Pyruvate dehydrogenase activity and acetyl group accumulation during exercise after different diets

Pyruvate dehydrogenase activity (PDHa) and acetyl group accumulation were examined in human skeletal muscle at rest and during exercise after different diets. Five males cycled at 75% of maximal O2 uptake (VO2 max) to exhaustion after consuming a low-carbohydrate diet (LCD) for 3 days and again 1-2 wk later for the same duration after consuming a high-carbohydrate diet (HCD) for 3 days. Resting PDHa was lower after a LCD (0.20 +/- 0.04 vs. 0.69 +/- 0.05 mmol.min-1.kg wet wt-1; P < 0.05) and coincided with a greater intramuscular acetyl-CoA-to-CoASH ratio, acetyl-CoA content, and acetylcarnitine content. PDHa increased during exercise in both conditions but at a lower rate in the LCD condition compared with the HCD condition (1.46 +/- 0.25 vs. 2.65 +/- 0.23 mmol.min-1.kg wet wt-1 at 16 min and 1.88 +/- 0.20 vs. 3.11 +/- 0.14 at the end of exercise; P < 0.05). During exercise muscle acetyl-CoA and acetylcarnitine content and the acetyl-CoA-to-CoASH ratio decreased in the LCD condition but increased in the HCD condition. Under resting conditions PDHa was influenced by the availability of fat or carbohydrate fuels acting through changes in the acetyl-CoA-to-CoASH ratio. However, during exercise the activation of PDHa occurred independent of changes in the acetyl-CoA-to-CoASH ratio, suggesting that other factors are more important.

Energy metabolism and adenine nucleotide degradation in twitch-stimulated rat hindlimb during ischemia-reperfusion

The purpose of this study was to characterize twitch tension and energy metabolism in ischemic, stimulated rat hindlimb to determine its suitability as a rapid time course model of ischemia-reperfusion injury. After 15 min equilibration, rat hindlimbs were stimulated (1-Hz twitches, 0.2 ms pulse duration, 15 V) for 5 min (control, n = 8). This twitch protocol was maintained throughout the ischemic and reperfusion periods. The control period was followed by 5, 20, or 40 min of ischemia (ligation of femoral artery and vein) or 40 min of ischemia with 0, 5, or 20 min of reperfusion (removal of ligature). The soleus [89% slow oxidative (SO)] and the white gastrocnemius [WG; 91% fast glycolytic (FG)] were analyzed for phosphocreatine (PCr), adenine nucleotides, glycogen, and glycolytic intermediates. Ischemia was characterized by progressive decreases in twitch tension, high-energy phosphagens, total adenine nucleotides (TAN), and glycogen. Also, energy metabolism was altered at a greater rate in WG than in soleus. Reperfusion resulted in a recovery in PCr and lactate, with little change in ATP, TAN, or glycogen. The inability to resynthesize adenine nucleotides and glycogen during reperfusion is characteristic of damaged skeletal muscle. The extent of the metabolic alterations in SO and FG muscles during twitch stimulation was comparable with previously reported noncontracting ischemia protocols of 2-4 and 4-7 h in length, respectively. The present study demonstrates that twitch stimulation of ischemic skeletal muscle is a useful model for inducing rapid metabolic changes and an ischemic insult comparable to prolonged noncontracting ischemia-reperfusion models.

Caffeine attenuates the exercise-induced increase in plasma [K+] in humans

This study examined the dose-response effects of caffeine on plasma K+ balance during prolonged exercise. Two series of experiments were performed. In series A, 1 h after ingestion of 9 mg/kg dextrose (placebo) or 9 mg/kg caffeine, eight subjects cycled at 78% of peak O2 consumption until exhaustion; in series B, in four trials, 1 h after ingestion of 0, 3, 6, or 9 mg/kg caffeine, eight subjects ran on a treadmill at 85% of peak O2 consumption until exhaustion. Blood was sampled from an antecubital vein for analysis of hematocrit, plasma concentrations of epinephrine ([Epi]) and norepinephrine, and [K+]. The change in plasma volume was calculated from hematocrit. During exercise, there was a net addition of K+ to and a net loss of fluid from the plasma compartment. Caffeine had no effect on plasma volume and norepinephrine concentration during exercise. In series A and B 9 mg/kg caffeine and in series B 6 mg/kg caffeine resulted in a significant attenuation of the increase in plasma [K+] with exercise. In series A increases in plasma [Epi] were 1.4- to 2-fold greater during exercise with caffeine than with placebo. At exhaustion, plasma [Epi] was twofold higher with caffeine (10.1 +/- 2.3 nM) than with placebo (5.3 +/- 0.8 nM), whereas plasma [K+] was only 4.88 +/- 0.18 meq/l with caffeine compared with 5.37 +/- 0.14 meq/l with placebo. It is concluded that caffeine attenuates the increase in plasma [K+] during exercise by stimulation (via one of its metabolites or by increased [Epi]) of tissue Na-K pump activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Erythrocyte ion regulation across inactive muscle during leg exercise

