S. Kozlowski

Greater serum GH response to arm than to leg exercise performed at equivalent oxygen uptake.

SummaryThe aim of this study was to provide information concerning the mechanism of exercise-induced stimulation of growth hormone (GH) release in human subjects. For this reason serum GH as well as some hemodynamic variables and blood concentrations of noradrenaline (NA), insulin (IRI), lactate (LA), glucose (BG), and free fatty acids (FFA) were determined in seven healthy male subjects exercising on a bicycle ergometer with arms or legs and running on a treadmill at equivalent oxygen consumption levels. Significantly greater increases in serum GH concentration accompanied arm exercises than those observed during the leg exercises. This was accompanied by greater increases in heart rate, blood pressure, and plasma NA and blood lactate concentrations. Serum IRI decreased during both leg exercises and did not change during the arm exercise. There were no differences in BG and plasma FFA concentrations between the three types of exercise.The role of humoral and neural signals responsible for the greater GH response to arm exercise is discussed. The findings are consistent with the hypothesis that neural afferent signals sent by muscle “metabolic receptors” participate in the activation of GH release during physical exercise. It seems likely that the stimulation of these chemoreceptors is more pronounced when smaller muscle groups are engaged at a given work load. However, a contribution of efferent impulses derived from the brain motor centres to the control system of GH secretion during exercise is also possible.

Plasma testosterone and catecholamine responses to physical exercise of different intensities in men

SummaryPlasma testosterone, noradrenaline, and adrenaline concentrations during three bicycle ergometer tests of the same total work output (2160 J·kg−1) but different intensity and duration were measured in healthy male subjects. Tests A and B consisted of three consecutive exercise bouts, lasting 6 min each, of either increasing (1.5, 2.0, 2.5 W·kg−1) or constant (2.0, 2.0, 2.0 W·kg−1) work loads, respectively. In test C the subjects performed two exercise bouts each lasting 4.5 min, with work loads of 4.0 W·kg−1. All the exercise bouts were separated by 1-min periods of rest.Exercise B of constant low intensity resulted only in a small increase in plasma noradrenaline concentration. Exercise A of graded intensity caused an increase in both catecholamine levels, whereas, during the most intensive exercise C, significant elevations in plasma noradrenaline, adrenaline and testosterone concentrations occurred. A significant positive correlation was obtained between the mean value of plasma testosterone and that of adrenaline as well as noradrenaline during exercise.It is concluded that both plasma testosterone and catecholamine responses to physical effort depend more on work intensity than on work duration or total work output.

Thermoregulation in hyperhydrated men during physical exercise

SummaryThe influence of hyperhydration on thermoregulatory function was tested in 8 male volunteers. The subjects performed cycle exercise in the upright position at 52%VO2max for 45 min in a thermoneutral (Ta=23° C) environment. The day after the control exercise the subjects were hyperhydrated with tap water (35 ml · kg−1 of body weight) and then performed the same physical exercise as before.Total body weight loss was lower after hyperhydration (329±85 g) than during the control exercise (442±132 g),p<0.05. The decrease in weight loss after hyperhydration was probably due to a decrease in dripped sweat (58±64 and 157±101 g,p<0.05). With hyperhydration delay in onset of sweating was reduced from 5.8±3.2 to 3.7±2.0 min (p<0.05), and rectal temperature increased less (0.80±0.20 and 0.60±0.10° C,p<0.01). The efficiency of sweating was higher in hyperhydrated (81.4%) than in euhydrated subjects (57.1%),p<0,01. It is concluded that hyperhydration influences thermoregulatory function in exercising men by shortening the delay in onset of sweating and by decreasing the quantity of dripped sweat. As a result, the increases in body temperature in hyperhydrated exercising men are lower than in normally hydrated individuals.

Mechanism of sympathetic activation during prolonged physical exercise in dogs

It seems likely that depletion of body carbohydrates may account for the rise in the sympathetic activity during prolonged exercise, since glucose given during or before exercise reduces the increase in plasma catecholamines. The aim of the present study was to find out whether the increase in plasma noradrenaline (NA) in response to exercise can be reduced by 1. increasing of the amount of carbohydrate available for metabolism without producing hyperinsulinemia and 2. by inhibition of afferent activity from hepatic glucoreceptors. The study was performed on dogs which exercised whilst receiving either the intravenous fructose infusion (2.2 mmol/min) or a slow glucose infusion (0.25 mmol/min) which was given either via the portal or a peripheral vein. Fructose infusion reduced the muscle glycogen depletion during exercise and reduced the increase in plasma NA and glycerol concentrations without altering the blood glucose or insulin levels. The exercise-induced increases in plasma NA and gycerol concentrations were significantly smaller with intraportal than with peripheral glucose infusion but there were no differences between these two cases in the concentration of glucose in the systemic circulation. These findings indicate that the reduction of the plasma NA response to physical effort under conditions of increased carbohydrate availability cannot be attributed to the inhibitory effect of insulin on sympathetic activity and provide evidence for the participation of hepatic glucoreceptors in the control of the sympathetic activity during exercise.

