Physiological Zoology

Rainbow trout exposed to unbuffered water of pH 10.5 initially showed significant increases in blood pH, plasma cortisol and glucose, partial pressure of NH3 (PNH3), NH4+, and HCO3- values as well as loss of plasma Cl-, reduced partial pressure of CO2 (PCO2), and inhibition of total ammonia excretion rate. After the first day, fish resisted further change, and new levels were established (for blood pH and plasma PCO2 and PNH3 levels) or imbalances corrected, either partially (for total ammonia excretion) or completely (for plasma Cl-, HCO3-, cortisol, and glucose values). During the 7-d exposure, 80% of fish in unbuffered water survived, but in buffered water (0.75 mmol L(-1) glycine-buffered KOH at pH 10.5), survival was only 50% after 3 d, and ion regulatory failure was evident. Fish in buffered and unbuffered alkaline waters had similar total ammonia excretion rates, which suggests that glycine-buffered KOH was not sufficient to significantly reduce gill boundary layer acidification. After 7 d in unbuffered alkaline water, 30% of total ammonia excretion was linked with an amiloride-sensitive (0.1 mmol L(-1)) Na+ uptake mechanism. Treatment of alkaline-exposed trout with waterborne acetazolamide (1.5 mmol L(-1)) indicated that gill boundary layer H+ production, through hydration of CO2, had a role in excretion of total ammonia. Exposure to 4-acetamino-4'-isothiocyantostilbene-2,2'-disulphonic acid (SITS; 0.1 mmol L(-1)) following 24-h exposure to unbuffered alkaline water resulted in increased plasma HCO3- and lowered plasma Cl- concentrations, indicating the role of branchial Cl-/HCO3- exchange in regaining Cl- lost and eliminating the HCO3- accumulated during exposure to alkaline water.

Plasma alkaline phosphatase and inorganic phosphorus levels were determined for 52 nestling Spanish imperial eagles from two wild populations and 22 captive adults and subadults (10 adults and 12 subadults). The exact age was known for all birds. Mean alkaline phosphatase and inorganic phosphorus were higher in chicks than in the captive adults and subadults. Sex differences were not observed, and nestlings from different populations showed similar values. No significant regression described the relationship between age and alkaline phosphatase or inorganic phosphorus throughout the nestling period. However, alkaline phosphatase and inorganic phosphorus decreased significantly throughout the subadult period, with age explaining 98.2% and 50.5% of the variation in alkaline phosphatase and inorganic phosphorus levels, respectively. Non-fully-ossified zones were measured in frontal bones of another 12 subadult eagles that died at known ages. Ossification increased throughout the subadult period and was significantly correlated with expected levels of alkaline phosphatase or inorganic phosphorus (i.e., values predicted from the regression equations derived from the first analysis). Minimum alkaline phosphatase levels and full ossification of the cranial roof coincided with puberty onset. We conclude that, in subadult Spanish imperial eagles, decreasing alkaline phosphatase and inorganic phosphorus values are related to the ossification of frontal bones, although a contribution of other unknown processes of late ossification cannot be excluded, and alkaline phosphatase (but not inorganic phosphorus) may be a useful parameter for age-predicting purposes.

The feeding energetics of the Antarctic spiny plunderfish (Harpagifer antarcticus) were examined with respect to the effect of both ration size and animal size. Fish of different sizes were fed single meals at one of two ration levels (2.5% wet body mass and satiation) to determine the maximum aerobic scope that could be elicited by the specific dynamic action. The excretion rates of ammonia, urea, and fluorescamine-positive substances were also monitored. Neither fish size nor ration had any effect on the factorial aerobic scope of feeding, which suggests that cellular metabolic processes associated with feeding were satiated by relatively small meals. The factorial scope in ammonia excretion was affected by both ration and fish size, indicating that respiration and excretion respond to a meal independently. The duration of the specific dynamic action response (240-390 h) increased with fish size but not ration, whereas both the time to reach the peak oxygen consumption and the duration of the ammonia response increased with ration but not fish size. The percentage of the ingested energy that was expended following feeding (the specific dynamic action coefficient) was high at low rations (approximately 56%) but lower (roughly 10%) at satiation rations. This is because the absolute energetic cost of processing a meal was largely independent of meal size. The change in O:N ratios after feeding was very ration-dependent; at low rations, O:N ratios increased, whereas at satiation rations, the O:N ratios decreased.

