Living on the edge: Glucocorticoid physiology in desert iguanas (Dipsosaurus dorsalis) is predicted by distance from an anthropogenic disturbance, body condition, and population density

Ecological factors, such as habitat quality, influence the survival and reproductive success of free-living organisms. Urbanization, including roads, alters native habitat and likely influences physiology, behavior, and ultimately Darwinian fitness. Some effects of roads are clearly negative, such as increased habitat fragmentation and mortality from vehicle collision. However, roads can also have positive effects, such as decreasing predator density and increased vegetation cover, particularly in xeric habitats due to increased water run-off. Glucocorticoids are metabolic hormones that reflect baseline metabolic needs, increase in response to acute challenges, and may mediate endogenous resource trade-offs between survival and reproduction. Here we examined circulating concentrations of corticosterone (baseline and stress-induced) in desert iguanas (Dipsosaurus dorsalis) in relation to the distance from a major anthropogenic disturbance, a high-traffic road in Palm Springs, CA. Additionally, we analyzed body condition and population density as additional predictors of glucocorticoid physiology. Surprisingly, we found lower baseline CORT levels closer to the road, but no effect of distance from road on stress-induced CORT or stress responsiveness (difference between baseline and stress-induced concentrations). Both population density and body condition were negative predictors of baseline CORT, stress-induced CORT, and stress responsiveness. Given the known effect of roads to increase run-off and vegetation density, increased water availability may improve available forage and shade, which may then increase the carrying capacity of the habitat and minimize metabolic challenges for this herbivorous lizard. However, it is important to recognize that surfaces covered by asphalt are not usable habitat for iguanas, likely resulting in a net habitat loss.

Differential responses of the somatotropic and thyroid axes to environmental temperature changes in the green iguana

Growth hormone (GH), together with thyroid hormones (TH), regulates growth and development, and has critical effects on vertebrate metabolism. In ectotherms, these physiological processes are strongly influenced by environmental temperature. In reptiles, however, little is known about the direct influences of this factor on the somatotropic and thyroid axes. Therefore, the aim of this study was to describe the effects of both acute (48h) and chronic (2weeks) exposure to sub-optimal temperatures (25 and 18°C) upon somatotropic and thyroid axis function of the green iguana, in comparison to the control temperature (30-35°C). We found a significant increase in GH release (2.0-fold at 25°C and 1.9-fold at 18°C) and GH mRNA expression (up to 3.7-fold), mainly under chronic exposure conditions. The serum concentration of insulin-like growth factor-I (IGF-I) was significantly greater after chronic exposure (18.5±2.3 at 25°C; 15.92±3.4 at 18°C; vs. 9.3±1.21ng/ml at 35°C), while hepatic IGF-I mRNA expression increased up to 6.8-fold. Somatotropic axis may be regulated, under acute conditions, by thyrotropin-releasing hormone (TRH) that significantly increased its hypothalamic concentration (1.45 times) and mRNA expression (0.9-fold above control), respectively; and somatostatin (mRNA expression increased 1.0-1.2 times above control); and under chronic treatment, by pituitary adenylate cyclase-activating peptide (PACAP mRNA expression was increased from 0.4 to 0.6 times). Also, it was shown that, under control conditions, injection of TRH stimulated a significant increase in circulating GH. On the other hand, while there was a significant rise in the hypothalamic content of TRH and its mRNA expression, this hormone did not appear to influence the thyroid axis activity, which showed a severe diminution in all conditions of cold exposure, as indicated by the decreases in thyrotropin (TSH) mRNA expression (up to one-eight of the control), serum T4 (from 11.6±1.09 to 5.3±0.58ng/ml, after 2weeks at 18°C) and T3 (from 0.87±0.09 to 0.05±0.01ng/ml, under chronic conditions at 25°C), and Type-2 deiodinase (D2) activity (from 992.5±224 to 213.6±26.4fmolI(125)T4/mgh). The reduction in thyroid activity correlates with the down-regulation of metabolism as suggested by the decrease in the serum glucose and free fatty acid levels. These changes apparently were independent of a possible stress response, at least under acute exposure to both temperatures and in chronic treatment to 25°C, since serum corticosterone had no significant changes in these conditions, while at chronic 18°C exposure, a slight increase (0.38 times above control) was found. Thus, these data suggest that the reptilian somatotropic and thyroid axes have differential responses to cold exposure, and that GH and TRH may play important roles associated to adaptation mechanisms that support temperature acclimation in the green iguana.

