Nutritional aspects of post exercise skeletal muscle glycogen synthesis in horses: a comparative review

Carbohydrate (CHO) stored in the form of skeletal muscle glycogen is the main energy source for glycolytic and oxidative ATP production during vigorous exercise in mammals. In man, horse and dog both short-term high intensity and prolonged submaximal exercise deplete muscle glycogen. In horses, however, muscle glycogen synthesis is 2-3-fold slower than in man and rat, even when a diet high in soluble CHO is fed. There appear to be significant differences in CHO and glycogen metabolism between horses and other mammals, and it is becoming increasingly clear that many conclusions drawn from human exercise physiology do not apply to horses. This review aims to provide a comprehensive, comparative summary of the research on muscle glycogen synthesis in horse, man and rodent. Species differences in CHO uptake and utilisation are examined and the issues with feeding high soluble CHO diets to horses are discussed. Alternative feeding strategies, including protein and long and short chain fatty acid supplementation and the importance of rehydration, are explored.

Frusemide results in an extracellular to intracellular fluid shift in horses

REASONS FOR PERFORMING STUDY

Frusemide (Lasix) is commonly used diuretic in horse racing and equine clinical practice. While pharmacology, pharmacodynamics, renal and haematological effects of frusemide have been studied in horses, its effects on the distribution of fluid within the horse remain unknown.

OBJECTIVE

To quantify the effects of frusemide on extracellular and intracellular fluid shifts.

METHODS

Horses were infused with 1 mg/kg body mass (n = 7) or 2 mg/kg (n = 9) i.v. frusemide. Total body water (TBW), extracellular fluid volume (ECFV) and plasma volume (PV) were measured using D2O, NaSCN and Evans blue dilution. Change in ECFV was assessed from the change in plasma [protein] and from repeated infusion/dilution of NaSCN.

RESULTS

Frusemide resulted in a 0.020 +/- 0.002 l/kg decrease in TBW within 120 min. At 120 min after frusemide infusion the ECFV losses were nearly double the TBW losses, therefore ECFV loss in excess of TBW loss is seen as an increase in ICFV.

CONCLUSIONS

Frusemide resulted in a net shift of fluid (electrolytes and water) from the extracellular to intracellular fluid compartment.

POTENTIAL RELEVANCE

The fluid shifts that occur within horses administered frusemide has not previously been characterised. The intracellular shift of fluid is of performance and clinical significance.

Do multiple ionic interactions contribute to skeletal muscle fatigue?

During intense exercise or electrical stimulation of skeletal muscle the concentrations of several ions change simultaneously in interstitial, transverse tubular and intracellular compartments. Consequently the functional effects of multiple ionic changes need to be considered together. A diminished transsarcolemmal K(+) gradient per se can reduce maximal force in non-fatigued muscle suggesting that K(+) causes fatigue. However, this effect requires extremely large, although physiological, K(+) shifts. In contrast, moderate elevations of extracellular [K(+)] ([K(+)](o)) potentiate submaximal contractions, enhance local blood flow and influence afferent feedback to assist exercise performance. Changed transsarcolemmal Na(+), Ca(2+), Cl(-) and H(+) gradients are insufficient by themselves to cause much fatigue but each ion can interact with K(+) effects. Lowered Na(+), Ca(2+) and Cl(-) gradients further impair force by modulating the peak tetanic force-[K(+)](o) and peak tetanic force-resting membrane potential relationships. In contrast, raised [Ca(2+)](o), acidosis and reduced Cl(-) conductance during late fatigue provide resistance against K(+)-induced force depression. The detrimental effects of K(+) are exacerbated by metabolic changes such as lowered [ATP](i), depleted carbohydrate, and possibly reactive oxygen species. We hypothesize that during high-intensity exercise a rundown of the transsarcolemmal K(+) gradient is the dominant cellular process around which interactions with other ions and metabolites occur, thereby contributing to fatigue.

Effects of dietary strong acid anion challenge on regulation of acid-base balance in sheep1

The acid-base status of the extracellular fluid is directly affected by the concentrations of strong basic cations and strong acid anions that are absorbed into the bloodstream from the diet. The objective of this study was to develop and characterize a model for dietary acid challenge in sheep by decreasing the dietary cation-anion difference (DCAD) using NutriChlor (HCl-treated canola meal), an anionic feed supplement. Ten fully fleeced sheep (Rideau-Arcott, 54.3 +/- 6.7 kg of BW) were fed either a control supplement [200 g/d of canola meal, DCAD = 184 mEq/kg of DM, calculated as (Na+ + K+) - (Cl- + S2-)] or an anionic supplement (AS; 200 g/d of NutriChlor, DCAD = -206 mEq/kg of DM) offered twice daily at 0700 and 1100 in a randomized complete block design. The sheep were individually housed and limit-fed a basal diet of dehydrated alfalfa pellets (22% CP and 1.2 Mcal of NE(g)/kg, DM basis) at 1.1 kg of DM/d offered twice daily at 1000 and 1300. Two days before the beginning of the experiment, the sheep were fitted with vinyl catheters (0.86-mm i.d., 1.32-mm o.d.) in the left jugular vein to facilitate blood sampling. Blood and urine samples were obtained daily from 1100 to 1130 on d 1 through 9 and at 0700, 1000, 1300, 1600, and 1900 on d 10. Blood was analyzed for hematocrit, plasma pH, gases, strong ions, and total protein. Urine samples were analyzed for pH. The AS induced a nonrespiratory acid-base disturbance associated with lower (P < 0.05) plasma pH (7.47 vs. 7.39), lower (P < 0.05) urine pH (8.13 vs. 6.09), and lower (P < 0.05) strong ion difference (42.5 vs. 39.5). The AS reduced (P < 0.05) the concentration of plasma glucose, base excess, and bicarbonate and increased (P < 0.05) the concentration of K+ and Cl-. Lowering DCAD increased (P < 0.05) Ca2+ concentrations in plasma by 13%. In conclusion, this dietary model successfully induced a significant acid-base disturbance in sheep. Although the acidifying effects of negative DCAD in the diet may have short-term prophylactic effects of elevating the concentration of Ca2+ in plasma, negative DCAD may have detrimental effects on acid-base balance.

Effects of prepartum administration of a monensin controlled release capsule on rumen pH, feed intake, and milk production of transition dairy cows

Effects of prepartum administration of a monensin controlled release capsule (CRC) on rumen pH, dry matter intake, and milk production during the transition period and early lactation were determined in 16 multiparous Holstein cows. Cows were divided into blocks of 2 depending on calving date. Cows were fed either a close-up dry cow or a lactating cow total mixed ration ad libitum. Rumen pH was monitored continuously using indwelling probes. Monensin did not affect average daily rumen pH, time below pH 6, time below pH 5.6, area below pH 6, and area below pH 5.6 throughout the experiment. Average daily pH, time below pH 6, and time below pH 5.6 before calving were 6.62, 65.6 min/d, and 17.6 min/d, respectively, and did not differ among the weeks before calving. Average daily pH, time below pH 6, and time below pH 5.6 were 6.19, 443.3 min/d, and 115.5 min/d, respectively, during the first week after calving, and were 6.36, 204.3 min/d, and 52.4 min/d, respectively, during the sixth week after calving. In the weeks after calving, average daily pH showed a quadratic increase, time below pH 6 showed a quadratic decrease, and time below pH 5.6 showed a linear decrease. Monensin did not affect dry matter intake and daily yields of milk, milk fat, and milk protein. Results suggest that prepartum administration of a monensin CRC did not increase rumen pH in multiparous cows fed the experimental diets during the transition period and early lactation.

Hydration of exercised standardbred racehorses assessed noninvasively using multi-frequency bioelectrical impedance analysis

REASONS FOR PERFORMING STUDY

In human and animal clinical practice, multi-frequency bioelectrical impedance analysis (MF-BIA) is increasingly used as a diagnostic tool to assess hydration of intra-and extracellular fluid compartments. Accurate determination of changes in hydration status within individuals over time has remained problematic due to the requirement for complete impedance-frequency relationships at the time points of interest.