ADH and thermal sweating.

SummarySweating responses to heat exposure were compared in healthy subjects pretreated with pitressin or alcohol and in the control group. Between the three groups, there were no consistent differences in the rate of sweating expressed both as a total body weight loss during 2-h heat exposure and in mg of sewat per skin area covered by a paper disc. Likewise, there were no differences in the sweat osmolality or electrolyte concentration. There was also no evidence of inverse correlation between plasma ADH level and rate of sweat secretion or its concentration when pooled data of all subjects were analyzed. It was concluded that ADH did not substantially affect thermal sweating in men.

Lipoprotein lipase activity in the skeletal muscle during physical exercise in dogs

Changes in lipoportein lipase activity/LPLA/ in the quadriceps femoris muscle were followed in ten dogs during 3-hour treadmill exercise and 2-hour post-exercise recovery period. During the first hour of exercise muscle LPLA steeply increased. Subsequently the enzyme activity tended to plateau. After exercise LPLA decreased to the pre-exercise value within 1 hour. It is concluded that exercise increases the ability of skeletal muscles to remove triglycerides from the circulation.

The effect of intraventricular thyroxine administration on body temperature in dogs at rest and during physical exercise

Infusion of 1 μg thyroxine into the left cerebral ventricle of the dog did not change body temperature at rest, but it caused significantly higher increases in Tre during physical exercise.

Antagonistic effects of vasopressin and hypervolemia on osmotic reactivity of the thirst mechanism in dogs

SummaryThe reactivity of the thirst mechanism to osmotic stimuli was examined in conscious dogs 1. under control conditions, 2. after raising the plasma vasopressin (PADH) level to about 30 μU/ml by intravenous infusion of the hormone, 3. after expansion of the blood volume by 15% by an intravenous infusion of dextran solution, and 4. after a simultaneous increase ofPADH and blood-volume expansion.The osmotic thirst threshold was significantly lowered by the elevation ofPADH and augmented by an expansion of blood volume, whereas no significant changes were observed when the increase inPADH and expansion of blood volume were applied simultaneously.The interactions between body-fluid osmolarity, blood volume, and vasopressin in regulation of water intake are discussed.

Sympathetic Activity During Prolonged Physical Exercise in Dogs: Control of Energy Substrate Utilization

The activation of sympathetic nervous system during exercise has been well documented [3, 8, 10, 16, 17, 20]. There are few data however on the activity of adrenergic system during prolonged muscular work. Karki [14] described distinctly increased catecholamine excretion during long-term exercise in human subjects. The studies of Juchmes et al. [13] showed that during one-hour exercise in men plasma noradrenaline (NA) increased progressively whereas, adrenaline (A) did not display a uniform pattern of response. In rats after long-term exhaustive exercise a decrease in plasma NA [6] and the depletion of catecholamine stores in the adrenal medulla [11] were described.

Enhanced glucose availability for working muscles reduces exercise hyperthermia in dogs

SummaryBody temperature and metabolic responses to 2 h treadmill exercise in dogs given glucose intravenously (25–30 mg·kg−1· min−1 throughout the run) were compared with those measured in the same animals with elevated plasma FFA concentrations (soya bean oil ingestion+intravenous heparin) and in control experiments (24 h fasting). In comparison with control conditions enhanced glucose availability for the working muscles caused a reduction in the exercise-induced increases in both rectal (by 0.9± 0.11° C) and muscle (by 0.9±0.16° C) temperatures, a lower rate of oxygen uptake (by 16%) and an elevated respiratory exchange ratio. A tendency towards enhanced body temperature responses to exercise, accompanied by increases inVO2 and cardiac frequency was noted in dogs with elevated plasma FFA concentrations as compared with the control animals. The estimated amount of heat effectively dissipated from the body, expressed as a fraction of heat load (thermoregulatory efficiency) was significantly higher in dogs infused with glucose (0.962±0.0035), than in the controls (0.947± 0.0043) and those with elevated plasma FFA concentrations (0.931±0.0029).It is concluded that the increased contribution of carbohydrates to the energy yield during exercise results in a marked attenuation of hyperthermia, associated with a reduced metabolic rate and improved thermoregulatory efficiency.