Behavioral thermoregulation by hypoxic rats.

To address whether a shift in hypothalamic thermal setpoint might be a significant factor in induction of hypoxic hypothermia, behavioral thermoregulation was examined in 7 female Sprague-Dawley rats implanted with radiotelethermometers for deep body temperature (Tb) measurement in a thermocline during normoxia (PO2 = 125 torr) and hypoxia (PO2 = 60 torr). Normoxic rats (TNox) selected a mean ambient temperature of 19.7 +/- 1.4 (SE) degrees C and maintained Tb at 37.0 +/- 0.2 degrees C. Hypoxic rats selected a significantly higher ambient temperature (THox = 28.6 +/- 2.2 degrees C) but maintained Tb significantly lower at 35.5 +/- 0.3 degrees C. Without a thermal gradient (ambient temperature = 25 degrees C), Tb during hypoxia was 35.4 +/- 0.4 degrees C. The maintenance of a lower body temperature during hypoxia through behavioral thermoregulation despite having warmer temperatures available supports the hypothesis that the thermoregulatory setpoint of hypoxic rats is shifted to promote thermoregulation at a lower Tb, effectively reducing oxygen demand when oxygen supply is limited.

Effect of temperature on chemical control of ventilation in Mexican black iguanas.

The impact of temperature on the chemical control of respiration in the Mexican black iguana Ctenosaura pectinata was examined by measuring ventilatory responses to graded hypoxia with and without 2.9% inspired CO2 at 25, 30, and 35 degrees C. Black iguanas increased pulmonary ventilation in response to hypoxia by increasing both tidal volume and respiratory frequency. Breathing 2.9% CO2 stimulated an increased pulmonary ventilation primarily through increases in tidal volume. The fractional O2 concentration at which ventilation began to increase (hypoxic threshold) varied with temperature, increasing from 0.067 at 25 degrees C to 0.085 at 30 degrees C and 0.112 at 35 degrees C. At 35 degrees C, breathing 2.9% CO2 promoted a further, although statistically insignificant, shift in the ventilatory hypoxic threshold to approximately 0.130 fractional inspired O2 concentration. A "gasping" ventilatory pattern was also observed, the frequency of which increased with progressive hypoxia and increasing temperature. These results suggest that the chemical control of ventilation in this lizard is susceptible to changes in temperature, although the mechanism underlying the temperature sensitivity remains obscure.

Thermoregulatory responses of diabetic rats.

1. Thermal responses and skin microcirculation were measured in streptozotocin-induced diabetic (SD) rats during acute and chronic exposure to ambient (Ta) temperatures ranging from about 5 to 35 degrees C. 2. At 28 degrees C, SD rats had higher rate of oxygen consumptions (VO2), tail skin blood flow (SKBF), but lower rectal temperatures (Tre) than saline-injected controls. 3. Chronic exposure of the SD rats to 35 and 5 degrees C caused a sharp rise and decline in Tre, respectively. 4. At 35 degrees C, hyperthermia in the SD rats was associated with greater increase in VO2 than controls, but changes in SKBF were similar in both groups. 5. At 5 degrees C, VO2 changed similarly in both the SD and control rats, but vasoconstriction was greater in the controls. 6. The data suggest that hypothermia in SD rats may be associated with impairment of vasoconstriction and hyperthermia may be related to an increase VO2 not accompanied by greater vasodilation.

Allometry of red cell oxygen binding and hematology in larvae of the salamander, Ambystoma tigrinum.

Very few studies have attempted to relate blood characteristics to body mass within, rather than between, species. Thus, respiratory and hematological properties of the blood of larval tiger salamanders (Ambystoma tigrinum) have been measured in animals ranging in body mass from approximately 2 to 112 g. This amphibian species was chosen because larvae of very different body mass may be of similar developmental stage, minimizing interference from ontogenetic factors. Mean corpuscular volume, mean corpuscular Hb, hematocrit, blood Hb concentration, blood O2 capacity and Hill's n were all positively correlated with body mass (P less than 0.005). Blood O2 affinity (P50) and Bohr shift were not significantly correlated with body mass (P greater than 0.20). The findings are discussed in the context of the general effect of body mass upon metabolic rate in vertebrates, as well as environmental and biological factors specific to larvae of the tiger salamander.

Elevation of the panting threshold of the desert iguana, Dipsosaurus dorsalis, during dehydration: potential roles of changes in plasma osmolality and body fluid volume.

