Correlation between the heat resistance of the provisional and definitive muscle tissue during the metamorphosis crisis in frogs

Using tadpoles of the lake frog Rana ridibunda Pall. during metamorphosis, a study was made of the heat resistance of the provisional muscle tissue of the tail and of that of two definitive muscles belonging to low-resistant (musculus iliofibularis) and high-resistant (musculus gastrocnemius) groups. It has been shown that during the late metamorphosis a statistically significant direct relation exists between the heat resistance of the provisional muscle tissue of the tail and definitive m. iliofibularis. A comparison with the earlier published data points to a positive correlation between each of these two characteristics and the survival of larvae at high injurious temperature.     

The relation of changes in the individual levels of heat resistance of muscle tissue to their initial values during heat acclimation of Asellus aquaticus

1. 1.|In hog slater, Asellus aquaticus, five extremities were consecutively isolated in the course of heat acclimation to study the pattern of changes in the level of the heat resistance of muscle tissue of each single specimen.

2. 2.|The initial response of the population, during acclimation, is for the muscle resistance of different individuals to become less varied. Then a simultaneous increase in tissue resistance occurs in all ammals, which is complete by the 6th day of acclimation. Afterwards the heat resistance of muscles in the majority of animals shows little change and then, in spite of maintenance of acclimation, it starts to return to its initial level.

3. 3.|Thermal acclimation causes a temporary decrease in the variability of the heat resistance of the muscle tissue and also a temporary stabilization of this physiological characteristic to a new level. This phenomenon is a phenotypical masking of genotypic differences in a physiological characteristic in the population studied during changes in environmental temperature.

4. 4.|At all the stages of acclimation the relation of individual increases in cellular heat resistance to their initial levels follows a hyperbolic exponential equation. This implies that to a rise in environmental temperature a population responds as an integral functional system.

Skeletal muscle metabolism during exercise is influenced by heat acclimation

The influence of heat acclimation on skeletal muscle metabolism during submaximal exercise was studied in 13 healthy men. The subjects performed 30 min of cycle exercise (70% of individual maximal O2 uptake) in a cool [21 degrees C, 30% relative humidity (rh)] and a hot (49 degrees C, 20% rh) environment before and again after they were heat acclimated. Aerobic metabolic rate was lower (0.1 l X min-1; P less than 0.01) during exercise in the heat compared with the cool both before and after heat acclimation. Muscle and plasma lactate accumulation with exercise was greater (P less than 0.01) in the hot relative to the cool environment both before and after acclimation. Acclimation lowered (P less than 0.01) aerobic metabolic rate as well as muscle and plasma lactate accumulation in both environments. The amount of muscle glycogen utilized during exercise in the hot environment did not differ from that in the cool either before or after acclimation. These findings indicate that accumulation of muscle lactate is increased and aerobic metabolic rate is decreased during exercise in the heat before and after heat acclimation; increased muscle glycogen utilization does not account for the increased muscle lactate accumulation during exercise under extreme heat stress; and heat acclimation lowers the aerobic metabolic rate and muscle and blood lactate accumulation during exercise in a cool as well as a hot environment.     

Thyroid activity in common frogs and the heat resistance of their bodies and muscle tissue

Thyroid activity of single individuals of the grass frog has been studied during winter hybernation (2-5 degrees) and heat acclimation (15 degrees) in relation to the heat resistance of the organism and muscle tissue at 2-5 degrees. A positive correlation has been observed between thyroid activity and the organismal heat resistance. At 15 degrees thyroid activity increases. It is the highest in females with a low initial heat resistance of the organism. It is assumed that changes in the heat resistance of the organism and muscle tissue are controlled by the thyroid gland. However, no correlation has been found between the initial (winter) level of the heat resistance of muscle tissue and the activity of thyroid.     

Effects of dietary sodium on body and muscle potassium content during heat acclimation

It has been suggested that renal conversion of sodium (Na+) during training in hot environments results in potassium (K+) deficiencies. This investigation examined the influence of two levels of dietary Na+ intake (399 vs 98 mmol X d-1) on intramuscular, urinary, sweat, and whole body K+ homeostasis. Nine unacclimated, untrained males underwent heat acclimation during two 8 day dietary-exercise regimens (40.1 +/- 0.1 degrees C, 23.5 +/- 0.4% RH). Both diets resulted in depressed urinary K+ excretion. Sweat K+ and muscle K+ concentrations were not altered by diets or acclimation. The whole body stores of Na+ increased 31.1% (+916.8 mmol) during the high Na+ diet and decreased 7.8% (-230.4 mmol) during the low Na+ diet; whole body stores of K+ increased 4.1% (+137.6 mmol) during the high Na+ diet and increased 3.4% (+113.6 mmol) during the low Na+ diet. This dietary-acclimation protocol did not result in whole-body or intramuscular K+ deficits and offers no evidence to support previous claims that dietary sodium levels affect K+ balance.     