OBJECTIVES

To use MF-BIA in 13 Standardbred racehorses and 7 'endurance' research horses to determine if MF-BIA could be used to track changes in total body water (TBW), intracellular fluid volume (ICFV) and extracellular fluid volume (ECFV) resulting from exercise.

METHODS

Jugular venous blood was sampled at rest and for 2-13 h following exercise. TBW, ECFV and plasma volume (PV) were measured at rest using indicator dilution techniques (D2O, thiocyanate and Evans Blue, respectively). TBW, ECFV, ICFV and PV were correlated to impedance measures and predictive equations used to determine hydration status from MF-BIA measures.

RESULTS

TBW loss continued throughout the recovery period, and was primarily borne by the ECF compartment at 90 min of recovery.

CONCLUSIONS

MF-BIA predictions of compartmental hydration status were significantly correlated to measured/calculated decreases in these compartments.

POTENTIAL RELEVANCE

Practical applications for MF-BIA in horses include monitoring of hydration status during transport and competition, assessment of body compostion, clinical health assessment and critical care management.

Effects of mild heat stress and grain challenge on acid-base balance and rumen tissue histology in lambs1

The effect of heat stress (HS) and grain challenge (GC) on acid-base balance and rumen tissue histology in lambs was investigated using 24 yearling wether lambs (58 +/- 4.5 kg of BW) in a 2 x 2 factorial experiment with repeated measures for day (10, 14, and 17) of sampling. The factors were temperature [thermoneutral zone (TN) vs. HS] and diet (control vs. GC). Lambs were blocked by BW and assigned to 1 of 4 treatments in temperature-controlled rooms: 1) TN (temperature = 18 to 20 degrees C; relative humidity = 30%; 2) TN + GC; 3) HS (temperature = 35 degrees C for 9 h/d, 20 degrees C for 15 h/d; relative humidity = 40%); and 4) HS + GC. Venous blood samples were collected at 1800 on the first day of GC (d 10), in the middle of GC (d 14), and at the end of the trial (d 17) by jugular venipuncture and analyzed for pH, gases, hematocrit, plasma ions, and total protein. After all measurements in live animals were taken on d 17, lambs were slaughtered, and tissue samples were obtained from the ventral sac of the rumen for histological assessment. Except for the concentration of plasma glucose (P = 0.04) and total protein (P < 0.01), there were no (P > 0.05) diet x temperature interactions. With HS, the concentration of Na+ and Cl- in the control group decreased at d 14 and then increased by d 17, and respiration rates in the control group decreased linearly (P < 0.05). Compared with the control group, respiration rates and the concentration of Cl- in the GC lambs increased linearly over time, whereas the concentration of Na+ decreased linearly (P < 0.05) across time. Under HS, the partial pressure of carbon dioxide, total carbon dioxide, the partial pressure of oxygen and oxygen saturation, and the concentration of Mg2+, glucose, and HCO3- showed quadratic (P < 0.05) responses with time. In both treatments, DMI, base excess of extracellular fluid, base excess of blood, and standard bicarbonate increased linearly (P < 0.05), and hematocrit, plasma protein, Ca2+, anion gap, and plasma strong ion difference decreased linearly (P < 0.05) across day. Compared with the control group, the GC group had decreased papillae count in the ruminal ventral sac (1.3 vs. 1.5; P < 0.05). These results suggest that under HS the acidifying effects of GC on acid-base balance in lambs were counteracted in the short-term through respiratory adaptation.

The Effects of Subacute Ruminal Acidosis on Sodium Bicarbonate-Supplemented Water Intake for Lactating Dairy Cows

Four multiparous ruminally fistulated Holstein dairy cows were used in an 8-wk experiment utilizing a repeated measures block design to determine the effects of subacute ruminal acidosis (SARA) on supplemented water intake. Animals were subjected to SARA, which was induced by replacing 25% of the ad libitum intake of the total mixed ration (dry matter basis) with 50:50 wheat:barley pellets utilizing a grain challenge model. Cows had free choice from 2 water bowls. One bowl contained water with sodium bicarbonate (SB) supplemented at 2.5 g/L. The other bowl contained unsupplemented water. Ruminal pH was monitored continuously during the trial using indwelling pH probes. The induction of SARA reduced daily mean ruminal pH and increased the duration when ruminal pH was below 6. The total mixed ration intake by the cows decreased during the SARA periods. The overall preference for SB-supplemented water did not change, as the preference ratio was similar during the control and SARA periods. During the period of greatest ruminal pH depression, total water intake was higher during the SARA periods than during the control periods. During SARA, there was no difference in the preference of a SB water source to unsupplemented water. During the period of day with the most severe ruminal pH depression, the lactating dairy cows subjected to SARA increased their total water intake.

Short communication: Effects of subacute ruminal acidosis on free-choice intake of sodium bicarbonate in lactating dairy cows

The effect of inducing subacute ruminal acidosis (SARA) on the free-choice intake of sodium bicarbonate (SB) was investigated in four midlactation Holstein cows in a switchover experiment with four 1-wk periods. The SARA was induced by replacing 25% of the ad libitum intake of total mixed ration (TMR) with pellets containing 50% ground wheat and 50% ground barley and restricting access to TMR from 0700 to 1700 h. Control consisted of feeding TMR ad libitum. Powdered SB was provided for ad libitum consumption. Rumen pH was measured continuously using indwelling pH probes. Induction of SARA reduced (P < 0.05) the average daily rumen pH from 6.08 to 5.87, increased (P < 0.05) the average duration of rumen pH below 6 from 547 min x d(-1) to 916 min x d(-1), and increased (P < 0.05) the average duration of rumen pH below 5.6 from 132 min x d(-1) to 397 min x d(-1) (P < 0.05) but did not significantly affect SB intake. Average intake of SB was 26.8 g x d(-1) during SARA and 34.5 g x d(-1) during control. These low SB intakes must not have substantially affected rumen pH. Sodium bicarbonate intake differed significantly (P < 0.05) between cows. These data indicate that cows did not select SB in order to attenuate SARA.

Effects of a subacute ruminal acidosis model on the diet selection of dairy cows

Two experiments were conducted to study the effects of a subacute ruminal acidosis (SARA) model on diet choice in dairy cows. In the first experiment, 25% of the ad libitum dry matter intake (DMI) of the total mixed ration (TMR) was replaced with wheat-barley pellets (WBP, 50% ground wheat, 50% ground barley). Rumen pH was measured continuously via in-dwelling probes in 4 mid to late lactation cows. This diet change reduced rumen pH by 0.14 +/- 0.02 pH units (mean +/- SE) and increased time below pH 6.0, from 319 +/- 36 min(-1) to 641 +/- 36 min(-1). Hence, the nutritional model successfully induced SARA. The second experiment determined if inducing SARA increases the feed preference for long alfalfa hay compared with alfalfa pellets. The 2 wk of inducing SARA were separated by 1 control wk. Four cows on either SARA and control diets were given a choice of 2 feeds, 2 times per d, for 30 min. The preference ratios (PR = Amount of Hay consumed/Amount of Hay + Pellets consumed) for alfalfa hay during two SARA weeks was greater (0.85 +/- 0.03) compared with the control week (0.60 +/- 0.03). In SARA weeks, average rumen pH was 0.23 +/- 0.03 units lower, and time below pH 6.0 and 5.6 was higher compared to control. These results suggest that when given a choice of feeds, dairy cows alter their diet selection to attempt to attenuate SARA.