We examined oxygen consumption and total evaporative water loss of aardwolves (Proteles cristatus) at temperatures within and below their thermal neutral zone during both summer and winter. During summer (December), body masses of aardwolves averaged 8.1 +/- 0.7 kg (+/-1 standard deviation). Within their thermal neutral zone, oxygen consumption was 2,194 +/- 443 mL O2 h-1, or 1,058 kJ d-1. The relationship between oxygen consumption (VO2, mL O2 h-1) and ambient temperature (Ta, degree C) below the lower critical temperature was VO2 = 6,310-178 (Ta). During winter (August), aardwolves had an average mass of 7.8 +/- 0.7 kg and a basal metabolic rate of 1,844 +/- 224 mL O2 h-1, or 889 kJ d-1. Below the thermal neutral zone, VO2 = 4,308-116 (Ta). Basal metabolic rate and the slope of the line relating oxygen consumption to ambient temperature were both significantly higher in summer than in winter. Evaporative water loss increased with air temperature for both seasons but was higher in summer than winter. Wet thermal conductance was relatively constant below the thermal neutral zone, but was significantly higher in summer (0.022 +/- 0.001 mL O2 g-1 h-1 degree C-1) than in winter (0.015 +/- 0.001 mL O2 g-1 h-1 degree C-1).

Nectarivore sugar preferences and nectar composition in the Cape Floristic Kingdom (southern Africa) differ from trends reported for analogous systems in America and Europe in that sugarbirds and sunbirds show no aversion to sucrose, which is the dominant nectar sugar in many of their food plants. To elucidate the physiological bases (if any) of nectarivore sugar preferences, we determined apparent sugar absorption efficiencies in a passerine endemic to this region, the Cape sugarbird Promerops cafer. Apparent absorption efficiencies for the three major nectar sugars, sucrose, glucose, and fructose, were extremely high (> 99%), as in other specialized avian nectarivores. Xylose, a pentose sugar recently reported in the nectar of some Proteaceae, was absorbed and/or metabolized inefficiently, with a mean of 47.1% of ingested sugar recovered in cloacal fluid. We did not measure the proportions of xylose that were absorbed and/or metabolized. We also compared three methods of estimating absorption efficiency: (1) measurements of total sugar in cloacal fluid with refractometry, without correction for differences between volumes of ingesta and excreta; (2) the same measurements combined with correction for volume differences; and (3) HPLC analyses quantifying individual sugars in cloacal fluid, with correction for volume differences. Refractometry has been frequently used in previous studies. For all sugars except xylose, method 1 yielded results similar to those obtained with method 2, but the convergence was artifactual, and we do not recommend use of this method. Apparent absorption efficiencies calculated with method 2 underestimated true absorption efficiency, because refractometry measures nonsugar solutes, but this error is biologically significant only when efficiencies are low.

Terrestrial amphibians take up water by abducting the hind limbs and pressing a specialized portion of the ventral skin to a moist surface, using a characteristic behavior called the water absorption response. An assay of the water absorption response was used to quantify physiological factors associated with thirst and water uptake. Dramatic changes in the water absorption response resulted from subtle changes in hydration state and from altering the reserve water supply in the urinary bladder. The water absorption response could be induced by intraperitoneal and intracerebroventricular injection of angiotensin II, demonstrating that components of the renin-angiotensin system on both sides of the blood-brain barrier have a dipsogenic function in amphibians. These experiments also demonstrated that the water absorption response could be influenced by changes in barometric pressure. Toads avoided the water absorption response on hyperosmotic substrates, and behavioral experiments showed that the amphibian skin served a sensory function similar to that of the lingual epithelium of mammals. The water absorption response assay has enormous potential as a tool for the investigation of physiological processes and sensory capabilities of amphibians.

