitive ATPase activity were not affected at any duration of exercise in SR isolated from superficial fibers of these muscles; however, the Mg2+-dependent ATPase activity was increased after 45 min and exhaustive exercise bouts. The alterations in SR function could not be attributed to disrupted vesicles or differential contamination in the SR from exercise groups and were reinforced by similar changes in Ca2+ uptake in crude muscle homogenates.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between parvalbumin content and the speed of relaxation in chronically stimulated rabbit fast-twitch muscle

The time courses of changes in parvalbumin (PA) content, isometric twitch tension, and half-relaxation time (1/2 RT) were studied in rabbit tibialis anterior muscle following chronic 10 Hz nerve stimulation of 1-21 days. Up to 5 days stimulation had no effect on PA content, but it induced a slight (10-15%) increase in the 1/2 RT. This change occurred together with the previously observed 50% decrease in Ca2+-uptake by the SR (Leberer et al. 1987). While prolonged stimulation produced no further decrease in the Ca2+-uptake by the SR, PA content declined after 5 days of stimulation. The reduction in PA content was accompanied by a progressive lengthening of the 1/2 RT. However, the increase in 1/2 RT was particularly pronounced after PA had fallen below 50% of its normal value. A 90% reduction in PA coincided with a 60% increase in the 1/2 RT. By this time the staircase phenomenon, normally observed in fast-twitch muscle, was completely abolished. Although the changes in PA content and 1/2 RT were not linearly related, these results suggest that PA plays an important role in the relaxation process of mammalian fast-twitch muscle.

Decrease in myosin light chain kinase activity of rabbit fast muscle by chronic stimulation

Analysis of myosin light chain kinase (MLCK) activity in tibialis anterior muscles of the rabbit revealed that chronic stimulation at a frequency of 10 Hz for 24 h per day reduced the enzyme activity in a time-dependent manner. Since fast twitch muscle contains significantly more myosin light chain kinase than slow twitch muscle, the observed reductions are consistent with the type of fast-to-slow transformation observed for other type-specific muscle characteristics. The present data also indicate that the stimulation-induced decrease in MLCK activity precedes the fast-to-slow conversion of the myosin molecule as judged by pyrophosphate-polyacrylamide gel electrophoresis.

Cardiac contractility, cAMP concentration, cAMP-dependent protein kinase, and phosphorylase activation during acute pressure overload

The relationship between increases in myocardial contractility and cAMP and protein kinase activity were studied for hearts of normal rats and those with altered sympathectic capacity produced by the combined treatments of adrenalectomy, and 6-hydroxydopamine and propranolol injections. Increases in myocardial contractility, evaluated from intra-ventricular pressure changes, were produced by occlusion of the ascending aorta for 15, 20, or 25 s. Resting peak left ventricular pressure and the rate of rise of left ventricular pressure were lower (P less than 0.05) in sympathectomized animals, however, aortic occlusion abolished these differences. Time to peak tension and the relationship between end-diastolic pressure and developed pressure were unchanged by sympathectomy. ATP and CP concentrations in freeze clamped samples of the myocardium were lower (P less than 0.05) in both groups after aortic occlusion whereas lactate was elevated (P less than 0.05). Sympathectomy delayed and reduced the magnitude of the increase in the phosphorylase a/a + b ratio produced by aortic occlusion. Myocardial cAMP concentration was increased in the normal rats but decreased in sympathectomized animals after aortic occlusion. cAMP-dependent protein kinase activity followed the pattern of cAMP. The results demonstrate that heart possesses the capacity to increase its contractility to an acute, short-term overload even when devoid of sympathetic control.