An integrative, in situ approach to examining K+ flux in resting skeletal muscle

The contributions of Na+/K+-ATPase, K+ channels, and the NaK2Cl cotransporter (NKCC) to total and unidirectional K+ flux were determined in mammalian skeletal muscle at rest. Rat hindlimbs were perfused in situ via the femoral artery with a bovine erythrocyte perfusion medium that contained either 86Rb or 42K, or both simultaneously, to determine differences in ability to trace unidirectional K+ flux in the absence and presence of K+-flux inhibitors. In most experiments, the unidirectional flux of K+ into skeletal muscle (J(in)K) measured using 86Rb was 8-10% lower than J(in)K measured using 42K. Ouabain (5 mM) was used to inhibit Na+/K+-ATPase activity, 0.06 mM bumetanide to inhibit NKCC activity, 1 mM tetracaine or 0.5 mM barium to block K+ channels, and 0.05 mM glybenclamide (GLY) to block ATP-sensitive K+ (K(ATP)) channels. In controls, J(in)K remained unchanged at 0.31 +/- 0.03 micromol x g(-1) x min(-1) during 55 min of perfusion. The ouabain-sensitive Na+/K+-ATPase contributed to 50 +/- 2% of basal J(in)K, K+ channels to 47 +/- 2%, and the NKCC to 12 +/- 1%. GLY had minimal effect on J(in)K, and both GLY and barium inhibited unidirectional efflux of K+ (J(out)K) from the cell through K+ channels. Combined ouabain and tetracaine reduced J(in)K by 55 +/- 2%, while the combination of ouabain, tetracaine, and bumetanide reduced J(in)K by 67 +/- 2%, suggesting that other K+-flux pathways may be recruited because the combined drug effects on inhibiting J(in)K were not additive. The main conclusions are that the NKCC accounted for about 12% of J(in)K, and that K(ATP) channels accounted for nearly all of the J(out)K, in resting skeletal muscle in situ.

Ouabain stimulates unidirectional and net potassium efflux in resting mammalian skeletal muscle

The present study compared ouabain-sensitive unidirectional K+ flux into (JinK) and out of (JoutK) perfused rat hindlimb skeletal muscle in situ and mouse flexor digitorum brevis (FDB) in vitro. In situ, 5 mM ouabain inhibited 54 +/- 4% of the total JinK in 28 +/- 1 min, and increased the net and unidirectional efflux of K+ within 4 min. In contrast, 1.8 mM ouabain inhibited 40 +/- 8% of the total JinK in 38 +/- 2 min, but did not significantly affect JoutK. In vitro, 1.8 and 0.2 mM ouabain decreased JinK to a greater extent (83 +/- 5%) than in situ, but did not significantly affect 42K loss rate compared with controls. The increase in unidirectional K+ efflux (JoutK) with 5 mM ouabain in situ was attributed to increased K+ efflux through cation channels, since addition of barium (1 mM) to ouabain-perfused muscles returned JoutK to baseline values within 12 min. Perfusion with 5 mM ouabain plus 2 mM tetracaine for 30 min decreased JinK 46 +/- 9% (0.30 +/- 0.03 to 0.16 +/- 0.02 micromol x min(-1) x g(-1)), however tetracaine was unable to abolish the ouabain-induced increase in unidirectional K+ efflux. In both rat hindlimb and mouse FDB, tetracaine had no effect on JoutK. Perfusion of hindlimb muscle with 0.1 mM tetrodotoxin (TTX, a Na+ channel blocker) decreased JinK by 15 +/- 1%, but had no effect on JoutK; subsequent addition of ouabain (5 mM) decreased JinK a further 32 +/- 2%. The ouabain-induced increase in unidirectional K+ efflux did not occur when TTX was perfused prior to and during perfusion with 5 mM ouabain. We conclude that 5 mM ouabain increases the unidirectional efflux of K+ from skeletal muscle through a barium and TTX-sensitive pathway, suggestive of voltage sensitive Na+ channels, in addition to inhibiting Na+/K+-ATPase activity.

Paraxanthine, a caffeine metabolite, dose dependently increases [Ca(2+)](i) in skeletal muscle

It was hypothesized that the caffeine derivative paraxanthine results in subcontracture increases in intracellular calcium concentration ([Ca(2+)](i)) in resting skeletal muscle. Single fibers obtained from mouse flexor digitorum brevis were loaded with a fluorescent Ca(2+) indicator, indo 1-acetoxymethyl ester. After a stable baseline was recorded, the fiber was superfused with physiological salt solution (Tyrode) containing 0.5, 1.0, 2.5, or 5 mM paraxanthine, resulting in [Ca(2+)](i) increases of 6.4 +/- 2.5, 9.7 +/- 3.6, 26.8 +/- 11.7, and 39.6 +/- 9.6 nM, respectively. The increases in [Ca(2+)](i) were transient and were also observed with exposure to 5 mM theophylline and theobromine. Six fibers were exposed to 5 mM paraxanthine followed by 5 mM paraxanthine in the presence of 10 mM procaine (sarcoplasmic reticulum Ca(2+) release channel blocker). There was no increase from baseline [Ca(2+)](i) when fibers were superfused with paraxanthine and procaine, suggesting that the sarcoplasmic reticulum is the primary Ca(2+) source in the paraxanthine-induced response. In separate experiments, intact flexor digitorum brevis (n = 13) loaded with indo 1-acetoxymethyl ester had a significant increase in [Ca(2+)](i) with exposure to 0.01 mM paraxanthine. It is concluded that physiological and low pharmacological concentrations of paraxanthine result in transient, subcontracture increases in [Ca(2+)](i) in resting skeletal muscle, the magnitude of which is related to paraxanthine concentration.

Heat storage in horses during submaximal exercise before and after humid heat acclimation

The effect of humid heat acclimation on thermoregulatory responses to humid and dry exercise-heat stress was studied in six exercise-trained Thoroughbred horses. Horses were heat acclimated by performing moderate-intensity exercise for 21 days in heat and humidity (HH) [34.2-35.7 degrees C; 84-86% relative humidity (RH); wet bulb globe temperature (WBGT) index approximately 32 degrees C]. Horses completed exercise tests at 50% of peak O(2) uptake until a pulmonary arterial temperature (T(pa)) of 41.5 degrees C was attained in cool dry (CD) (20-21.5 degrees C; 45-50% RH; WBGT approximately 16 degrees C), hot dry (HD 0) [32-34 degrees C room temperature (RT); 45-55% RH; WBGT approximately 25 degrees C], and HH conditions (HH 0), and during the second hour of HH on days 3, 7, 14, and 21, and in HD on the 18th day (HD 18) of heat acclimation. The ratios of required evaporative capacity to maximal evaporative capacity of the environment (E(req)/E(max)) for CD, HD, and HH were approximately 1.2, 1.6, and 2.5, respectively. Preexercise T(pa) and rectal temperature were approximately 0.5 degrees C lower (P < 0. 05) on days 7, 14, and 21 compared with day 0. With exercise in HH, there was no effect of heat acclimation on the rate of rise in T(pa) (and therefore exercise duration) nor the rate of heat storage. In contrast, exercise duration was longer, rate of rise in T(pa) was significantly slower, and rate of heat storage was decreased on HD 18 compared with HD 0. It was concluded that, during uncompensable heat stress in horses, heat acclimation provided modest heat strain advantages when E(req)/E(max) was approximately 1.6, but at higher E(req)/E(max) no advantages were observed.

Total body water and ECFV measured using bioelectrical impedance analysis and indicator dilution in horses

The purposes of this study were 1) to determine the compartmentation of body water in horses by using indicator dilution techniques and 2) to simultaneously measure bioelectrical impedance to current flow at impulse current frequencies of 5 and 200 kHz to formulate predictive equations that could be used to estimate total body water (TBW), extracellular fluid volume (ECFV), and intracellular fluid volume (ICFV). Eight horses and ponies weighing from 214 to 636 kg had catheters placed into the left and right jugular veins. Deuterium oxide, sodium thiocyanate, and Evans blue were infused for the measurement of TBW, ECFV, and plasma volume (PV), respectively. Bioelectrical impedance was measured by using a tetrapolar electrode configuration, with electrode pairs secured above the knee and hock. Measured TBW, ECFV, and PV were 0.677 +/- 0.022, 0.253 +/- 0.006, and 0.040 +/- 0.002 l/kg body mass, respectively. Strong linear correlations were determined among measured variables that allowed for the prediction of TBW, ECFV, ICFV, and PV from measures of horse length or height and impedance. It is concluded that bioelectrical impedance analysis (BIA) can be used to improve the predictive accuracy of noninvasive estimates of ECFV and PV in euhydrated horses at rest.