Mature Erythrocytes of Iguana iguana (Squamata, Iguanidae) Possess Functional Mitochondria

Electron microscopy analyses of Iguana iguana blood preparations revealed the presence of mitochondria within erythrocytes with well-structured cristae. Fluorescence microscopy analyses upon incubation with phalloidin-FITC, Hoechst 33342 and mitochondrial transmembrane potential (Δψ_m)-sensitive probe MitoTracker Red indicated that mitochondria i) widely occur in erythrocytes, ii) are polarized, and iii) seem to be preferentially confined at a "perinuclear" region, as confirmed by electron microscopy. The analysis of NADH-dependent oxygen consumption showed that red blood cells retain the capability to consume oxygen, thereby providing compelling evidence that mitochondria of Iguana erythrocytes are functional and capable to perform oxidative phosphorylation.

Characterization of pituitary growth hormone and its receptor in the green iguana (Iguana iguana)

Pituitary growth hormone (GH) has been studied in most vertebrate groups; however, only a few studies have been carried out in reptiles. Little is known about pituitary hormones in the order Squamata, to which the green iguana (gi) belongs. In this work, we characterized the hypophysis of Iguana iguana morphologically. The somatotrophs (round cells of 7.6-10 μm containing 250- to 300-nm secretory granules where the giGH is stored) were found, by immunohistochemistry and in situ hybridization, exclusively in the caudal lobe of the pars distalis, whereas the lactotrophs were distributed only in the rostral lobe. A pituitary giGH-like protein was obtained by immuno-affinity chromatography employing a heterologous antibody against chicken GH. giGH showed molecular heterogeneity (22, 44, and 88 kDa by SDS-PAGE/Western blot under non-reducing conditions and at least four charge variants (pIs 6.2, 6.5, 6.9, 7.4) by isoelectric focusing. The pituitary giGH cDNA (1016 bp), amplified by PCR and RACE, encodes a pre-hormone of 218 aa, of which 190 aa correspond to the mature protein and 28 aa to the signal peptide. The giGH receptor cDNA was also partially sequenced. Phylogenetic analyses of the amino acid sequences of giGH and giGHR homologs in vertebrates suggest a parallel evolution and functional relationship between the GH and its receptor.

Identification of a novel splicing form of amelogenin gene in a reptile, Ctenosaura similis

Amelogenin, the major enamel matrix protein in tooth development, has been demonstrated to play a significant role in tooth enamel formation. Previous studies have identified the alternative splicing of amelogenin in many mammalian vertebrates as one mechanism for amelogenin heterogeneous expression in teeth. While amelogenin and its splicing forms in mammalian vertebrates have been cloned and sequenced, the amelogenin gene, especially its splicing forms in non-mammalian species, remains largely unknown. To better understand the mechanism underlying amelogenin evolution, we previously cloned and characterized an amelogenin gene sequence from a squamate, the green iguana. In this study, we employed RT-PCR to amplify the amelogenin gene from the black spiny-tailed iguana Ctenosaura similis teeth, and discovered a novel splicing form of the amelogenin gene. The transcript of the newly identified iguana amelogenin gene (named C. Similis-T2L) is 873 nucleotides long encoding an expected polypeptide of 206 amino acids. The C. Similis-T2L contains a unique exon denominated exon X, which is located between exon 5 and exon 6. The C. Similis-T2L contains 7 exons including exon 1, 2, 3, 5, X, 6, and 7. Analysis of the secondary and tertiary structures of T2L amelogenin protein demonstrated that exon X has a dramatic effect on the amelogenin structures. This is the first report to provide definitive evidence for the amelogenin alternative splicing in non-mammalian vertebrates, revealing a unique exon X and the splicing form of the amelogenin gene transcript in Ctenosaura similis.