Exercise-induced fibre type transitions with regard to myosin, parvalbumin, and sarcoplasmic reticulum in muscles of the rat

Effects of a long-term, high intensity training program upon histochemically assessed myofibrillar actomyosin ATPase, myosin composition, peptide pattern of sarcoplasmic reticulum (SR), and parvalbumin content were analysed in muscles from the same rats which were used in a previous study (Green et al. 1983). Following 15 weeks of extreme training, an increase in type I and type II A fibres and a decrease in type II B fibres occurred both in plantaris and extensor digitorum longus (EDL) muscles. In the deep portion of vastus lateralis (VLD), there was a pronounced increase from 10 +/- 5% to 27 +/- 11% in type I fibres. No type I fibres were detected in the superficial portion of vastus lateralis (VLS) both in control and trained animals. An increase in slow type myosin light chains accompanied the histochemically observed fibre type transition in VLD. Changes in the peptide pattern of SR occurred both in VLS and VLD and suggested a complete transition from type II B to II A in VLS and from type II A to I in VLD. A complete type II A to I transition in the VLD was also suggested by the failure to detect parvalbumin in this muscle after 15 weeks of training. Changes in parvalbumin content and SR tended to precede the transitions in the myosin light chains. Obviously, high intensity endurance training is capable of transforming specific characteristics of muscle fibres beyond the commonly observed changes in the enzyme activity pattern of energy metabolism. The time courses of the various changes which are similar to those in chronic nerve stimulation experiments, indicate that various functional systems of the muscle fibre do not change simultaneously.

Alterations in liver mitochondrial function as a result of fasting and exhaustive exercise

The effect of exercise upon liver mitochondria structure and function was examined in fasted and fed rats, following a single run to exhaustion on a motor-driven treadmill. Exercise alone and exercise coupled with fasting both produced a significant decrease in the amount of hexokinase bound to the mitochondria, as well as reduction in the ADP/O ratio and acceptor control index measured in the presence of succinate. The mitochondria of the exercised animals, when exposed to freeze-fracture analysis while in state 3, displayed fewer deflections in the fracture plane between the inner and outer membrane than those isolated from control animals. This suggests that fewer contacts existed between the two membranes. Measurements based upon the binding of 8-anilinonaphthalene 1-sulphonate indicated that there was an increase in the net negative charge on the surface of the mitochondrial membranes of the exercised animals. All of these effects could be mimicked by incubation of mitochondria from control animals with free fatty acids. This fact, coupled with the observation that washing of the mitochondria with a solution comprising 5% (w/v) albumin could reverse all of the consequences of exercise, suggests that these alterations in mitochondrial structure and function may be the result of the increase in plasma free fatty acids that accompanies long-term exercise. Furthermore, the observation that the exercise-induced changes are dynamic and readily reversible indicates that the mitochondria were not necessarily damaged, but rather that the coupling of oxidative phosphorylation may be subject to physiological regulation.

Phosphorylation of myosin light chain in skeletal and smooth muscles

Significant differences in the properties of myosin light chain phosphorylation in skeletal and smooth muscles may be important in considering the role of myosin phosphorylation in contraction. Repetitive, low-frequency stimulation of fast-twitch skeletal muscle resulted in phosphorylation of myosin light chain. Some of the factors leading to phosphorylation under these conditions include 1) partial activation of myosin light chain kinase with each stimulus, 2) a slow rate (t1/2 = 1 s) of inactivation of the kinase activity, and 3) a very slow rate of dephosphorylation by myosin light chain phosphatase. Myosin light chain phosphorylation was correlated with potentiation of isometric twitch tension in posttetanic potentiation and staircase responses. Stimulation of contraction in bovine tracheal smooth muscle by the cholinergic agonist carbachol was correlated with phosphorylation of the myosin light chain. The initial rate and maximum extent of phosphorylation during the first minute of stimulation was dependent on the concentration of carbachol. Phosphate incorporation into light chain declined after 1 min whereas isometric tension was maintained. beta-Adrenergic inhibition of tension development was accompanied by a decrease in the rate and extent of phosphorylatable myosin light chain phosphorylation but was not associated with reduced affinity of myosin light chain kinase for calcium-calmodulin.