Dehydration of the desert iguana, Dipsosaurus dorsalis, resulted in a progressive elevation in the magnitude of the skin temperature necessary to elicit thermal panting (i.e., the panting threshold). Panting threshold increased from 43.4 +/- 0.8 degrees C at 100% initial body weight (IBW) to 45.4 +/- 1.2 degrees C at 90% IBW to 45.7 +/- 0.9 degrees C at 80% IBW. Plasma osmolality showed no significant change with dehydration to 80% IBW. Changes in plasma osmolality, whether induced by NaCl or non-ionic sucrose loading, had a significant impact on panting threshold. Increasing plasma osmolality resulted in an elevation of panting threshold while decreasing plasma osmolality resulted in lower panting thresholds. Decreasing body fluid volume by exsanguination of 1 ml whole blood/100 g body weight resulted in a mean increase in panting threshold by 0.7 +/- 0.2 degrees C. Volume loading with 160 mM NaCl (approximately isosmotic) had no significant effect on panting threshold. These data suggest that plasma osmolality and decreases in body fluid volume may be potent modulators of panting threshold during periods of water deprivation. However, at least in desert iguanas, increases in plasma osmolality would not appear to be an important factor in the elevation of panting threshold during dehydration to 80% IBW.

Control of panting in the desert iguana: roles for peripheral temperatures and the effect of dehydration.

Although it is generally held that panting is a physiological mechanism for the regulation of brain temperature during heat stress, a number of studies have pointed to the importance of peripheral input for the initiation of the panting response in a variety of animals. By presenting ambient heat loads of 47 degrees, 54 degrees, 58 degrees, and 65 degrees C, and measuring skin, ear and core temperatures of the desert iguana, Dipsosaurus dorsalis, at the onset of panting, we found that the skin temperature at panting onset was independent of ambient heat load. This suggests that skin (peripheral) temperature is the body temperature on which the central thermoregulatory center cues to initiate thermal panting. Peripheral temperature control of panting was retained when the plasma osmolality of the desert iguana was increased by 100 mOsm/kg H2O to simulate dehydration. Dehydration to 80% initial body weight (IBW) resulted in a progressive increase in panting threshold (skin) from 42 degrees C for untreated lizards to 42.5 degrees C at 90% IBW to 43.3 degrees C at 80% IBW. Injection of 80% IBW lizards with a volume of 10 mM NaCl equivalent to weight loss resulted in a decrease in panting threshold to 40.8 degrees C. Injection with 1% body weight 3000 mM NaCl produced a dramatic increase in panting threshold to 45.9 degrees C. These data suggest that the desert iguana responds to dehydration by elevating panting threshold, thus promoting water conservation. These data also suggest that changes in plasma osmolality may be involved in the "setting" of panting threshold.

Static lung compliance during the development of the bullfrog, Rana catesbeiana.

The static compliance of excised lungs was measured during the metamorphic development of the bullfrog, Rana catesbeiana. Absolute static compliance of excised lungs increased with developmental stage from 0.02(+/- 0.01 SD) ml/cm H2O at Taylor-Kollros (TK) stage III to 34.67(+/- 15.00 SD) ml/cm H2O for adults. When static lung compliance was standardized by dividing by the lung volume at 4 cm H2O transmural pressure (approximately maximal capacity), lung compliance was relatively constant during early development and increased at the onset of metamorphic climax (TK stage XX). Although the absolute lung compliances of developing tadpoles are small, the volume-specific compliances are three to four times greater than those of some mammals. The compliance-independent index of hysteresis also increased during development from 0.0008(+/- 0.0006 SD) cm at TK stage V to 1.36(+/- 1.21 SD) cm for adults suggesting increases in pulmonary structural complexity and/or an increase in the production and secretion of a pulmonary surfactant-like substance. These developmental changes in pulmonary compliance and hysteresis mirror the increasing utility of the lung for gas exchange during the metamorphosis of the bullfrog.

Allosteric control of pyrimidine transport in Hymenolepis diminuta: an unusual kinetic isotope effect.

A thymine-uracil transport system is present in the plasma membrane brush-border of the tapeworm, Hymenolepis diminuta. The relation between initial uptake and substrate concentration of either thymine or uracil was sigmoidal when 2-14C-labelled substrates were used. In contrast, absorption kinetics of methyl-14C- and 3H-labelled substrates were hyperbolic. Since there was no metabolism of the labelled substrates during the incubation period, these differences indicate that the sigmoidal kinetics of pyrimidine transport in this organism is an isotopic effect associated with the presence of the 14C label at the 2 position in the pyrimidine ring.

The role of diet in glycogen storage by juvenile bullfrogs prior to overwintering.

Juvenile bullfrogs from natural populations doubled their glycogen stores during their predormancy period, while their blood glucose and lipid stores declined. Glycogen stores were replenished in natural populations as food consumption decreased. Some liver glycogen was stored during fasting if the bullfrogs were collected in the storage phase of their energy reserve cycle, but the amount of glycogen stored was increased by feeding.