Fecal glucocorticoid response to environmental stressors in green iguanas (Iguana iguana)

Quantification of glucocorticoid metabolites in feces has been shown to be a powerful tool in evaluating well-being in vertebrates. Little is known however about the hypothalamic-pituitary-adrenal axis response to stressors, and consequent glucocorticoid excretion, in reptiles. In a longitudinal study, fecal corticosterone metabolite (FCM) levels in green iguanas (Iguana iguana) were quantified during periods of rest and exposure to hypothesized stressors. FCM quantification was combined with behavioral analysis to further contextualize the measured increases. It was shown that both daily 5-minute handling/restraint, as well as housing devoid of climbing opportunity, resulted in increased FCM excretion. Behavioral analysis suggested that the iguanas were chronically stressed by the lack of climbing opportunity, whereas handling may have induced only a transient stress response. The experimental design, using repeated periods of stressor-exposure, also revealed a facilitating effect, where the two stressors potentiated one another. Furthermore, the order of the two stressors was found to be important. The study provides insight into the functioning of the hormonal stress response in green iguanas, and to the refining of their housing and handling.

Human disturbance alters endocrine and immune responses in the Galapagos marine iguana (Amblyrhynchus cristatus)

Anthropogenic disturbance is a relevant and widespread facilitator of environmental change and there is clear evidence that it impacts natural populations. While population-level responses to major anthropogenic changes have been well studied, individual physiological responses to mild disturbance can be equally critical to the long-term survival of a species, yet they remain largely unexamined. The current study investigated the impact of seemingly low-level anthropogenic disturbance (ecotourism) on stress responsiveness and specific fitness-related immune measures in different breeding stages of the marine iguana (Amblyrhynchus cristatus). Specifically, we found stress-induced elevations in plasma corticosterone among tourist-exposed populations relative to undisturbed populations. We also found changes in multiple immunological responses associated with stress-related effects of human disturbance, including bacterial killing ability, cutaneous wound healing, and hemolytic complement activity, and the responses varied according to reproductive state. By identifying health-related consequences of human disturbance, this study provides critical insight into the conservation of a well-known species that has a very distinct ecology. The study also broadens the foundation of knowledge needed to understand the global significance of various levels of human disturbance.

The energetic savings of sleep versus temperature in the Desert Iguana (Dipsosaurus dorsalis) at three ecologically relevant temperatures

One of the proposed ecological functions of sleep is to conserve energy. The majority of studies that support this theory have been done on endothermic animals whose body temperatures drop during sleep due to the reduced neurological control of thermoregulation. In the present study, we examined typical temperatures to which the Desert Iguana, Dipsosaurus dorsalis, is exposed to in the field and found that mean high temperatures ranged from 24-58 degrees C throughout the active portion of the year. We also examined the ecological savings that sleep could provide for this ectothermic iguana using a closed system respirometer. We found that laboratory-acclimated iguanas are able to save significantly more (27.6%) energy by sleeping than by being awake and that field iguanas also had significant savings of energy (69.1%) while asleep. However, iguanas could save more energy by remaining awake at cooler temperatures than by sleeping at warmer temperatures. In addition, we found no correlation for time of night with metabolic rate. Our study supports the hypothesis that one potential function of sleep is to conserve energy.

Gene expression and tissue distribution of cytoglobin and myoglobin in the Amphibia and Reptilia: possible compensation of myoglobin with cytoglobin in skeletal muscle cells of anurans that lack the myoglobin gene

Cytoglobin (Cygb), a recently discovered vertebrate cytoplasmic heme-binding globin, is considered to be in a clade with vertebrate myoglobin (Mb), which is exclusively distributed in the cytoplasm of cardiac and skeletal muscles as an oxygen storage protein. GenBank databases (NCBI and JGI) and gene synteny analyses showed the absence of the Mb gene (mb) in two anuran amphibians, Xenopus laevis and X. tropicalis. Here we conducted comparative studies on the gene expression and tissue distribution of Cygb and Mb in anuran and reptilian tissues. Cygb and Mb genes were cloned from a reptile, iguana (Iguana iguana). Two types of cygb (cygb-1 and -2) were cloned, with lengths of 1066 and 1034 bp, and 196 and 193 amino acid residues, respectively. Their nucleotide and amino acid sequence identities were 90 and 87%, respectively. The Mb gene covered 1416 bp with an open reading frame of 465 bp, giving rise to a 154 amino acid protein. The distal ligand-binding histidine at E7, the proximal heme-binding histidine at F8, and the phenylalanine residue at CD1 were conserved in Mb and Cygb. The nucleotide and amino acid sequence identity of I. iguana cygb-1 and cygb-2 against X. laevis cygb were approximately 67% and 65%, respectively. RT-PCR demonstrated that X. laevis cygb was uniquely expressed in the heart and skeletal muscles, and faintly in the liver and spleen, which was quite contrasted with Iguana and the other vertebrates, where mb is exclusively expressed in the heart and skeletal muscles. Immunohistochemical analyses showed the distribution of Cygb in the cytoplasm of skeletal muscle cells. Interestingly, Cygb in the heart was localized in the nuclei. Considering the absence of mb in the Anura, we hypothesize that Cygb in muscle cells of anurans compensates for the lack of Mb for the storage and intracellular transportation of oxygen.