The effect of low frequency stimulation on myosin light chain phosphorylation in skeletal muscle

Phosphorylation of the P-light chain of myosin from skeletal muscle by myosin light chain kinase is dependent upon calmodulin and Ca2+. Investigations were performed to determine if the rapid Ca2+ transients that occur during low frequency repetitive stimulation are sufficient to activate myosin light chain kinase with significant P-light chain phosphorylation. In addition, P-light chain phosphorylation was correlated with potentiation of isomeric twitch tension (staircase phenomenon). Stimulation of rat gastrocnemius muscle at 5 Hz in situ results in a time-dependent phosphorylation of the P-light chain of myosin. Initially, there was a rapid rate of phosphorylation within the first 10 muscle twitches (0.19 to 0.40 mol of phosphate/mol of P-light chain) followed by a slower rate of phosphorylation. These data indicate that myosin light chain kinase can be activated during repetitive stimulation at a low frequency in the range that occurs in vivo, despite the fact that the muscle is in the relaxed state during most of the period between each stimulation. Potentiation of isometric twitch tension was found to be temporally correlated to light chain phosphorylation at 5 Hz. It is postulated that the transient changes in intracellular Ca2+ concentration associated with low frequency stimulation are sufficient to activate myosin light chain kinase, and, furthermore, the magnitude of the potentiation of isometric twitch tension may be related to the extent of phosphorylation of myosin during a stimulus train.

Exercise-induced glycogenolysis in sympathectomized rats

The role of the adrenergic system in regulating glycogenolysis during exercise was studied in rats. Alterations in the adrenergic system were produced by injections of 6-hydroxydopamine (6-OHD), surgical removal of the adrenal medulla (ADMX), or the combination of ADMX and 6-OHD injection. Exercise was treadmill running at 22 m/min for 60 min. Colonic temperature averaged 2.8 degrees C higher in the exercised than control rats. Exercise reduced the glycogen of the liver and skeletal muscles of all groups. The glycogen concentrations of the soleus and red portion of the gastrocnemius muscles of the ADMX and ADMX-6-OHD groups were about 3.8 and 2.5 times higher after exercise than those of the normal-exercised rats. Glycogen depletion of the white portion of the gastrocnemius muscle was similar for all exercised groups. 6-OHD treatment depleted the catecholamines of the myocardium. These results demonstrate that glycogen depletion during exercise occurs in rats devoid of adrenergic control. However, differences between types of skeletal muscle suggest that factors other than the adrenergic system may be involved in controlling glycogen metabolism during exercise.

Lipolysis and cAMP accumulation in adipocytes in response to physical training

Adenylate cyclase activity is lower in membrane preparations of fat cell homogenates from exercise-trained compared with sedentary rats (J. Appl. Physiol.: Respirat. Environ, Exercise Physiol. 42: 884-888, 1977). In the present investigations lipolysis and cyclic adenosine 3':5'-monophosphate (cAMP) accumulation were measured in isolated parametrial fat cells prepared from sedentary and trained rats. The purpose of these investigations was to determine whether the normal catecholamine-induced increases in cAMP accumulation is affected in isolated adipocytes from endurance-trained rats. The increases in cAMP accumulation in response to isoproterenol (0.01-10 microM) was reduced in fat cells isolated from trained rats. However, glycerol release in response to the same hormonal challenge was greater in these adipocytes. cAMP phosphodiesterase activity measured at 0.125 and 1.025 microM cAMP was greater in the particulate fraction of fat cell homogenates obtained from trained rats as compared with their sedentary counterparts. Hormone-sensitive lipase activity was reduced in crude fat pad homogenate preparations from trained rats if the animals were killed at rest. However, if the animals were run to exhaustion immediately prior to being killed, there were no differences in the hormone-sensitive lipase activity between preparations from trained and nontrained rats. These data indicate that, although cAMP accumulation by isolated fat cells in response to isoproterenol is markedly lower in trained rats, lipolysis and hormone-sensitive lipase activation is not reduced.