Ion concentration changes in whole blood, plasma, and erythrocytes across inactive muscle were examined in eight healthy males performing four 30-s bouts of maximal isokinetic cycling with 4 min rest between each bout. Blood was sampled from the arm brachial artery and deep antecubital vein during the intermittent exercise period and for 90 min of recovery. Arterial and venous erythrocyte lactate concentration ([Lac-]) increased from 0.3 +/- 0.1 to 12.5 +/- 1.3 (p < 0.01) and 1.1 +/- 0.4 to 8.5 +/- 1.5 mmol/L (p < 0.01), respectively, returning to control values during recovery. Arterial and venous plasma [Lac-] increased from 1.5 +/- 0.2 to 27.7 +/- 1.8 and from 1.3 +/- 0.4 to 25.7 +/- 3.5 mmol/L, respectively, and was greater than erythrocyte [Lac-] throughout exercise and recovery. Arterial and venous [K+] increased in erythrocytes from 119.5 +/- 5.1 to 125.4 +/- 4.6 (p < 0.01) and from 113.6 +/- 1.7 to 120.6 +/- 7.1 mmol/L, respectively, decreasing to control during recovery. In arterial and venous plasma, [K+] increased from 4.3 +/- 0.1 to 6.1 +/- 0.2 (p < 0.01) and from 4.5 +/- 0.2 to 5.3 +/- 0.2 mmol/L (p < 0.01), respectively, decreasing to control during recovery. The efflux of Lac- out of erythrocytes against an electrochemical concentration gradient suggests the presence of an active transport system. Efflux of K+ from erythrocytes as blood passes across inactive muscle affords an important adaptation to the K+ release from muscle activated in heavy exercise.

Blood ion regulation during repeated maximal exercise and recovery in humans

We investigated the ionic changes in arterial (a) and femoral venous (fv) blood that accompany muscle fatigue with repeated maximal exercise. Measurements were made on separated plasma and hemolysed whole blood to quantify the relative contributions of plasma and erythrocytes to this acid-base challenge. Five healthy males performed four 30-s bouts of maximal isokinetic cycling exercise, with 4 min of rest between bouts, and recovery was followed for 90 min. In whole blood, maximal increases in [K+]a amounted to 10 +/- 2.0 meq/l and in [K+]fv to 7 +/- 4.3 meq/l and occurred at the end of bout 2. Whole blood lactate concentration ([Lac-]) peaked at 15.3 +/- 1.39 ([Lac-]a) and 16.7 +/- 1.59 meq/l ([Lac-]fv) at the end of bout 4. In plasma, peak [Lac-]a and [Lac-]fv were both 21 meq/l at the end of bout 4. Plasma [H+]a increased from 36 +/- 1.0 neq/l at rest to 44 +/- 2.9 neq/l at the end of the first bout of exercise; 80% of this increase was due to a 2.9 meq/l decrease in arterial strong ion difference ([SID]), and 20% was due to an increase in plasma protein ([Atot]a); a reduction in arterial PCO2 to 29 mmHg had an alkalinizing effect. In contrast, plasma [H+]fv increased from 39 +/- 0.5 neq/l at rest to 93 +/- 4.1 neq/l, with an increase in PfvCO2 to 97 +/- 7 mmHg contributing 75%, a decrease in [SID]fv 15%, and an increase in [Atot]fv 10% to the increase in [H+]fv. In later exercise bouts, the relative contributions of [SID]a, [Atot]a, and arterial PCO2 to plasma [H+]a were similar, but the contribution of [SID]fv to [H+]fv increased and that of femoral venous PCO2 decreased, with the contribution of [Atot]fv remaining unchanged (8-12%). During exercise and recovery, the changes in both arterial and femoral venous PCO2 and [K+] were more rapid than changes in [Lac-], and the time course of whole blood [K+] was slower than that of plasma [K+]. Erythrocytes may play an important role in regulating plasma [Lac-] and [K+] with intense exercise.