Pharmacokinetics of inhaled anesthetics in green iguanas (Iguana iguana)

OBJECTIVE

To test the hypothesis that differences in anesthetic uptake and elimination in iguanas would counter the pharmacokinetic effects of blood:gas solubility and thus serve to minimize kinetic differences among inhaled agents.

ANIMALS

6 green iguanas (Iguana iguana).

PROCEDURES

Iguanas were anesthetized with isoflurane, sevoflurane, or desflurane in a Latin-square design. Intervals from initial administration of an anesthetic agent to specific induction events and from cessation of administration of an anesthetic agent to specific recovery events were recorded. End-expired gas concentrations were measured during anesthetic washout.

RESULTS

Significant differences were not detected for any induction or recovery events for any inhalation agent in iguanas. Washout curves best fit a 2-compartment model, but slopes for both compartments did not differ significantly among the 3 anesthetics.

CONCLUSIONS AND CLINICAL RELEVANCE

Differences in blood:gas solubility for isoflurane, sevoflurane, and desflurane did not significantly influence differences in pharmacokinetics for the inhalation agents in iguanas.

PHARMACOKINETICS OF KETOPROFEN IN THE GREEN IGUANA (IGUANA IGUANA) FOLLOWING SINGLE INTRAVENOUS AND INTRAMUSCULAR INJECTIONS

The nonsteroidal antiinflammatory drug ketoprofen (KTP) is a commonly used antiinflammatory and analgesic agent in reptile medicine, but no studies documenting its pharmacokinetics in this species have been published. Ketoprofen was administered as a racemic mixture to green iguanas (Iguana iguana) intravenously (i.v.) and intramuscularly (i.m.) at 2 mg/kg. Pharmacokinetic analyses were performed and indicated that ketoprofen in iguanas administered by the intravenous route has a classical two-compartmental distribution pattern, a slow clearance (67 ml/ kg/hr) and a long terminal half-life (31 hr) compared to ketoprofen studies reported in mammals. When delivered by the intramuscular route, bioavailability was 78%. These data indicate the daily dosing that is generally recommended for reptile patients, as an extrapolation from mammalian data, may be more frequent than necessary.

Metabolic support of moderate activity differs from patterns seen after extreme behavior in the desert iguana Dipsosaurus dorsalis

This study examined glucose and lactate metabolism in an iguanid lizard, Dipsosaurus dorsalis, during rest and after activity patterned on field behavior (15 s of running at 1 m/s). Metabolite oxidation and incorporation into glycogen by the whole animal, the liver, and oxidative and glycolytic muscle fibers were measured using (14)C- and (13)C-labeled compounds. Results showed that lactate metabolism is more responsive to changes that occurred between rest and recovery, whereas glucose appears to play a more steady state role. After activity, lactate oxidation produced 57 times as much ATP during 1 h of recovery than did glucose oxidation. However, lactate oxidation rates were elevated for only 30 min after activity, while glucose oxidation remained elevated beyond 1 h. Lactate was the primary source for glycogen synthesis during recovery, and glucose was the main glycogenic substrate during rest. This study supports previous research showing that brief activity in D. dorsalis is primarily supported by glycolysis and phosphocreatine breakdown, but it also suggests that there may be less of a reliance on glycolysis and a greater reliance on phosphocreatine than previously shown. The findings presented here indicate that the metabolic consequences of the behaviorally relevant activity studied are less severe than has been suggested by studies using more extreme activity patterns.