Heat acclimation improves regulation of plasma volume and plasma Na(+) content during exercise in horses

This study determined the plasma volume (PV) and ion responses to heat acclimation and exercise in six trained Thoroughbred horses during 21 days of exposure to heat and humidity (33 degrees C, 83% relative humidity) for 4 h/day. During the 2nd h on days 0, 3, 7, 14, and 21, horses performed a standardized treadmill test, running at 50% of peak O(2) uptake until pulmonary artery temperature reached 41.5 degrees C. Heat acclimation resulted in an increase in PV from 21.3 +/- 1.1 liters on day 0 to 24.3 +/- 1.0 liters on day 14, returning to 22.6 +/- 0.9 liters on day 21. The corresponding total plasma protein contents were 1,273 +/- 53, 1,455 +/- 81, and 1,377 +/- 57 g, respectively, and increases in total plasma Na(+) plus Cl(-) content were 5,145 +/- 126, 5,749 +/- 146, and 5,394 +/- 114 mmol, respectively. Thus changes in PV were accompanied by direct changes in plasma protein and osmolyte contents. With exercise on day 0, PV decreased by 7.1 +/- 0.7% at 5 min of exercise and remained decreased (-6.7 +/- 1.3%) at 5 min of recovery. By day 21, PV decreased significantly less than on day 0 (by 5.2 +/- 0.9% at 5 min of exercise), was decreased by only 2.0 +/- 1.6% at 5 min of recovery, and was fully restored at 15 min of recovery. Plasma Na(+) concentration increased 3 meq/l during the first 5 min of exercise and was normalized by 5 min of recovery on day 0 and by end exercise on day 21. It is concluded that improved ability to regulate PV during exercise in response to heat acclimatization is associated with an increased PV and an improved conservation of Na(+).

NaHCO(3) and KHCO(3) ingestion rapidly increases renal electrolyte excretion in humans

This paper describes and quantifies acute responses of the kidneys in correcting plasma volume, acid-base, and ion disturbances resulting from NaHCO(3) and KHCO(3) ingestion. Renal excretion of ions and water was studied in five men after ingestion of 3.57 mmol/kg body mass of sodium bicarbonate (NaHCO(3)) and, in a separate trial, potassium bicarbonate (KHCO(3)). Subjects had a Foley catheter inserted into the bladder and indwelling catheters placed into an antecubital vein and a brachial artery. Blood and urine were sampled in the 30-min period before, the 60-min period during, and the 210-min period after ingestion of the solutions. NaHCO(3) ingestion resulted in a rapid, transient diuresis and natriuresis. Cumulative urine output was 44 +/- 11% of ingested volume, resulting in a 555 +/- 119 ml increase in total body water at the end of the experiment. The cumulative increase (above basal levels) in renal Na(+) excretion accounted for 24 +/- 2% of ingested Na(+). In the KHCO(3) trial, arterial plasma K(+) concentration rapidly increased from 4.25 +/- 0.10 to a peak of 7.17 +/- 0.13 meq/l 140 min after the beginning of ingestion. This increase resulted in a pronounced, transient diuresis, with cumulative urine output at 270 min similar to the volume ingested, natriuresis, and a pronounced kaliuresis that was maintained until the end of the experiment. Cumulative (above basal) renal K(+) excretion at 270 min accounted for 26 +/- 5% of ingested K(+). The kidneys were important in mediating rapid corrections of substantial portions of the fluid and electrolyte disturbances resulting from ingestion of KHCO(3) and NaHCO(3) solutions.

Exercise-induced stimulation of K(+) transport in human erythrocytes

The hypothesis was tested that exercise-induced changes in plasma composition stimulate unidirectional K(+) transport (J(in)K) in human red blood cells (RBCs). Ten men performed two 30-s high-intensity leg-cycling tests separated by 4 min of rest. Antecubital venous blood was sampled before exercise and at the end of the second exercise bout. RBCs were separated from true exercise plasma, (42)K was added to plasma, and RBC K(+) transport was studied in vitro at 37 degrees C. In the second part of the study, blood from nine healthy men studied in vitro at 37 degrees C was used to test the hypothesis that exercise-simulated (ES) plasma stimulates net K(+) transport and J(in)K (measured using (86)Rb) in human RBCs. The J(in)K of resting RBCs added to true exercise plasma was 1,574 +/- 200 (SE) micromol. h(-1). l(-1) vs. 1,236 +/- 256 micromol. h(-1). l(-1) in true resting plasma at 2 min (controls). In true exercise and ES plasma, J(in)K was increased through activation of the ouabain-sensitive Na(+)-K(+) pump and the bumetanide-sensitive Na(+)-K(+)-2Cl(-) cotransporter. Increases in plasma osmolality and K(+), H(+), and epinephrine concentrations independently and in combination stimulated K(+) transport into human RBCs. In a third series of experiments, in which ES plasma K(+) concentration was continuously measured during the first 5 min of incubation of RBCs, a 1.6 +/- 0.3 mmol/l decrease in plasma K(+) concentration occurred during the first 2 min. It is concluded that RBCs transport K(+) at elevated rates in response to exercise-induced changes in plasma composition.

K+ transport in resting rat hind-limb skeletal muscle in response to paraxanthine, a caffeine metabolite

This study tested the hypothesis that paraxanthine, a caffeine metabolite, stimulates skeletal muscle potassium (K+) transport by an increase in Na+ -K+ ATPase activity. The unidirectional transport of K+ into muscle (J(in)K) was studied using a perfused rat hind limb technique. Using 12 hind limbs, we examined the response to 20 min of paraxanthine perfusion (0.1 mM), followed by 20 min perfusion with 0.1 mM paraxanthine and 5 mM ouabain (n = 5) to irreversibly inhibit Na+ -K+ ATPase activity. Paraxanthine stimulated J(in)K by 23+/-5% within 20 min. Ouabain abolished the paraxanthine-induced stimulation of J(in)K, suggesting the increase in K+ uptake was due to activation of the Na+ -K+ ATPase. To confirm the role of the Na+ -K+ ATPase, 14 hind limbs were perfused for 20 min with 5 mM ouabain prior to 20 min perfusion with 0.1 mM paraxanthine and 5 mM ouabain (n = 6). Ouabain alone resulted in a 41+/-7% decrease in J(in)K within 15 min. Inhibition of ouabain-sensitive J(in)K prevented the paraxanthine-induced increase in J(in)K. Hind limbs (n = 3) were also perfused with 0.1 mM paraxanthine for 60 min to examine the response to longer duration paraxanthine perfusion. The paraxanthine-induced increase in J(in)K continued for the entire 60 min. In another series, hind limbs were perfused with 0.01 (n = 9), 0.1 (n = 9), or 0.5 (n = 6) mM paraxanthine for 15 min. There was no concentration-dependent relationship between J(in)K and paraxanthine concentration, and 0.01, 0.1, and 0.5 mM paraxanthine increased J(in)K similarly (25+/-5, 22+/-4, and 27+/-6%, respectively). The effect of paraxanthine on J(in)K could not be reversed by subsequent perfusion with paraxanthine-free perfusate. Caffeine (0.05-1.0 mM) had no effect on K+ transport. It is concluded that paraxanthine increases J(in)K in resting skeletal muscle by stimulating ouabain-sensitive Na+ -K+ ATPase activity.