Contribution of erythrocytes to the control of the electrolyte changes of exercise

Five healthy males performed four 30-s bouts of maximal isokinetic cycling with 4 min rest between each bout. Arterial and femoral venous blood was sampled during and for 90 min following exercise. During exercise, arterial erythrocyte [K+] increased from 117.0 +/- 6.6 mequiv./L at rest to 124.2 +/- 5.9 mequiv./L after the second exercise bout. Arterial erythrocyte [K+] returned to the resting values during the first 5 min of recovery. No significant change was observed in femoral venous erythrocyte [K+]. Arterial erythrocyte lactate concentration ([Lac-]) increased during exercise from 0.2 +/- 0.1 mequiv./L peaking at 9.5 +/- 1.5 mequiv./L at 5 min of recovery, after which the values returned to control. Femoral venous erythrocyte [Lac-] changed in a similar fashion. Arterial erythrocyte [Cl-] rose during exercise to 76 +/- 3 mequiv./L and returned to resting values (70 +/- 2 mequiv./L) by 25 min recovery. During exercise there was a net flux of Cl- into the erythrocyte. We conclude that erythrocytes are a sink for K+ ions leaving working muscles. Furthermore, erythrocytes function to transport Lac- from working muscle and reduce plasma acidosis by uptake of Cl-. The erythrocyte uptake of K+, Lac-, and Cl- helps to maintain a concentration difference between plasma and muscle, facilitating diffusion of Lac- and K+ from the interstitial space into femoral venous plasma.

Lactate metabolism in inactive skeletal muscle during lactacidosis

Contributions of carbohydrate and fat metabolism to the removal of a lactate (Lac-) load were quantified in inactive soleus (SL), plantaris (PL), and white gastrocnemius (WG) rat hindlimb muscle. Male Sprague-Dawley rats were perfused for 60 min with normal perfusate (NP, n = 8) or a high-lactate perfusate (LP, n = 8), simulating ionic conditions found in arterial blood and plasma after intense exercise: Lac- = 11.0 mM, K+ = 7.88 mM, and pH = 7.15. Metabolite fluxes across the hindlimb were calculated from blood flow and arteriovenous differences. In NP, Lac- was continuously released (2.9 +/- 0.2 mumol.min-1 x 100 g-1). However, in LP, a rapid and significant uptake of Lac- increased muscle Lac- fivefold to 39.6 +/- 1.1, 33.1 +/- 2.2, and 28.8 +/- 1.7 mumol/g dry wt in SL, PL, and WG, respectively. Glucose and O2 uptakes were similar during LP and NP perfusion. Glycerol release increased eightfold to 3.3 +/- 0.7 mumol.min-1 x 100 g-1 in response to LP. Muscle ATP, creatine phosphate, glycogen, glycolytic intermediate, and triacylglycerol concentrations did not change. However, muscle lactate-to-pyruvate ratios were elevated in all muscles of the LP group postperfusion, indicating changes in the mass action ratio at the pyruvate dehydrogenase reaction. In LP, of 80 mumol of Lac- taken up, 11% was accounted for by increased muscle Lac-, 12-24% was oxidized, and 5% may have been involved in glycerol release. The remaining Lac- may have been involved in metabolic cycling along the glyconeogenic-glycolytic pathway and/or in triacylglycerol-free fatty acid substrate cycling.