Diurnal and nocturnal differences in hypothalamic-pituitary-adrenal axis function in Galápagos marine iguanas

Temporal modulation of the stress response is a ubiquitous characteristic of animals. Here, we investigate possible mechanisms underlying daily changes in corticosterone release in an ectotherm model system. Earlier work indicated that free-living Galápagos marine iguanas (Amblyrhynchus cristatus) have lower corticosterone concentrations during the night than during the day. This could result from: (i) a lower circadian secretion of adrenocorticotropic hormone (ACTH) as seen in mammals; (ii) from an increase in corticosterone negative feedback; or (iii) reflect lower metabolic activity during the night when core body temperature falls (from 35 degrees C during the day to as low as 21 degrees C during the night). To begin to distinguish between these three possibilities, exogenous ACTH was used to compare diel differences in adrenocortical tissue responsiveness, and dexamethasone was used to compare diel differences in the efficacy of corticosterone negative feedback. Low levels of exogenous ACTH (30 IU/kg body weight) potently stimulated both daytime and nighttime corticosterone release. Dexamethasone (1 mg/kg) inhibited only daytime, but not nighttime endogenous corticosterone release. Because the response to ACTH was similar between day and night we suggest that a simple lowering of core body temperature cannot explain the nighttime reduction in corticosterone release. However, the failure of negative feedback at night suggests that the response is not equivalent to the controlled downregulation seen in mammals.

Regulation of Gut Function Varies with Life‐History Traits in Chuckwallas (Sauromalus obesus: Iguanidae)

We examined the effects of hibernation and fasting on intestinal glucose and proline uptake rates of chuckwallas (Sauromalus obesus) and on the size of organs directly or indirectly related to digestion. These lizards show geographic variation in body size and growth rate that parallels an elevational gradient in our study area. At low elevation, food is available only for a short time during the spring; at high elevation, food may also be available during summer and autumn, depending on rainfall conditions in a given year. We hypothesized that low-elevation lizards with a short season of food availability would show more pronounced regulation of gut size and function than high-elevation lizards with prolonged or bimodal food availability. Hibernating lizards from both elevations had significantly lower uptake rates per milligram intestine for both nutrients, and lower small intestine mass, than active lizards. The combination of these two effects resulted in significantly lower total nutrient uptake in hibernating animals compared to active ones. The stomach, large intestine, and cecum showed lower masses in hibernators, but these results were not statistically significant. The heart, kidney, and liver showed no difference in mass between hibernating and nonhibernating animals. Lizards from low elevations with a short growing season also showed a greater increase in both uptake rates and small intestine mass from the hibernating to the active state, compared to those from high elevations with longer growing seasons. Thus, compared to those from long growing season areas, lizards from short growing season areas have equal uptake capacity during hibernation but much higher uptake capacity while active and feeding. This pattern of regulation of gut function may or may not be an adaptive response, but it is consistent with variation in life-history characteristics among populations. In areas with a short season, those lizards that can extract nutrients quickly and then reduce the gut will be favored; in areas where food may be available later in the year, those lizards that maintain an active gut would be favored. While other researchers have found much greater magnitudes of gut regulation when making comparisons among species, we find the different patterns of change in gut function between different populations of chuckwallas particularly intriguing because they occur within a single species.

IgH-2 cells: a reptilian model for apoptotic studies

Regulation of proper cell number in tissues depends upon a balance between cell proliferation and cell death. The process of apoptosis has thus far been studied in a variety of multicellular organisms from humans to higher plants. In order to broaden our perspective and identify another metazoan system with which to deepen our understanding of the function and evolution of the apoptotic machinery, we have characterized cell death in a reptilian cell line. We show that the death of IgH-2 Iguana (Iguana iguana) heart cells [Clark, H.F., Cohen, M.M., Karzon, D.T., 1970. Characterization of reptilian cell lines established at incubation temperatures of 23 to 36 degrees. Proc. Soc. Exp. Biol. Med. 133, 1039-1047.] is, in response to DNA damaging agents, accompanied by classic morphological changes of apoptosis including detachment from the substrate, cell shrinkage, nuclear pyknosis and externalization of the plasma membrane phospholipid phosphatidylserine. Our biochemical studies show that the death of IgH-2 cells is accompanied by internucleosomal DNA fragmentation and activation of caspases. Our studies with the pan-caspase inhibitor zVAD.fmk implicate caspases in the apoptotic process we observe. This work represents the first detailed molecular and biochemical analysis of apoptosis in cells of an organism of class Reptilia and establishes IgH-2 cells as a suitable model system with which to investigate the phenomenon of caspase dependent apoptosis and the apoptotic machinery in a reptilian model.