In an earlier study, we found that yellow-rumped warblers had in vitro active uptake rates of D-glucose that were only a few percent of the glucose absorption rate achieved at the whole-animal level. Here we used a pharmacokinetic technique to test whether a substantial amount of sugar can be absorbed passively. We used yellow-rumped warblers (Dendroica coronata), known for their seasonal frugivory, freely feeding on a synthetic mash formulated with naturally occurring concentrations of D-glucose. Birds absorbed 89.8% +/- 1.0% (SE) of the D-glucose in the mash. When fed the same mash with trace-labeled 3H L-glucose, the stereoisomer that does not interact with the intestinal Na(+)-glucose cotransporter, 3H appeared in plasma, an indication that this stereoisomer of glucose was absorbed. We used 3H levels in plasma and excreta in a pharmacokinetic model to calculate L-glucose extraction efficiency (i.e., the percent absorbed). Calculated mean extraction efficiency for the passively absorbed L-glucose averaged 91% +/- 23%. Our finding of considerable passive absorption reconciles the in vitro and in vivo results for D-glucose absorption and is in concert with results from five other avian species. The passive pathway appears to provide birds with an absorptive process that can respond quickly to changing luminal concentration and that is energetically inexpensive to maintain and modulate in real time but that may bear a cost. Less discriminate passive absorption might increase vulnerability to toxins and thus constrain foraging behavior and limit the breadth of the dietary niche.

Metabolic consequences of osmotic stress were investigated in the toad Bufo viridis. Toads were acclimated either to terrestrial conditions in the absence of free water or to being partially immersed in 250 mmol L-1 NaCl, which was achieved by gradually increasing the salinity of the bath. This slow acclimation evoked little metabolic response, whereas the immediate osmotic challenge of water restriction resulted in a significant increase in the concentration of urea in the plasma and in liver glycogen. Urea accumulation, involving a transient increase in its rate of synthesis, allows the toads to lower their body water potential and thereby to absorb soil-bound water. The metabolic cost of this response is reduced by conserving the resulting by-product, glucose, as glycogen stored in the liver for future use.

Channel catfish were collected on 11 different dates from October 1991 to July 1993 and acclimated in the laboratory to 7 degrees C, 15 degrees C, or 25 degrees C for 6 wk. Hepatosomatic index, mg protein mg-1 DNA, total liver DNA and protein, and the activities of liver glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, lactate dehydrogenase, and malate dehydrogenase were measured to examine seasonal variation in the acclimation response. Liver and muscle cytochrome oxidase and lactate dehydrogenase activities were measured to compare tissue-specific responses. Hepatosomatic indexes of fall and winter channel catfish were highest at 7 degrees C, with values at 15 degrees C higher than at 25 degrees C, while spring and summer fish had the highest values at 15 degrees C, with values at 7 degrees C higher than those at 25 degrees C. Acclimation patterns for total liver protein and DNA, mg protein mg-1 DNA, and glycogen were generally higher in cold temperatures but varied seasonally in an unpredictable manner. Glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, and malate dehydrogenase demonstrated positive acclimation in the fall and winter; fish collected in the spring and summer showed little or inverse acclimation. Liver lactate dehydrogenase activity showed little or no positive compensation at any time of the year. Cytochrome oxidase activity showed positive acclimation in muscle but not liver. All liver enzymes, even those that showed marginal acclimation on a protein basis, showed positive acclimation when activity was expressed on a whole-liver basis.

Physiological acclimation that alters enzyme activity can compensate for the effect of temperature on function and may be achieved by altering enzyme concentration. This study uses phylogenetic analyses to investigate the evolutionary history of and to test several hypotheses about acclimation responses among all the glycolytic enzymes. These hypotheses are that (1) acclimation increases enzyme concentration at lower temperatures to compensate for reduced activity; (2) equilibrium enzymes tend to show acclimation responses; and (3) acclimation responses are more common in species whose populations experience either large temporal or geographical temperature variations. Using maximal activities as indices of enzyme concentration, the presence of acclimation responses in all the glycolytic enzymes in the heart ventricle was determined for five species in the teleost genus Fundulus. Three of these species are distributed along the steep thermal cline of the North American Atlantic coast, and thus these species experience both seasonal and geographical variation in temperature. The other two species are found in the Gulf of Mexico and experience seasonal variation similar to the Atlantic species but no geographical variation in temperature. Two Atlantic coast species, Fundulus heteroclitus and Fundulus majalis, have unique derived acclimation responses. No derived acclimation responses occur in the Gulf species. A conserved response in hexokinase was observed within one subgenus comprising both Atlantic and Gulf species. In F. heteroclitus, enolase responded to acclimation, and in F majalis, aldolase, triphosphate isomerase, and lactate dehydrogenase had acclimation responses. These enzymes are equilibrium enzymes, and the concentrations of all of them increase at lower temperatures, which would compensate for the effect of temperature on enzyme activity. The compensatory changes all occur in the Atlantic species and may be a mechanism for species to expand their ranges. These data suggest that physiological acclimation is evolutionarily labile.