Potassium regulation during exercise and recovery

The concentrations of extracellular and intracellular potassium (K+) in skeletal muscle influence muscle cell function and are also important determinants of cardiovascular and respiratory function. Several studies over the years have shown that exercise results in a release of K+ ions from contracting muscles which produces a decrease in intracellular K+ concentrations and an increase in plasma K+ concentrations. Following exercise there is a recovery of intracellular K+ concentrations in previously contracting muscle and plasma K+ concentrations rapidly return to resting values. The cardiovascular and respiratory responses to K+ released by contracting muscle produce some changes which aid exercise performance. Increases in the interstitial K+ concentrations of contracting muscles stimulate CIII and CIV afferents to directly stimulate heart rate and the rate of ventilation. Localised K+ release causes a vasodilatation of the vascular bed within contracting muscle. This, together with the increase in cardiac output (through increased heart rate), results in an increase in blood flow to isometrically contracted muscle upon cessation of contraction and to dynamically contracting muscle. This exercise hyperaemia aids in the delivery of metabolic substrates to, and in the removal of metabolic endproducts from, contracting and recovering muscle tissues. In contrast to the beneficial respiratory and cardiovascular effects of elevations in interstitial and plasma K+ concentrations, the responses of contracting muscle to decreases in intracellular K+ concentrations and increases in intracellular Na+ concentrations and extracellular K+ concentrations contribute to a reduction in the strength of muscular contraction. Muscle K+ loss has thus been cited as a major factor associated with or contributing to muscle fatigue. The sarcolemma, because of changes in intracellular and extracellular K+ concentrations and Na+ concentrations on the membrane potential and cell excitability, contributes to a fatigue 'safety mechanism'. The purpose of this safety mechanism would be to prevent the muscle cell from the self-destruction which is evident upon overload (metabolic insufficiency) of the tissues. The net loss of K+ and associated net gain of Na+ by contracting muscles may contribute to the pain and degenerative changes seen with prolonged exercise. During exercise, mechanisms are brought into play which serve to regulate cellular and whole body K+ homeostasis. Increased rates of uptake of K+ by contracting muscles and inactive tissues through activation of the Na(+)-K+ pump serve to restore active muscle intracellular K+ concentrations towards precontraction levels and to prevent plasma K+ concentrations from rising to toxic levels. These effects are at least partially mediated by exercise-induced increases in plasma catecholamines, particularly adrenaline.(ABSTRACT TRUNCATED AT 400 WORDS)

The roles of ion fluxes in skeletal muscle fatigue

Intense muscle contractions result in large changes in the intracellular concentrations of electrolytes. The purpose of this study was to examine the contributions of changes in intracellular strong ions to calculated changes in steady-state membrane potential (Em) and muscle intracellular H+ concentration ([H+]i). A physicochemical model is used to examine the origin of the changes in [H+]i during intense muscle contraction. The study used the isolated perfused rat hindlimb intermittently stimulated to contract at high intensity for 5 min. This resulted in significant K+ depletion of both slow (soleus) and fast (white gastrocnemius, WG) muscle fibers and a release of K+ and lactate (Lac-) into venous perfusate. The major contributor to a 12- to 14-mV depolarization of Em in soleus and WG was the decrease in intracellular K+ concentration ([K+]i). The major independent contributors to [H+]i are changes in the concentrations of strong and weak ions and in CO2. Significant decreases in the strong ion difference [( SID]i) in both soleus and WG contributed substantially to the increase in [H+]i during stimulation. In WG the model showed that the decrease in [SID]i accounted for 35% of the increase in [H+]i (133-312 nequiv/L; pHi = 6.88-6.51) at the end of stimulation. Of the main contributors to decreased [SID]i, increased [Lac-]i and decreased [K+]i contributed 40 and 60%, respectively, to increased [H+]i, whereas a decrease in [PCr2-]i contributed to reduced [H+]i. It is concluded that decreased muscle [K+]i during intense contractions is the single most important contributor to reduced Em and increased [H+]i. Depletion of PCr2- simultaneous to the changes in [Lac-]i and [K+]i prevents larger increases in [H+]i and helps maintain the intracellular acid-base state.

Effects of alkalosis on muscle ions at rest and with intense exercise

The effects of metabolic and respiratory alkalosis (MALK and RALK) on intracellular strong ion concentrations ([ion]i) and muscle to blood ion fluxes were examined at rest and during 5 min of intense, intermittent tetanic stimulation in the isolated, perfused rat hindlimb. Compared with the control (C), perfusion of resting skeletal muscle during MALK and RALK significantly increased [Cl-]i and [Na+]i, and RALK significantly lowered [K+]i; these changes, however, did not affect initial hindlimb force production. In both resting and stimulated muscle, the intracellular ion changes corresponded to appropriate perfusate to muscle ion fluxes. At rest, changes in slow-twitch soleus were greater than in fast-twitch white gastrocnemius (WG), but stimulation-induced changes in [Lac]i and [K+]i were greater in WG. At the end of stimulation [K+]i and [Mg2+]i had decreased less in MALK than in C and RALK, particularly in plantaris and WG muscles. Compared with C, the muscle to perfusate flux of Lac- increased by 37% in MALK and 27% in RALK. This was associated with significantly less Lac- accumulation in all muscles in MALK than in RALK, which, in turn, had significantly less lactate than C. Lactate efflux from contracting skeletal muscle was significantly correlated with an uptake of Cl- by muscle. It is concluded that extracellular alkalosis alters skeletal muscle intracellular ionic composition and increases Lac- efflux from skeletal muscle. In agreement with other studies, lactate release appears to occur by both ionic and molecular transport processes. Alkalosis had no apparent effect on muscle performance with this preparation.