Intraspecific allometry of standard metabolic rate in green iguanas, Iguana iguana

To study the allometric relationship between standard metabolic rate and body mass (mass range 16-3627 g) in green iguanas, Iguana iguana (n=32), we measured rates of oxygen consumption (V(O(2))) at 30 degrees C during scotophase. The relationship could be described as: V(O(2))(ml h(-1))=0.478W(0.734). The resulting mass exponent was similar to the 3/4 power commonly used in interspecific curves (P>0.05), but differed from a proposed intraspecific value of 2/3 (P<0.05). The mass exponents of male (n=8) and female (n=11) iguanas did not differ (P>0.05). The mass adjusted V(O(2)) was higher than predicted from generalized squamate curves. The mean mass exponent of intra-individual allometric equations of iguanas (n=7) at varying masses during ontogeny did not differ from that of the pooled equation, indicating that scaling of V(O(2)) is similar for both between and within individuals. Thermal acclimation, compensatory changes in V(O(2)) with prolonged exposure to a constant temperature, was not observed in juvenile iguanas (n=11) between 1 and 5 weeks of acclimation at 30 degrees C.

Minimum alveolar concentration of isoflurane in green iguanas and the effect of butorphanol on minimum alveolar concentration

OBJECTIVE

To determine minimum alveolar concentration (MAC) of isoflurane in green iguanas and effects of butorphanol on MAC.

DESIGN

Prospective randomized trial.

ANIMALS

10 healthy mature iguanas.

PROCEDURE

in each iguana, MAC was measured 3 times: twice after induction of anesthesia with isoflurane and once after induction of anesthesia with isoflurane and IM administration of butorphanol (1 mg/kg [0.45 mg/lb]). A blood sample was collected from the tail vein for blood-gas analysis at the beginning and end of the anesthetic period. The MAC was determined with a standard bracketing technique; an electrical current was used as the supramaximal stimulus. Animals were artificially ventilated with a ventilator set to deliver a tidal volume of 30 mL/kg (14 mL/lb) at a rate of 4 breaths/min.

RESULTS

Mean +/- SD MAC values during the 3 trials (2 without and 1 with butorphanol) were 2.0 +/- 0.6, 2.1 +/- 0.6, and 1.7 +/- 0.7%, respectively, which were not significantly different from each other. Heart rate and end-tidal partial pressure of CO2 were also not significantly different among the 3 trials. Mean +/- SD heart rate was 48 +/- 10 beats/min; mean end-tidal partial pressure of CO2 was 22 +/- 10 mm Hg. There were no significant differences in blood-gas values for samples obtained at the beginning versus the end of the anesthetic period.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that the MAC of isoflurane in green iguanas is 2.1% and that butorphanol does not have any significant isoflurane-sparing effects.

The cardiac anesthetic index of isoflurane in green iguanas

OBJECTIVE

To determine the cardiac anesthetic index (CAI) of isoflurane in green iguanas and whether butorphanol affected the CAI.

DESIGN

Prospective randomized controlled trial.

ANIMALS

7 healthy mature iguanas.

PROCEDURE

In 5 iguanas, CAI was determined after induction of anesthesia with isoflurane alone, and in 5 iguanas, CAI was determined after induction of anesthesia with isoflurane and IM administration of butorphanol (1 mg/kg [0.45 mg/lb]). Three iguanas underwent both treatments. Animals were equilibrated for 20 minutes at 1.5 times the minimum alveolar concentration (MAC) of isoflurane and observed for evidence of cardiovascular arrest. If there was no evidence of cardiovascular arrest, end-tidal isoflurane concentration was increased by 20%, and animals were allowed to equilibrate for another 20 minutes. This process was repeated until cardiovascular arrest occurred or vaporizer output could no longer be consistently increased. The CAI was calculated by dividing the highest end-tidal isoflurane concentration by the MAC.

RESULTS

None of the iguanas developed cardiovascular arrest and all survived. Mean +/- SD highest end-tidal isoflurane concentration during anesthesia with isoflurane alone (9.2 +/- 0.60%) was not significantly different from mean concentration during anesthesia with isoflurane and butorphanol (9.0 +/- 0.43%). The CAI was > 4.32.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that the CAI of isoflurane in green iguanas is > 4.32 and not affected by administration of butorphanol. Isoflurane appears to be a safe anesthetic in green iguanas.