Equine sweating responses to submaximal exercise during 21 days of heat acclimation

This study examined sweating responses in six exercise-trained horses during 21 consecutive days (4 h/day) of exposure to, and daily exercise in, hot humid conditions (32-34 degrees C, 80-85% relative humidity). On days 0, 3, 7, 14, and 21, horses completed a standardized exercise test on a treadmill (6 degrees incline) at a speed eliciting 50% of maximal O(2) uptake until a pulmonary artery temperature of 41.5 degrees C was attained. Sweat was collected at rest, every 5 min during exercise, and during 1 h of standing recovery for measurement of ion composition (Na(+), K(+), and Cl(-)) and sweating rate (SR). There was no change in the mean time to reach a pulmonary artery temperature of 41.5 degrees C (range 19.09 +/- 1.41 min on day 0 to 20.92 +/- 1.98 min on day 3). Peak SR during exercise (ml. m(-2). min(-1)) increased on day 7 (57.5 +/- 5. 0) but was not different on day 21 (48.0 +/- 4.7) compared with day 0 (52.0 +/- 3.4). Heat acclimation resulted in a 17% decline in SR during recovery and decreases in body mass and sweat fluid losses during the standardized exercise test of 25 and 22%, respectively, by day 21. By day 21, there was also a 10% decrease in mean sweat Na(+) concentration for a given SR during exercise and recovery; this contributed to an approximately 26% decrease in calculated total sweat ion losses (3,112 +/- 114 mmol on day 0 vs. 2,295 +/- 107 mmol on day 21). By day 21, there was a decrease in sweating threshold ( approximately 1 degrees C) but no change in sweat sensitivity. It is concluded that horses responded to 21 days of acclimation to, and exercise in, hot humid conditions with a reduction in sweat ion losses attributed to decreases in sweat Na(+) concentration and SR during recovery.

Increased flow rate and papaverine increase K+ exchange in perfused rat hind-limb skeletal muscle

This study tested the hypothesis that increases in perfusate flow rate result in increased rates of unidirectional and net K+ transport in rat hind-limb skeletal muscle at rest. Ten neurally and vascularly isolated hind limbs, with arterial and venous catheters placed proximal to the popliteal region, were perfused for 10-min periods at flow rates (presented in a random order) of 0.27, 0.42, 0.63, 0.84, or 1.05 mL x min(-1) x g(-1). Potassium extraction and unidirectional K+ influx were determined using 42K, and arterial perfusion pressure was measured continuously. Increases in flow rate resulted in decreases in K+ extraction and increases in unidirectional K+ influx, unidirectional K+ efflux, and net K+ efflux. The increases in K+ flux were associated with increases in oxygen uptake, glucose uptake, and lactate release. In separate experiments (n = 5), the vasodilator papaverine (10(-4) M) did not further vasodilate the vasculature of resting hind limbs, suggesting that the hind limbs in this preparation were fully vasodilated. Papaverine, at constant flow, resulted in a nearly 1.5-fold increase in K+ extraction, a doubling of unidirectional K+ influx, and increases in unidirectional K+ efflux and net K+ efflux. It is concluded that physiological increases in flow rate result in increases in K+ transport in isolated, perfused rat hind-limb skeletal muscle. Furthermore, papaverine appeared to induce an increase in skeletal muscle membrane permeability to K+.

Exercise in the heat: thermoregulatory limitations to performance in humans and horses

This paper reviews the limits to exercise imposed by increases in ambient, hypothalamic, and contracting skeletal muscle temperature in humans and horses. Like humans, horses frequently compete in hot environments, yet their high mass-specific rate of heat production and low mass-specific surface area for heat dissipation places them at a great disadvantage compared to humans. Exercise in hot conditions increases the rate of body heat storage and reduces the time required to reach a critical hypothalamic temperature that results in voluntary fatigue. This critical temperature appears to be associated with dysfunction of the brain's motor control centres. The ensuing voluntary cessation of exercise appears to coincide with temperature-induced alterations in skeletal muscle function with increased requirement for anaerobic ATP provision. The duration of exercise that can be performed before this critical temperature is reached can be increased by ingesting fluids, of a volume at least equal to that lost in sweat, within 60 min prior to and during exercise. Emerging research in the area of skeletal muscle heat dissipative mechanisms involves heat-induced increases in muscle sympathetic nerve activity, producing stimulation of CIII and CIV afferent nerve stimulation, and heat-induced release of nitric oxide within skeletal muscle and skin, producing muscle and skin vasodilation.

Role of skeletal muscle in plasma ion and acid-base regulation after NaHCO3 and KHCO3 loading in humans

This paper examines the time course of changes in plasma electrolyte and acid-base composition in response to NaHCO3 and KHCO3 ingestion. It was hypothesized that skeletal muscle is involved in the correction of the ensuing plasma disturbance by exchanging ions, gasses, and fluids between cells and extracellular fluids. Five male subjects, with catheters in a brachial artery and antecubital vein, ingested 3.57 mmol/kg body mass NaHCO3 or KHCO3. While seated, blood samples were taken 30 min before ingestion of the solution, at 10-min intervals during the 60-min ingestion period, and periodically for 210 min after ingestion was complete. Blood was analyzed for gases, hematocrit, plasma ions, and total protein. With NaHCO3, arterial plasma Na+ concentration ([Na+]) increased from 143 +/- 1 to 147 +/- 1 (SE) meq/l, H+ concentration ([H+]) decreased by 6 +/- 1 neq/l, and PCO2 increased by 5 +/- 1 mmHg. There was no detectable net Na+ uptake by tissues. An increased plasma strong ion difference ([SID]) accounted fully for the decrease in plasma [H+]. With KHCO3, K+ concentration increased from 4.25 +/- 0.10 to 7.17 +/- 0.13 meq/l, plasma volume decreased by 15.5 +/- 2.3%, [H+] decreased by 4 +/- 1 neq/l, and there was no change in PCO2. The decrease in [H+] in the KHCO3 trial primarily arose in response to the increased [SID]. Net K+ uptake by tissues accounted for 37 +/- 5% of the ingested K+. In conclusion, ingestion of NaHCO3 and KHCO3 produced markedly different fluid and ionic disturbances and associated regulatory responses by skeletal muscle. Accordingly, the physicochemical origins of the acid-base disturbances also differed between treatments. The tissues did not play a role in regulating plasma [Na+] after ingestion of NaHCO3. In contrast, the net influx of K+ to tissues played an important role in removing K+ from the extracellular compartment after ingestion of KHCO3.

Sarcoplasmic reticulum responses to repeated sprints are affected by conditioning of horses

Sarcoplasmic reticulum (SR) responses to repeated sprints and to physical conditioning were studied in 10 Quarter Horses. Exercise tests (four repeated sprints on a treadmill) were conducted before and after 12 wk of sprint conditioning. Muscle samples from the middle gluteal muscle were taken before and after each exercise test, and SR vesicles were isolated. Calcium uptake was determined spectrophotometrically using antipyrylazo III, and Ca2+-ATPase activity was determined using an enzyme-linked optical assay. Conditioning increased calcium uptake rate and Ca2+-ATPase activity by 14 and 38%, respectively, before exercise and by 25 and 26% after exercise. Exercise decreased calcium uptake rate and Ca2+-ATPase activity by 37 and 27%, respectively, before conditioning and by 28 and 21% after conditioning. Decreases in calcium uptake and Ca2+-ATPase activity of SR have been associated with fatigue during exercise, and this association is strengthened by the moderating effect of conditioning.