Role of nonworking muscle on blood metabolites and ions with intense intermittent exercise

The exchange of ions between blood and inactive muscle was studied in healthy males (n = 11) who performed four 30-s bouts of maximum isokinetic leg exercise, with 4 min of rest between bouts; recovery was followed for 90 min. Blood was sampled from the brachial artery and antecubital vein from inactive arm, and biopsies were taken from the nonworking deltoid muscles. During exercise, whole blood and plasma arteriovenous (a-v) differences across the arm showed increased net uptake of lactate (Lac-), K+, Na+, Cl-, glycerol, and O2. Arm glucose uptake also increased during exercise and the last 30 min of recovery, but there was no change in muscle glycogen content. Deltoid intracellular Lac- concentration [( Lac-]) increased 2.5-fold to 8.2 +/- 1.4 meq/l during exercise and 25 min of recovery. Despite evidence of uptake from blood and plasma a-v differences, deltoid intracellular K+, Na+, and Cl- concentrations did not change. During recovery, arm a-v differences showed no Lac- release, but Cl- and Na+ were released to venous blood for at least 90 min of recovery. During repeated exercise, the elevated Lac- uptake (a-v [Lac-] is 5-8 meq/l) and a HCO3- release (a-v HCO3- concentration is -8-11 meq/l) by the arm was suggestive of a possible anion exchange (Lac-/HCO3-). It is concluded that during heavy exercise non-working muscles take up and oxidize significant amounts of Lac-. Other inactive tissues also play a role in the regulation of ion and metabolite concentrations of blood.

Renal responses to exercise-induced lactic acidosis

Five healthy males performed four 30-s bouts of maximal exercise, separated by 4 min of rest, on an isokinetic cycle ergometer. Arterial blood and urine samples were taken from indwelling catheters at rest, immediately postexercise, and for 90 min of recovery. Inulin was continuously infused to measure glomerular filtration rate (GFR). Arterial plasma [Na+], [K+], and [Cl-] increased (P less than 0.05) with exercise; plasma lactate concentration ([Lac-]) increased from 1.3 +/- 0.2 to 21.0 +/- 1.0 (SE) meq/l (P less than 0.05). A significant decrease in the GFR occurred after exercise and during recovery associated with reductions in renal Na+ and K+ excretion (P less than 0.05). Renal excretion of Lac- reached a maximum of 293 +/- 79.4 mu eq.kg-1.h-1 (P less than 0.05), with Cl- excretion reaching a minimum of 4.8 +/- 0.95 mu eq.kg-1.h-1 (P less than 0.05). Urine [Lac-] was 189 +/- 25.6 meq/l, and urine [Cl-] was 6 +/- 1.7 meq/l at 30 min of recovery. There was a curvilinear relationship between urine [Cl-] and [Lac-] (r = -0.86; P less than 0.0001). Net Lac- production was estimated from arterial [Lac-] and after assuming a distribution volume. Less than 2% (13.1 meq) of the total estimated Lac- produced (678 meq) was excreted in the urine. Decreases in urine [Cl-] act to limit the fall in urine pH that accompanies increases in urine [Lac-].

Muscle glycogenolysis and H+ concentration during maximal intermittent cycling

The relationships between muscle glycogenolysis, glycolysis, and H+ concentration were examined in eight subjects performing three 30-s bouts of maximal isokinetic cycling at 100 rpm. Bouts were separated by 4 min of rest, and muscle biopsies were obtained before and after bouts 2 and 3. Total work decreased from 20.5 +/- 0.7 kJ in bout 1 to 16.1 +/- 0.7 and 13.2 +/- 0.6 kJ in bouts 2 and 3. Glycogenolysis was 47.2 and 15.1 mmol glucosyl U/kg dry muscle during bouts 2 and 3, respectively. Lower accumulations of pathway intermediates in bout 3 confirmed a reduced glycolytic flux. In bout 3, the work done represented 82% of the work in bout 2, whereas glycogenolysis was only 32% of that in bout 2. Decreases in ATP and phosphocreatine contents were similar in the two bouts. Muscle [H+] increased from 195 +/- 12 to 274 +/- 19 nmol/l during bout 2, recovered to 226 +/- 8 nmol/l before bout 3, and increased to 315 +/- 24 nmol/l during bout 3. Muscle [H+] could not be predicted from lactate content, suggesting that ion fluxes are important in [H+] regulation in this exercise model. Low glycogenolysis in bout 3 may be due to an inhibitory effect of increased [H+] on glycogen phosphorylase activity. Alternately, reduced Ca2+ activation of fast-twitch fibers (including a possible H+ effect) may contribute to the low overall glycogenolysis. Total work in bout 3 is maintained by a greater reliance on slow-twitch fibers and oxidative metabolism.