Selective absorption of carotenoids in the common green iguana (Iguana iguana)

In the animal kingdom, species-specific differences with regard to the absorption of intact carotenoids are observed. The causes of these differences are not entirely understood. To investigate the absorption of selected carotenoids, 20 juvenile green iguanas (Iguana iguana) were fed a carotenoid deficient basal diet for 56 days. Thereafter, the iguanas were assigned to receive a basal diet supplemented with different carotenoids (80 mg/kg diet) such as beta-carotene, canthaxanthin and apo-8'-carotenoic acid ethyl ester for 28 days. Changes in plasma carotinoid concentrations associated with the individual diets were used as indicators of carotenoid absorption. In both the experimental and control groups, only the oxygenated carotenoids (xanthophylls), lutein, zeaxanthin and canthaxanthin, were found in the plasma. Canthaxanthin and apo-8'-carotenoic acid ethyl ester were readily absorbed and recovered from the plasma. However, the supplementation of beta-carotene caused no increase in plasma beta-carotene concentration. Additionally, beta-carotene, canthaxanthin or apo-8'-carotenoic acid ethyl ester did not affect the concentrations of retinol and alpha-tocopherol in plasma. In conclusion, the study demonstrates that iguanas appear to be selective accumulators of polar xanthophylls. The iguana might, therefore, be a valuable model to investigate the selectiveness of carotenoid absorption as well as the function of xanthophylls in animals.

Evidence for facilitated lactate uptake in lizard skeletal muscle

To understand more fully lactate metabolism in reptilian muscle, lactate uptake in lizard skeletal muscle was measured and its similarities to the monocarboxylate transport system found in mammals were examined. At 2 min, uptake rates of 15 mmol l–1 lactate into red iliofibularis (rIF) were 2.4- and 2.2-fold greater than white iliofibularis (wIF) and mouse soleus, respectively. α-Cyano-4-hydroxycinnamate (15 mmol l–1) caused little inhibition of uptake in wIF but caused a 42–54 % reduction in the uptake rate of lactate into rIF, suggesting that much of the lactate uptake by rIF is via protein-mediated transport. N-ethymaleimide (ETH) (10 mmol l–1) also caused a reduction in the rate of uptake, but measurements of adenylate and phosphocreatine concentrations show that ETH had serious effects on rIF and wIF and may not be appropriate for transport inhibition studies in reptiles. The higher net uptake rate by rIF than by wIF agrees with the fact that rIF shows much higher rates of lactate utilization and incorporation into glycogen than wIF. This study also suggests that lactate uptake by reptilian muscle is similar to that by mammalian muscle and that, evolutionarily, this transport system may be relatively conserved even in animals with very different patterns of lactate metabolism.

Effect of activity duration on recovery and metabolic costs in the desert iguana (Dipsosaurus dorsalis)

The majority of elevated O(2) consumption associated with short and vigorous activity occurs during recovery, thus an assessment of associated metabolic costs should also examine the excess post-exercise oxygen consumption (EPOC). This study examined O(2) uptake during exercise, EPOC and distance traveled during 5-, 15-, 60- and 300-s sprints at maximal treadmill intensity in Dipsosaurus (N=10; 74.3+/-2.1 g). EPOC (0.08, 0.14, 0.23 and 0.18 ml O(2) g(-1), respectively) was large (80-99% of total elevated O(2) consumption) and increased significantly between 5 and 60 s. The cost of activity (C(act); ml O(2) g(-1) x km(-1)), intended to reflect the total net costs associated with the activity, was calculated as the total elevated O(2) consumption per unit distance traveled. C(act) decreased with activity duration due to proportionally larger increases in distance traveled relative to EPOC volume, and is predicted by the equation C(act)=14.7 x activity duration (s)(-0.24). The inclusion of EPOC costs provides an ecologically relevant estimate of the total metabolic cost of locomotor activity. C(act) exceeds standard transport costs at all durations examined due to the addition of obligate recovery costs. The differences are large enough to impact energy budget analyses for ectotherms.

Tissue and plasma enzyme activities in juvenile green iguanas

OBJECTIVE

To determine activities of intracellular enzymes in 8 major organs in juvenile green iguanas and to compare tissue and plasma activities.

ANIMALS

6 green iguanas < 1 year old.

PROCEDURE

Lysates of liver, kidney, epaxial muscle, heart, lung, spleen, small intestine, and pancreas were analyzed for alkaline phosphatase (ALP), lactate dehydrogenase (LDH), aspartate transaminase (AST), alanine transaminase (ALT), gamma-glutamyltransferase (GGT), creatine kinase (CK), glutamate dehydrogenase (GMD), and amylase (AMS) activities.