Distribution of lactate and other ions in inactive skeletal muscle: influence of hyperkalemic lactacidosis

The purpose of this study was to quantify changes in intracellular ion and acid-base status resulting from the net flux of ions between perfusate and noncontracting muscle of differing fibre type in response to a perfusate that simulated the ionic conditions seen during intense exercise. Isolated rat hind limbs were perfused for 80 min with a bovine erythrocyte perfusate. Two series of experiments were performed: a normal perfusate (NP, n = 8) or a lactacidotic perfusate (LP, n = 8) that simulated arterial plasma and blood composition during intense exercise ([Lac-] = 11.0 mequiv. L-1, [K+] = 7.5 mequiv. L-1, and nonvolatile acid concentration = 71 nequiv.L-1). Net ion fluxes were determined by the arteriovenous difference across the hind limb and perfusate flow. Muscle ion concentrations were measured in the soleus (SOL), plantaris (PLT), and white gastrocnemius (WG) muscles. In the NP group, small net effluxes of K+ and Lac- from muscle were observed, but there was no net flux of Na+ or CI-. During LP, an initial rapid net influx of Lac- into muscle (151.2 +/- 9.4 mu equiv. min-1. 100 g-1 at 5 min) was followed by a steady-state net influx of 24-37 mu equiv. min-1. 100 g-1 between 20 and 60 min. During LP, net influx of Na+, CI-, and K+ was greater than during NP and average 58.0 +/- 11.2, 30.0 +/- 7.5, and 7.5 +/- 1.9 mu equiv. min-1. 100 g-1, respectively. Following LP, muscle content of Na+ (WG only) and Lac- (WG, PLT, and SOL) was increased to a greater extent than following NP. The increased [Lac-]i contributed to an elevated [H+]i only in the slow oxidative SOL, consistent with the higher concentration of Lac- transporters, lower capacity to bind protons, and better regulation of [Na+]i in slow oxidative muscles. Calculated membrane potential (Em) was unchanged with NP but decreased on average from -76.2 +/- 1.2 to 63.4 +/- 2.2 mV with LP perfusion, with no difference among fibre types. The steady-state distribution of Lac- across the sarcolemma appears to be a function of both metabolic and transport processes; specifically, Lac- distribution was not fully explained by the membrane potential nor by the nonionic distribution of HLac as determined by the transmembrane pH gradient. It is concluded that inactive skeletal muscle modifies the ionic composition of blood perfusing the muscles. However, the altered ionic composition of these muscles may compromise their function as a result of an altered membrane potential in fast and slow muscles and increased [H+]i in slow oxidative muscles.

Metabolite accumulation increases adenine nucleotide degradation and decreases glycogenolysis in ischaemic rat skeletal muscle

Adenine nucleotides and glycogen are degraded in skeletal muscle during no-flow ischaemia. Past investigations have ascribed these metabolic changes to the severe energetic stress which arises with the removal of exogenous substrates (principally oxygen). We tested this hypothesis by measuring the high-energy phosphagen and glycogen contents of stimulated rat hindlimb muscles (1 twitch s-1) prior to and following 40 min of no-flow ischaemia or hypoxic perfusion without glucose (PaO2 = 4.6 +/- 0.1 torr, plasma glucose = 0.3 +/- 0.1 mmol L-1). Both experimental protocols eliminated exogenous substrate supply; however, the maintenance of flow during hypoxic perfusion ensured the removal of metabolic by-products. A period of forty minutes of skeletal muscle ischaemia was characterized by reductions in the total adenine nucleotide pool, phosphocreatine and glycogen in the slow oxidative soleus, fast oxidative-glycolytic plantaris and the fast glycolytic white gastrocnemius. Compared to ischaemia, the total adenine nucleotide pool was higher (by 7.2-13.3 mumol g-1 dry wt) and the glycogen content lower (by 10.0-16.6 mumol g-1 dry wt) in skeletal muscle exposed to hypoxic perfusion without glucose. The ability of hypoxic perfusion to attenuate TAN degradation and augment glycogenolysis can be attributed to metabolic by-product removal. By limiting muscle lactate and PCO2 accumulation, hypoxic perfusion without glucose attenuates cellular acidification; this could in turn limit AMP deaminase activation and glycogen phosphorylase inhibition. We conclude that the ischaemia-induced alterations in adenine nucleotide and glycogen metabolism arise in response to the elimination of exogenous substrates and to the accumulation of metabolic by-products.

Adaptations to daily exercise in hot and humid ambient conditions in trained thoroughbred horses

The objectives of this study were to: 1) determine the effects of heat and high relative humidity (RH) on the clinical and physiological responses of horses during and after daily exercise training and 2) determine whether repeated exposure to, and exercise in, the heat would result in improved thermal tolerance (heat acclimation). Six trained Thoroughbred horses completed 1 h of submaximal exercise in cool, dry conditions (CD) and during a daily 4 h period of exposure to high heat and humidity (HH, room temperature = 33-35 degrees C, RH = 80-85%) for 22 days. Rectal temperature (Tre) and heart rate (HR) were measured before, during and after exercise and respiratory rate (RR) was measured before exercise and during a 2 h recovery. In HH, the rate of rise in Tre was significantly higher than in CD. However, by HH Day 5, Tre before, during and after exercise was significantly lower than on HH Day 1. The day-to-day decrease in Tre during exercise was reflected in significant decreases in heat storage following exercise by HH Day 10 (910 +/- 47 kcal) when compared to HH Day 1 (1211 +/- 75 kcal). At rest, RR was initially higher in HH than CD, and a further increase in pre-exercise RR from HH Day 1 to Day 10 may have contributed to the lower pre-exercise Tre. Recovery RR was higher after HH Day 1 and was associated with a lower end-of-exercise HR did not change in the 1 h before exercise in CD and did not differ from HH Days 1-22. By HH Day 10, mean HR during the latter part of exercise was lower than HH Day 1 and was not different from pre-exercise by 60 min of recovery. Pre-exercise body mass did not change during the 3 wk period and the decrease in body mass that occurred during the 4 h training period was significantly attenuated by HH Day 15 (9.8 +/- 0.8 kg) when compared to HH Day 1 (12.5 +/- 0.8 kg). Over the 3 week period of HH, mean 24 h water consumption increased from 26.0 +/- 2.1 litres to 39.5 +/- 3.2 litres, largely reflecting a 2-fold increase in water intake during the 4 h period of heat exposure. It is concluded that 3 weeks of daily exposure to, and exercise in, hot and humid ambient conditions resulted in a progressive reduction in thermal and cardiovascular strain. Furthermore, the reported physiological adaptations are consistent with an improved thermal tolerance (heat acclimation).

L-type Ca2+ channel and Na+/Ca2+ exchange inhibitors reduce Ca2+ accumulation in reperfused skeletal muscle

It is known that extracellular Ca2+ accumulates within skeletal muscle after prolonged periods of ischemia and reperfusion. In this study, we determined whether the L-type Ca2+ channel and the Na+/Ca2+ exchanger mediated Ca2+ influx and whether Ca2+ accumulation limited the metabolic and contractile recovery of reperfused skeletal muscle. Contracting rat hindlimbs (1-Hz twitch) exposed to 40 min of no-flow ischemia were reperfused with diltiazem (500 microM) or 3,4-dichlorobenzamil (300 microM) to block the Na+/Ca2+ exchanger and/or the L-type Ca2+ channel. High inhibitor concentrations were used to counter the binding of diltiazem and 3,4-dichlorobenzamil to albumin and red blood cells. Muscle Ca2+ accumulation, contractile function, and energy metabolism were assessed by measuring intracellular Ca2+ concentration ([Ca2+]i), Ca2+ influx, twitch tension, and high-energy phosphagens [ATP, total adenine nucleotides (TAN) and phosphocreatine (PCr)]. Compared with control reperfusion, diltiazem and 3,4-dichlorobenzamil reduced Ca2+ influx and attenuated the rise in [Ca2+]i in the fast-oxidative glycolytic plantaris (Pl) and the fast-glycolytic white gastrocnemius (WG). The inhibitor-induced decrease in Ca2+ influx was 1.5- to 2-fold greater with 3,4-dichlorobenzamil than with diltiazem. Coinciding with the reduced Ca2+ accumulation, diltiazem and 3,4-dichlorobenzamil enhanced the resynthesis of ATP (Pl and WG), PCr (Pl and WG), and TAN (Pl) compared with control reperfusion. 3,4-Dichlorobenzamil also augmented twitch-tension recovery. We conclude that Ca2+ accumulation during reperfusion 1) arises from L-type Ca2+ channel and Na+/Ca2+ exchange activation; and 2) impairs the metabolic and contractile recovery of skeletal muscle.