Substrate utilization in leg muscle of men after heat acclimation

Eight men were heat acclimated (39.6 degrees C and 29.2% rh) for 8 days to examine changes in substrate utilization. A heat exercise test (HET), (cycling for 60 min; 50% maximal O2 consumption) was performed before (UN-HET) and after (ACC-HET) the acclimation period. Muscle glycogen utilization (67.0 vs. 37.6 mmol/kg wet wt), respiratory exchange ratio (0.85 +/- 0.002 vs. 0.83 +/- 0.001), and calculated rate of carbohydrate oxidation (75.15 +/- 1.38 vs. 64.80 +/- 1.52 g/h) were significantly reduced (P less than 0.05) during the ACC-HET. Significantly lower (P less than 0.05) femoral venous glucose (15, 30, and 45 min) and lactate (15 min) levels were observed during the ACC-HET. No differences were observed in plasma free fatty acid (FFA) and glycerol concentrations or glucose, lactate and glycerol arteriovenous uptake/release between tests. A small but significant increase (P less than 0.05) above resting levels in FFA uptake was observed during the ACC-HET. Leg blood flow was slightly greater (P greater than 0.05) during the ACC-HET (4.64 +/- 0.13 vs. 4.80 +/- 0.13 l/min). These findings indicate a reduced use of muscle glycogen following heat acclimation. However, the decrease is not completely explained by a shift toward greater lipid oxidation or increased blood flow.     

Changes in the properties of the enzymes of energy allowance resulting from heat shock in lake frogs

A study was made of the activity and heat resistance of preparations of Na, K-ATPase, Mg-ATPase and succinate dehydrogenase of the lake frog R. ridibunda caused by the heat shock of animals. A decrease in the activity and an increase in the heat resistance of all the three enzymes studied were observed. The level of individual correlations between these parameters remained unchanged. An increase in the heat resistance of Na, K-ATPase occurs at the expense of a higher threshold of its thermal inactivation without changes in the affinity of this enzyme to Na+ and K+. Under discussion is the question of the functional significance of the changes observed.     

The effect of photoperiod on changes in plasma, glucose, cholesterol, and free fatty acids during cold acclimation in frogs

1. Many previous experiments on cold acclimation in frogs were carried out on animals maintained in total darkness. I tested the effect of photoperiod on changes in plasma FFA, glucose, and cholesterol during cold acclimation. 2. Hematocrit levels were lower whereas plasma FFA levels were higher in frogs kept in total darkness during cold acclimation. 3. Plasma glucose and plasma cholesterol levels were not affected by photoperiod during cold acclimation.     

Functional and morphological changes in the eccrine sweat gland with heat acclimation

Three adult male patas monkeys (11-15 kg) were heat acclimated by continuous exposure to an ambient temperature of 33 +/- 1 degree C at 13% relative humidity for 9 mo. During the last month, they were also exposed to 45 degrees C at 10% relative humidity for 4 h/day and 5 days/wk. Before and after 3 wk of acclimation, the animals were given a heat-tolerance test in which rectal (Tre) and mean skin (Tsk) temperatures, heart rate, and sweat rate (msw) were monitored during a 90-min exposure to 45 degrees C heat with 24% relative humidity under lenperone (1.0-1.4 mg/kg im) tranquilization. Maximal in vivo msw was also determined in response to subcutaneous injections (1 and 10% solutions) of methacholine (MCh). Before and after 9 wk and 9 mo of acclimation, sweat glands were dissected from biopsy specimens of the lateral calf, cannulated, and stimulated in vitro with MCh. Morphological measurements of isolated tubules were compared with maximal secretory rates produced by MCh stimulation. Three weeks of acclimation 1) reduced Tre and Tsk and increased msw during the heat tolerance test and 2) significantly increased maximal msw in response to MCh stimulation. Acclimation also increased (P less than 0.05) sweat gland size, as measured by tubular length and tubular volume. Maximal in vitro msw produced by MCh stimulation and msw per unit length of secretory coil also increased significantly. We conclude that heat acclimation increases the size of eccrine sweat glands and that these larger glands produce more sweat. They are also more efficient because they produce more sweat per unit length of secretory coil.     