Factors influencing hydrogen ion concentration in muscle after intense exercise

To assess the importance of factors influencing the resolution of exercise-associated acidosis, measurements of acid-base variables were made in nine healthy subjects after 30 s of maximal exercise on an isokinetic cycle ergometer. Quadriceps muscle biopsies (n = 6) were taken at rest, immediately after exercise, and at 3.5 and 9.5 min of recovery; arterial and femoral venous blood were sampled (n = 3) over the same time. Intracellular and plasma inorganic strong ions were measured by neutron activation and ion-selective electrodes, respectively; lactate concentration ([La-]) was measured enzymatically, and plasma PCO2 and pH were measured by electrodes. Immediately after exercise, intracellular [La-] increased to 47 meq/l, almost fully accounting for a reduction in intracellular strong ion difference ([SID]) from 154 to 106 meq/l. At the same time, femoral venous PCO2 increased to 100 Torr and plasma [La-] to 9.7 meq/l; however, plasma [SID] did not change because of a concomitant increase in inorganic [SID] secondary to increases in [K+], [Na+], and [Ca2+]. During recovery, muscle [La-] fell to 26 meq/l by 9.5 min; [SID] remained low (101 and 114 meq/l at 3.5 and 9.5 min, respectively) due almost equally to the elevated [La-] (30 and 26 meq/l) and reductions in [K+] (from 142 meq/l at rest to 123 and 128 meq/l). Femoral venous PCO2 rose to 106 Torr at 0.5 min postexercise and fell to resting values at 9.5 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of lungs and inactive muscle in acid-base control after maximal exercise

The pulmonary responses and changes in plasma acid-base status occurring across the inactive forearm muscle were examined after 30 s of intense exercise in six male subjects exercising on an isokinetic cycle ergometer. Arterial and deep forearm venous blood were sampled at rest and during 10 min after exercise; ventilation and pulmonary gas exchange variables were measured breath by breath during exercise and recovery. Immediately after exercise, ventilation and CO2 output increased to 124 +/- 17 1/min and 3.24 +/- 0.195 l/min, respectively. The subsequent decrease in CO2 output was slower than the decrease in O2 intake (half time of 105 +/- 15 and 47 +/- 4 s, respectively); the respiratory exchange ratio was greater than 1.0 throughout the 10 min of recovery. Arterial plasma concentrations of Na+, K+, and Ca2+ increased transiently after exercise. Arterial lactate ion concentration ([La-]) increased to 14-15 meq/l within 1.5 min and remained at this level for the rest of the study. Throughout recovery there was a positive arteriovenous [La-] difference of 4-5 meq/l, associated with an increase in the arteriovenous strong ion difference ([SID]) and by a large increase in the venous Pco2 and [HCO3-]. These findings were interpreted as indicating uptake of La- by the inactive muscle, leading to a fall in the muscle [SID] and increase in plasma [SID], associated with an increase in muscle PCO2. The venoarterial CO2 content difference was 38% greater than could be accounted for by metabolism of La- alone, suggesting liberation of CO2 stored in muscle, possibly as carbamate.(ABSTRACT TRUNCATED AT 250 WORDS)

Ion fluxes during tetanic stimulation in isolated perfused rat hindlimb

The present study examined the relationships between changes in intra- and extracellular concentrations of strong ions, the appearance of nonvolatile acid (NVA) in venous perfusate, and skeletal muscle fatigue during intense electrical stimulation. A one-pass system was used to perfuse an isolated rat hindlimb during 5 min of intermittent tetanic contractions. Initial isometric tensions averaged 2.85 kg/hindlimb and declined by 45% during 5 min. During stimulation, intracellular lactate concentration ([La-]i) increased by 2, 13, 15, and 21 meq/l of intracellular fluid in the soleus, plantaris, and red and white gastrocnemius. This was associated with a proportionate decrease in intracellular K+ ([K+]i) and Mg2+([Mg2+]i) concentrations and increased intracellular Na+ ([Na+]i) and Cl-([Cl-]i) concentrations. A stoichiometrically equivalent uptake of Na+ and Cl- from the perfusate peaked at 8.5 mu eq.min-1.g-1 at the end of the 5th min. The increase in plasma [K+] during the last 4 min of stimulation was constant at 0.5 mu eq.min-1.g-1. A significant reduction in intracellular strong ion difference of all muscles contributed directly to an increase in [H+] during stimulation. After the 1st min of stimulation the rate of appearance of NVA in venous perfusate exceeded that of the increase in venous plasma [La-] by 12-fold; this decreased to 2.7-fold at the end of 5 min. La- release and NVA appearance in venous perfusate was maximal at 3.1 and 9.7 mu eq.min-1.g wet wt-1 during the 4th min of stimulation. It is concluded that the changes in the intracellular concentrations of strong ions during intense contractile activity are the primary factors contributing to skeletal muscle fatigue.