Stimulation of Na+, K(+)-pump activity in skeletal muscle by methylxanthines: evidence and proposed mechanisms

Evidence is presented to support the hypothesis that submillimolar concentrations of methylxanthines stimulate Na+, K(+)-ATPase activity in skeletal muscle. Administration of methylxanthines to skeletal muscle results in plasma membrane hyperpolarization and increased rates of K+ uptake and Na+ efflux. These effects are both dose- and time-dependent and inhibited by blockers of the Na+, K+ ATPase. The mechanisms for stimulation of Na+, K(+)-ATPase activity and the signal transduction pathways are not known. The methylxanthine concentrations required for stimulation of Na+, K(+)-ATPase activity are less than those required to cause a 50% inhibition of phosphodiesterase activity, and therefore increases in cyclic AMP due to inhibition of the enzyme are not involved. Possible mechanisms by which methylxanthines may increase Na+, K(+)-ATPase activity include; (1) a role for increased intracellular [Ca2+]; (2) Ca2+ or adenosine-receptor-mediated increases in intracellular cyclic AMP; and (3) a direct action of methylxanthines on the Na+, K+ ATPase.

Preparing for and competing in the heat: the human perspective

This review provides an overview of the challenges that face man and horses when exercising in the heat. Some of the strategies that are used and are being developed for human athletes exercising in the heat are reviewed. There are many similarities between human and equine physiological responses to exercise in the heat; and equine exercise science may gain some useful insights from the training, fluid replacement and heat acclimatisation strategies used by human athletes. There are, however, some important differences that impact on the ability of horses to thermoregulate and to regulate fluid and electrolyte balance. The major differences are the low surface area to body mass ratio in horses compared to man; and the high metabolic capacity of equine skeletal muscle. These 2 factors may limit the ability of horses to dissipate heat when exercise is performed under hot conditions. Some of the more important equine differences are highlighted within the context of the "human perspective'.

Plasma volume and ions during exercise in cool, dry; hot, dry; and hot, humid conditions

We studied the effects of heat and relative humidity (RH) on plasma volume (PV) and ion responses to submaximal exercise and 60 min recovery in Thoroughbreds. Five horses were exercised at 50% of peak VO2 in cool, dry (CD, T = 22 degrees C, RH = 45-55%), and hot, humid (HH, T = 30-34 degrees C, RH = 80-85%) conditions until a pulmonary artery temperature of 41.5 degrees C was reached. Blood was obtained from the carotid artery. Body mass was measured at rest and after 30 min of recovery. The thermal conditions had no effect on the PV and ion responses during exercise and initial 30 min of recovery. Exercise resulted in an 8.5% decrease in PV within the first 2 min and, in the absence of changes in plasma [Na+] and [Cl-], was responsible for a 150 to 185 mmol decrease in plasma Na+ and Cl- contents. The decrease in PV was responsible for about 50% of the increase in packed cell volume (37% at rest; 51% at 2 min of exercise). Plasma [K+] and K+ content increased rapidly during the first 2 min of exercise. With cessation of exercise, plasma [K+] declined with a half time of about 2 min; recovery of PV and plasma Na+ and Cl- contents occurred with a half time of 10-15 min, with nearly complete recovery by 30 min. In the HD trial, between 30 and 60 min of recovery there were further decreases in PV and ion contents. It is concluded that with this exercise protocol, thermal stress had minimal influence on the rate and magnitude of exercise-induced changes in PV and ion contents. However, thermal stress during recovery from exercise may impede the restoration of PV and ion contents.

Sweating rate and sweat composition during exercise and recovery in ambient heat and humidity

The objective of this study was to determine the composition and extent of sweat losses during submaximal exercise under hot and humid conditions and to compare these findings with the same exercise protocol conducted under cool, dry and hot, dry conditions. Five Thoroughbred horses (age 3 to 6) completed exercise tests under each of 3 environmental conditions in random order: cool, dry (CD), room temperature (T) = 20 degrees C, relative humidity (RH) = 45-55%; hot, dry (HD), T = 32-34 degrees C, RH = 45-55%; and hot, humid (HH), T = 32-34 degrees C, RH = 80-85%. Horses exercised at 50% of their predetermined VO2max on a treadmill set at a 10% slope until attainment of a pulmonary artery blood temperature of 41.5 degrees C followed by a 60 min recovery. Sweat was collected from a sealed polyethylene pouch enclosing a 150 cm2 area on the lateral thorax. During exercise and the first 30 min of recovery, sweat fluid losses were 7.9 +/- 0.7 litres, 9.9 +/- 0.5 litres and 6.6 +/- 1.2 litres (mean +/- s.e.m.) for CD, HD and HH, respectively. Sweating rate (SR), calculated from sweat volume per unit area of enclosed skin, was lowest in CD and similar in HD and HH during exercise such that at end of exercise in HH (16.5 min) calculated sweat losses were approximately 5% and 32% higher than in HD and CD, respectively. In recovery, SR declined in all conditions but was significantly lower in CD (P < 0.05). Sweating was detectable until 30 min recovery in CD, 45 min recovery in HD and 60 min recovery in HH. Sweat composition and osmolality was different under the 3 environmental conditions and changed gradually during exercise and recovery in all conditions. Osmolality and [Na] was highest in HD and lowest in CD. During exercise, [Na] increased with increasing SR. Although exercise duration was significantly decreased in HH (16.5 +/- 1 min) when compared to HD (28 +/- 2 min) and CD (37 +/- 2 min), fluid and ion losses in HH were comparable to those in HD as a result of a high SR and prolonged sweating during recovery.

Thermal and cardiorespiratory responses of horses to submaximal exercise under hot and humid conditions

The objective of this study is to determine the effects of heat, and heat and high relative humidity (RH) on the thermal and cardiorespiratory responses to exercise and recovery. Five Thoroughbred horses (age 3 to 6) completed exercise tests under each of 3 environmental conditions: cool, dry (CD, room temperature (T) = 20 degrees C, RH = 45-55%), hot, dry (HD, T = 32-34 degrees C, RH = 45-55%) and hot, humid (HH, T = 32-34 degrees C, RH = 80-85%). Horses were exercised at a workload equal to 50% of VO2max on a treadmill set at a 10% slope until attainment of a pulmonary artery blood (PA) temperature of 41.5 degrees C followed by a 30 min walking recovery (0% slope), and a further 30 min standing recovery in the same environmental conditions. Blood (PA), rectal, skin (dorsal aspect of the thorax) and muscle (middle gluteal muscle) temperatures and heart rate were measured before, during and after exercise. Respiratory rate was measured before exercise and during the 60 min recovery period. Exercise duration for HD (mean +/- s.e. 28 +/- 2 min) and HH (16.5 +/- 1 min) was significantly (P < 0.05) decreased when compared with CD (37 +/- 2 min). The rate of increase in PA blood temperature was significantly higher in HH (0.26 +/- 0.03 degrees C/min) than in HD (0.17 +/- 0.04 degrees C/min) and CD (0.12 +/- 0.05 degrees C/min). Temperature in the middle gluteal muscle after 15 min of exercise was significantly higher in HH (41.9 +/- 0.3 degrees C) than in HD (40.7 +/- 0.25 degrees C) and CD (40.15 +/- 0.35 degrees C); whereas rectal temperature at the end of exercise was significantly lower in HH (39.1 +/- 0.3 degrees C) than in HD (40.1 +/- 0.3 degrees C) and CD (40 +/- 0.2 degrees C). The PA blood:skin temperature difference was significantly smaller in HD and HH than in CD. When compared with CD, temperatures at all sites were higher in HD and HH during the 60 min of recovery. Throughout exercise and recovery, heart rate was significantly higher in HH when compared with the other conditions. Post exercise respiratory rate was significantly higher in HD and HH than in CD throughout recovery. It was concluded that the added thermal loads of high temperature and relative humidity increased the rate of heat storage during exercise and delayed dissipation of heat during recovery. The impairment to heat dissipation was probably the result of a reduced capacity for heat transfer from the skin to the environment.