Plasma opioid peptide responses during heat acclimation in humans

Plasma beta-endorphin, Met-enkephalin and Peptide F immunoreactivity (ir) were measured at rest and following exercise on three days (days 1, 4, 8) of an eight day heat acclimation regime. Fourteen male subjects demonstrated physiological heat acclimation adaptations. Our data demonstrated a differential response of peripheral plasma levels of endogenous opioid peptides (EOP) to exercise in the heat. In addition, EOP did not follow the same time-course of other physiological adaptations as no differences (day 1 vs. 4 vs. 8) in resting or exercise levels were observed over the eight day heat acclimation regime. Significant increases in beta-endorphin ir (pre- to post-exercise) appear to reflect concomitant exercise-heat related changes. The increased peripheral levels of beta-endorphin were correlated to plasma levels of cortisol. Heat and exercise stress may result in a reduction of Met-enkephalin ir observed in peripheral plasma and might be due to degradation or a decrease in processing from the larger precursors. The differential responses of EOP suggest the possibility of separate physiological roles for these peptides during exercise in the heat but peripheral plasma levels of EOP do not appear to reflect acute heat acclimation changes.     

Sweat rate and sweat composition during heat acclimation

The purpose of this study was to determine local sweat rate (LSR) and sweat composition during heat acclimation (HA). For ten consecutive days of HA, eight participants cycled in 33 °C and 65% relative humidity at an intensity such that a rectal temperature of 38.5 °C was reached within ~40 min, followed by a 60-min clamp of this rectal temperature (i.e., controlled hyperthermia). Four participants extended HA by a 28-day decay period and five consecutive days of heat re-acclimation (HRA) using controlled hyperthermia. Sweat from the upper arm and upper back was collected three times during each heat exposure session. LSR and sweat sodium, chloride, lactate, and potassium concentrations were determined. Relative to HA day 1, LSR was increased at the final day of HA (day 10) (arm: +58%, P < 0.001; back: +36%, P < 0.05). Concentrations of sodium, chloride, and lactate significantly (P < 0.05) decreased to ~60% at HA day 10 compared to day 1 on the arm and back. Potassium concentration did not significantly differ on HA day 10 compared to day 1 (arm: +11%, P > 0.05; back: +8%, P > 0.05). The induction patterns of the sudomotor adaptations were different. Whilst LSR increased from HA day 8 on the arm and from HA day 7 on the back, sodium and chloride conservation already occurred from HA day 3 on both skin sites. Lastly, the sweat lactate reduction occurred from HA day 6 on the arm and back. Initial evidence is provided that adaptations were partly conserved after decay (28 days) and that a 5-day HRA may be sufficient to restore HA adaptations. In conclusion, ten days of exercise-induced HA using controlled hyperthermia led to increases in LSR and concomitant reductions of sweat sodium, chloride, and lactate concentrations, whilst potassium concentrations remained relatively constant.     

Regulation of submaxillary gland muscarinic receptors during heat acclimation

Binding properties of submaxillary gland muscarinic receptors and agonist-induced saliva secretion were studied in rats subjected to heat acclimation. The maximal binding capacity for the muscarinic antagonist N-[3H]methyl-4-piperidyl benzilate was increased from control value of 0.21 to 0.40 pmol/mg protein within 1-2 days of heat acclimation. The increase in the number of muscarinic receptors per gland (100%) was by far higher than the increase in tissue weight (20%), indicating higher density of receptors in the acinar cells of the treated rats. High levels of receptors coincided with the appearance of high-affinity binding sites for muscarinic agonists (oxotremorine, pilocarpine and carbamylcholine), and with reduced tissue sensitivity to pilocarpine. After 4-8 weeks of heat acclimation, the number of receptors as well as tissue response to pilocarpine returned to control levels. These results suggest a functional correlation between the transient upregulation muscarinic receptors in the submaxillary gland and the physiological activity in salivary secretion, and indicate that the high-affinity muscarinic receptors may attenuate saliva secretion during the initial phase of heat acclimation.