Effects of intense swimming and tetanic electrical stimulation on skeletal muscle ions and metabolites

The purpose of this study was to compare changes in ions and metabolites in four different rat hindlimb muscles in response to intense swimming exercise in vivo (263 +/- 33 s) (SWUM), and to 5 min (300 s) of tetanic electrical stimulation of artificially perfused rat hindlimbs (STIM). With both swimming and electrical stimulation, soleus (SOL) contents of creatine phosphate (CP), ATP, and glycogen changed the least, whereas the largest decreases in these metabolites occurred in the white gastrocnemius (WG). Lactate (La-) accumulation and glycogen breakdown were significantly greater in SWUM hindlimb muscles compared with STIM. The high arterial La- concentration [( La-] = 20 meq.l-1) in SWUM may have contributed to elevated muscle [La-], whereas one-pass perfusion kept arterial [La-] below 2 meq.l-1 in STIM. In SWUM, intracellular [Na+] increased significantly in the plantaris (PL), red gastrocnemius (RG), and WG, but not in SOL. [Cl-] increased, and [K+], [Ca2+], and [Mg2+] decreased in all muscles. In STIM, intracellular [K+], [Mg2+], and [Ca2+] decreased significantly, whereas [Na+] and [Cl-] increased in all muscles. Differences in the magnitude of ion and fluid fluxes between groups can be explained by the different methods of hindlimb perfusion. In conclusion, STIM is a useful model of in vivo energy metabolism and permits mechanisms of transsarcolemmal ion movements to be studied.

Intracellular ion content of skeletal muscle measured by instrumental neutron activation analysis

The intracellular contents of sodium (Na+), potassium (K+), calcium (Ca2+), magnesium (Mg2+), and chloride (Cl-) in rat hindlimb muscles (soleus, plantaris, white and red gastrocnemii) were measured by instrumental neutron activation analysis (INAA) and atomic absorption spectrophotometry (AAS). Muscle extracellular fluid volume (ECFV) was determined using [3H]mannitol, [14C]mannitol, [3H]polyethylene glycol (PEG, mol wt 900, PEG-900) or the chloride (Cl) method and intracellular fluid volume (ICFV) calculated. Rats were anesthetized with pentobarbital sodium. The muscles were biopsied, frozen in liquid nitrogen, freeze-dried, weighed, and transferred to vials for analysis. For a given muscle, ion contents measured by the two methods showed a consistent small difference which could not be explained. The PEG-900 space and the Cl method yielded a larger ECFV than did mannitol; it is concluded that PEG-900 and Cl overestimate ECFV. There were significant differences in total tissue water (TTW), ECFV, ICFV, and intracellular ion contents between the different muscle types. The fast glycolytic muscles (white gastrocnemius, plantaris) had lower TTW (758 ml/kg wet wt) and ECFV (6.5-8.5% TTW) but the highest ICFV; the soleus (slow oxidative fibers) had the highest TTW (766 ml/kg wet wt) and ECFV (10-15% TTW) but the lowest ICFV. The fast-twitch white gastrocnemius and plantaris muscles have a higher intracellular content of K+ and lower Na+ and Cl- than the slow-twitch soleus muscle. The technique of INAA provides a rapid and accurate means of determining intramuscular ion content in small samples of tissue.