Water and ion losses during the cross-country phase of eventing

Loss of total body water and ions during prolonged exercise can predispose the horse to health and performance problems. This study examined total body water (TBW) losses and extracellular (ECF) ion losses during the cross-country (XC) phase of Preliminary, Intermediate and Advanced Horse Trials and CCI level 3-day-events. Jugular venous blood samples and body mass (BM) were collected on 49 horses at rest, Pre-XC, Post-XC and following 30 min of recovery. Plasma was separated from blood cells within 10 min of collection. Plasma was analysed for [Na+], [K+], [Cl-], ionised [Ca+2], [glucose], [lactate] and packed cell volume (PCV) and plasma [protein] ([PP]). Distances ranged from 3000-6000 m for the XC phase with speeds of 500-570 m/min. In general, losses of TBW, Na+ and Cl- increased with increasing level of difficulty. Loss of TBW Post-XC ranged from 2-6.1% of resting values or 8.9-12.6 litres for the Preliminary level to mean +/- s.e. 20.4 +/- 1.8 litres for CCI. Losses of ECF ions ranged from 0.5-6.4% for Na+, 1.2-7.7% for Cl-, gains of 8% to losses of 23% for K+, and gains of 7% to losses of 11.7% for Ca+2 at 30 min Post-XC. There was little recovery following 30 min and deficits of 5.3 +/- 2.5 litres persisted overnight in horses that were tested. Plasma protein and PCV increased Post-XC and PCV remained elevated above resting values at 30 min recovery. It is concluded that significant water and ion losses occur and, in general, increase with increased level of difficulty. There was little change with 30 min of recovery and for some horses losses persisted overnight.

Origins of [H+] changes in exercising skeletal muscle

This brief review describes the main physicochemical factors that contribute to increases in intracellular hydrogen ion concentration ([H+]i) in mammalian skeletal muscle during high intensity exercise. High intensity exercise results in changes in the three main independent physicochemical variables: PCO2, the strong ion difference ([SID]), and total concentration of weak acids and bases ([Atot]), within the intracellular fluid compartment of contracting muscle that result in increased [H+]i. The decrease in [SID] contributes 62% to the increase in [H+]i, due to decreased [K+]i and increased [lactate]i; the decrease in phosphocreatine ([PCr2-]i) exerts an alkalinizing effect. The increase in [Atot], resulting primarily from increases in inorganic phosphate and creatine as a result of PCr2- breakdown, contributes 19% to the increase in [H+]i. An increase in the apparent proton dissociation constant (KA) for [Atot] contributes 7% to the increase in [H+]i. PCO2 is a relatively poor effector of changes in [H+]i, such that a 50-mmHg increase in PCO2 contributes only 12% to the increase in [H+]i during high intensity exercise.

Potassium regulation during exercise and recovery in humans: implications for skeletal and cardiac muscle

This review summarizes the main cellular mechanisms involved in potassium regulation in plasma and skeletal muscle during exercise. The effects of exercise-induced hyperkalemia and post-exercise hypokalemia on the cardiac action potential are reviewed in light of recent research on Na+ and K+ channel activity. Specific consideration is given to K+ release from contracting skeletal muscle, K+ uptake by contracting skeletal muscle, K+ uptake by non-contracting tissues during the period of exercise, and K+ uptake by skeletal muscle recovering from contractile activity. The onset of exercise is associated with a net release of K+ from contracting skeletal muscle that results in an increase in plasma [K+]. Resultant decreases in intracellular [K+] and increases in interstitial [K+] in contracting skeletal muscle have been implicated in the fatigue process. The rate and magnitude of increase in plasma [K+] is dependent on exercise intensity, trained state of the individual, and on drugs such as beta-adrenoceptor blockers and caffeine. During exercise, the uptake of K+ from the blood by non-contracting tissues may be important in preventing plasma [K+] from rising to excessive levels that will impair skeletal muscle and myocardial excitability and contractility. Cessation of exercise results in a rapid decrease in plasma [K+], often to 3 mEq/l or less with intense exercise, that may be maintained for prolonged periods. The rapid increases and decreases in plasma [K+] with onset and cessation of exercise, respectively, has been implicated in altered myocardial function and sudden cardiac death. Recent studies suggest that increases in catecholamines during exercise are cardioprotective to the arrhythmogenic effects of hyperkalemia.

K+ and Lac- distribution in humans during and after high-intensity exercise: role in muscle fatigue attenuation?

This review describes processes for the distribution of K+ ([K+]) and lactate concentrations ([Lac-]) that are released from contracting muscle at high rates during high-intensity exercise. This results in increased interstitial and venous [K+] and [Lac-] in contracting muscle. Large and rapid increases in plasma [K+] and [Lac-] result in the transport of these ions into red blood cells (RBCs). These ions are distributed to noncontracting tissues within both the plasma and RBC compartments of blood. The extraction of K+ and Lac- from the circulation by noncontracting tissue serves to markedly attenuate exercise-induced increases in plasma [K+] and [Lac-]. This apparent regulation of the plasma compartment by noncontracting tissues helps to maintain favorable concentration gradients for the net movement of [K+] and [Lac-] into the venous side of the microcirculation from interstitial fluids of contracting muscle. This provides conditions that 1) reduce the increase in interstitial [K+], thereby decreasing the magnitude and rate of sarcolemmal depolarization, and 2) favor the sarcolemmal transport of Lac- from within contracting muscle cells, thereby regulating intracellular osmolality and H+ concentration. On cessation of exercise, net K+ uptake by recovering muscle is rapid, with 90-95% recovery of intracellular [K+] within 3.5 min, indicating a very high rate of Na+-K+ pump activity. The K+ extracted by noncontracting tissues during exercise may be slowly released during recovery. During the initial minutes of recovery, recovering muscle continues to release Lac- into the circulation, and noncontracting tissues continue to extract Lac- for up to 30 min. The uptake of Lac- by noncontracting tissues results in elevated intracellular [Lac-]. There is no evidence that Lac- extracted by noncontracting tissues is subsequently released; it is probably metabolized within these cells. We conclude that the uptake of K+ and Lac- by RBCs and noncontracting tissues regulates ion homeostasis within plasma and the interstitial and intracellular compartments of contracting muscle. The regulatory processes help to maintain the function of active muscles by delaying the onset of fatigue during exercise and to restore homeostasis during recovery.

Plasma volume and ion regulation during exercise after low- and high-carbohydrate diets

This study compared plasma volume (PV) and ion regulation during prolonged exercise in control vs. glycogen-depleted (GD) conditions, with emphasis on the initial minutes of exercise. In two trials separated by 1-2 wk, four adult males cycled at 75% of peak oxygen consumption (VO2) until exhaustion (50 +/- 7 min for GD) or until the GD exhaustion time in the control trial. Blood was sampled from catheters placed in the brachial artery and retrograde in the femoral vein (fv). Arterial PV decreased rapidly and by 15 min PV was 83% (control) and 88% (GD) of initial. The decrease in PV was accompanied by a net osmotic flux of water from plasma and inactive tissues to contracting muscles. The significantly greater decrease in PV in control compared with GD was associated with a higher muscle lactate content (Lac-; 36 vs. 17 mumol/g dry wt, respectively). Increases in plasma [Cl-] and [Na+] were less than predicted from decreased PV, indicating net loss of these ions from the plasma compartment. Increases in arterial and fv [K+] were 50% greater than could be accounted for by decreased PV, corresponding with increased arterial and fv plasma K+ contents. The rapid net release of K+ and Lac- from contracting muscle during the first few minutes of exercise in both trials was abolished (control) or reversed (GD) within 15 min of beginning exercise.(ABSTRACT TRUNCATED AT 250 WORDS)

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.