The atypical excitatory effect of acetylcholine on cardiac ventricular muscle was investigated in the horned shark, Heterodontus portusjacksoni. Electrically paced ventricular strips produced a massive 391.45% (+/-26.39%) increase in basal force of contraction in response to exogenously applied acetylcholine. The response was similar in nature to that produced by applied adrenaline, which caused a 382.52% (+/-72.47%) increase. The response to acetylcholine was blocked by the muscarinic cholinoceptor antagonist atropine and the competitive beta-adrenoceptor antagonist propranolol and was reduced by bretylium, an agent known to inhibit the release of catecholamines from adrenergic nerves. These findings strongly suggest that acetylcholine mediates a localised release of a catecholamine via muscarinic cholinoceptors in shark heart. A cholinergically controlled catecholamine store has been proposed (cholinergic-adreno complex), implying that elasmobranchs may be capable of finer control of cardiac output than has previously been suspected. This complex may represent a transitional adrenergic state between humoral and neuronal regulation. The spontaneously beating atrium showed no evidence of such an excitatory response to applied acetylcholine but produced an atropine-sensitive slowing, a response typical of other vertebrates.

We examined the ionoregulatory capabilities of the blackskirt tetra (Gymnocorymbus ternetzi), which is native to ion-poor acidic waters of the Amazon River. Examination of Na+ uptake, which was only slightly sensitive to the uptake blocker amiloride, revealed several specializations for uptake in these waters. Kinetic analysis of Na+ uptake (at pH 6.5) revealed a high maximum rate of uptake and a low Michaelis-Menten constant, which allows the tetras to take up Na+ at high rates even at very low water levels. At pH 4.5, a pH where they experience sizable ion disturbances, they displayed several mechanisms to restore balance. Kinetic analysis at pH 4.5 revealed that the maximum uptake rate rose 67% while the Michaelis-Menten constant remained unchanged. Further tests showed that the upregulation of Na+ uptake occurred within 12 h in response to a doubling of Na+ efflux. Despite these specializations of the Na+ uptake mechanism, blackskirt tetras were not especially tolerant of low pH. Upon exposure to pH 4.0, they experienced a massive loss of Na+ due to a fourfold increase of Na+ efflux (relative to pH 5.0) and an 80% inhibition of uptake. Measurement of Na+ efflux in waters with different Ca2+ levels and in the presence of LaCl, a strong Ca2+ competitor, correlated the stimulation of Na+ efflux at low pH with a low branchial affinity for Ca2+. These tests indicate that blackskirt tetras possess abilities to resist the disruptive effects of moderately low pH but cannot survive in waters with a pH of 4.0 or less because of leaching of Ca2+ from branchial tight junctions, which stimulates ion losses.

High-elevation populations of many grasshopper species produce small adults in response to shortened growing seasons and cooler ambient temperatures. Mass-specific metabolic rate tends to increase with elevation, and several authors have argued that this is an adaptation to accelerate development. In the present study, the relationship of thermoregulation and metabolism was investigated in adults of the acridid grasshopper Xanthippus corallipes from six populations along an elevation gradient. Thermoregulation was measured in the field, and several lines of evidence suggested that afternoon body temperatures were actively maintained within each population. Populations were found to maintain stable afternoon body temperatures that correlate negatively with elevation. Elevation had a strong negative effect on adult mass. Mass-specific metabolic rates at 35 degrees and 45 degrees C correlated positively with elevation. However, population differences in mass explained most of the variation in mass-specific metabolic rates, and when mass was used as a covariate, the effect of elevation disappeared. Mass-specific metabolic rates at afternoon field body temperatures were estimated and found not to differ among populations. Thus, differences in thermoregulation offset the effect of mass on mass-specific metabolic rate across populations, such that X. corallipes adults exhibited a common mass-specific metabolic rate in the wild, independent of large population differences in mass and ambient temperatures.