Effects of alkalosis on skeletal muscle metabolism and performance during exercise

This study examined the effects of extracellular alkalosis on the metabolism and performance of perfused rat hindlimb muscles during electrical stimulation. Three acid-base conditions were used: control (C, normal acid-base state), metabolic alkalosis (MALK, increased bicarbonate concentration), and respiratory alkalosis (RALK, decreased PCO2). A one-pass system was used to perfuse the hindlimb via the femoral artery for 20 min at rest and during 5 min of tetanic stimulation via the sciatic nerve. The isometric tension generated by the gastrocnemius-plantaris-soleus muscle group was recorded. Arterial and venous perfusates were periodically sampled for substrate and metabolite measurements, and muscle samples were taken pre- and postperfusion. Peak isometric tensions in C, MALK, and RALK were similar: 3,367 +/- 107, 3,317 +/- 110, and 3,404 +/- 69 g, respectively. The rate of tension decay was also unaffected by alkalosis and represented 78 and 55% of the peak tension following 2 and 5 min of stimulation, respectively. Muscle O2 uptake, glycogen utilization, and total lactate (La-) production were similar following 5 min of stimulation in all conditions. However, alkalosis resulted in an enhanced La- release from working muscle (peak La- release: C, 15.5 +/- 1.1; MALK, 19.7 +/- 1.6; RALK, 18.3 +/- 2.2 mumol/min), and a 15-20% reduction in intramuscular La- accumulation. Alkalosis had no effect on muscle creatine phosphate and ATP concentrations. Thus, in the perfused rat hindlimb, alkalosis was not associated with changes in tetanic force or glycolysis, but La- release from the working muscle was enhanced by increased extracellular pH and bicarbonate.

Acid-base and respiratory properties of a buffered bovine erythrocyte perfusion medium

Current research in organ physiology often utilizes in situ or isolated perfused tissues. We have characterized a perfusion medium associated with excellent performance characteristics in perfused mammalian skeletal muscle. The perfusion medium consisting of Krebs-Henseleit buffer, bovine serum albumin, and fresh bovine erythrocytes was studied with respect to its gas-carrying relationships and its response to manipulation of acid-base state. Equilibration of the perfusion medium at base excess of -10, -5, 0, 5, and 10 mmol X L-1 to humidified gas mixtures varying in their CO2 and O2 content was followed by measurements of perfusate hematocrit, hemoglobin concentration, pH, Pco2, Cco2, Po2, and percent oxygen saturation. The oxygen dissociation curve was similar to that of mammalian bloods, having a P50 of 32 Torr (1 Torr = 133.3 Pa), Hill's constant n of 2.87 +/- 0.15, and a Bohr factor of -0.47, showing the typical Bohr shifts with respect to CO2 and pH. The oxygen capacity was calculated to be 190 mL X L-1 blood. The carbon dioxide dissociation curve was also similar to that of mammalian blood. The in vitro nonbicarbonate buffer capacity (delta [HCO3-] X delta pH-1) at zero base excess was -24.6 and -29.9 mmol X L-1 X pH-1 for the perfusate and buffer, respectively. The effects of reduced oxygen saturation on base excess and pH of the medium were quantified. The data were used to construct an acid-base alignment diagram for the medium, which may be used to quantify the flux of nonvolatile acid or base added to the venous effluent during tissue perfusions.

Cutaneous and renal responses to intravascular infusions of HCl and NH4Cl in the bullfrog (Rana catesbiana)

This study examined the ability of bullfrogs to correct a non-respiratory acidosis by renal and cutaneous mechanisms. Acidosis was induced by intravascular infusions of HCl (3 mmole/kg) or NH4Cl (4 mmole/kg). The acid load was removed primarily by increased renal excretion of NH4+, while urine pH and titratable buffer acid excretion changed little. Acid loading resulted in an increase in cutaneous permeability, shown by large ion losses and elevated water uptake across the skin. It is concluded that infused mineral acids were immediately buffered by the extracellular fluids, moved rapidly into the intracellular fluid compartment, and only later were slowly cleared.

Fine structure of the abdominal epidermis of the adult mudpuppy, Necturus maculosus (Rafinesque)

The ventral epidermis of adult Necturus maculosus has been studied using electron and light microscopy. Many larval characteristics of amphibian epidermal structure are retained in adult Necturus. The epidermis is a stratified epithelium consisting of four cell layers and five cell types. Major differences compared with other adult amphibians are: the absence of a well defined moulting cycle together with an apparently diminished synthetic and mitotic activity in the stratum germinativum; an outermost cell layer (stratum mucosum) that is unkeratinized and appears to synthesize a mucous layer; and numerous large club-shaped Leydig cells which span the epidermis between the cells of the stratum germinativum and stratum mucosum. The apical region of the stratum granulosum and stratum mucosum cells shows evidence of extensive synthesis. The stratum mucosum appears to be involved in the secretion of vesicular contents onto the outermost surface of the epithelium. The external surfaces of the stratum mucosum cells possess numerous microridges which are supported by an intricate network of cytofilaments in the apical region of these cells. The significance of these features is discussed in relation to the physiology and ecology of this species.