RESULTS

In general, low tissue enzyme activity coincided with low plasma activity. The CK activity was high in epaxial muscle and the heart and low in all other tissues tested. The AMS activity was found exclusively in the pancreas. Moderate LDH and AST activities were found in all tissues. Low ALT and ALP activities were found in a variety of tissues. Plasma and tissue activities of GGT and GMD were low or undetectable.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that plasma CK activity may be muscle specific in iguanas, but high values may not always indicate overt muscle disease. The AMS activity may be specific for the pancreas, but the wide range of plasma activity would likely limit its diagnostic usefulness. Activities of AST and LDH may reflect tissue damage or inflammation, but probably do not reflect damage to specific tissues or organs.

Dietary fiber influences nutrient utilization, growth and dry matter intake of green iguanas (Iguana iguana)

Herbivory is an uncommon feeding strategy in lizards. Appropriate diet formulations for captive lizards should be based on performance measures, yet few data are available on the effect of plant fiber on food intake, nutrient utilization and growth of captive herbivorous lizards. This study was conducted to determine the effect of three levels of dietary fiber on dry matter intake, nutrient and energy metabolizability and growth rate of the green iguana (Iguana iguana). Twenty-one captive iguanas were fed nutritionally complete diets containing three levels of dietary fiber: 19, 24, and 27% neutral detergent fiber. The iguanas were fed each diet for at least 12 wk, and total excreta were collected for 11.3 +/- 4.0 d (means +/- , range of 7 to 25 d). Diets and excreta were analyzed for dry matter, organic matter, gross energy, neutral detergent fiber, acid detergent fiber, and acid detergent lignin. The study was designed as a Latin square crossover. Across all diets, dry matter intake was proportional to body mass1.0 (BM). Growth rate was greater (P < 0. 05) when iguanas were fed the low and medium fiber diets (2.2 and 2. 4 g/d, respectively) than when fed the high fiber diet (1.4 g/d). However, mean daily dry matter intake of the three diets [7.2 g/(d. kg BM)] was not different. In general, digestibility of fiber fractions and the metabolizability of dietary energy decreased (P < 0.05) as the level of dietary fiber increased. These data suggest that a diet containing less than 27% neutral detergent fiber should be fed if rapid growth is to be sustained during intensive captive production of green iguanas.

Regulation of skeletal muscle metabolism in the lizard Dipsosaurus dorsalis by fructose-2,6-bisphosphate

Changes in liver and skeletal muscle fructose-2,6-bisphosphate (Fru-2,6-P2) concentrations were compared during fasting, exercise, and recovery in the lizard Dipsosaurus dorsalis and in outbred mice (Mus musculus). We present the first correlative evidence that suggests that a decrease in the content of Fru-2,6-P2 may mediate elevated gluconeogenesis in lizard skeletal muscle. Contents of Fru-2,6-P2 in lizard gastrocnemius and red and white iliofibularis (IF) were significantly lower (as much as 55% in white IF) during recovery from exhaustive exercise than at rest. Recovery from exhaustive exercise had no significant effect on Fru-2,6-P2 concentrations in any mouse muscle examined. Fasting significantly depressed lizard and mouse liver Fru-2,6-P2 contents and decreased lizard red IF by over 84% from the fed condition. Lizard red and white muscle fiber bundles incubated in 20 mM lactate had significantly lower Fru-2,6-P2 (94 and 61% depression, respectively) than those incubated in 8.5 mM glucose. These results are consistent with the hypothesis that Fru-2,6-P2 acts as a signal for controlling gluconeogenesis in lizard skeletal muscle.

Development of the axial cartilaginous skeleton in the regenerating tail of lizards

In order to study the pattern of growth of regenerating cartilage, H3-thymidine was injected in individuals of the lizards Anolis carolinensis and Lampropholis delicata during advanced stages of tail regeneration. Light and electron microscopic autoradiographical analysis showed that the regenerating cartilaginous tube was mostly derived from the interstitial proliferation of chondroblasts. Appositional growth is limited during the initial stages of cartilage formation. Later, after the differentiation of an inner and outer perichondrium, a few cells were added to the cartilaginous tube while cellular degeneration of the calcifying areas was in progress. In this way a constant diameter of the cartilaginous tube was maintained. In order to study the rate of production of matrix protein (collagen), H3-proline was administered to other samples of Lampropholis. To the best of the author's knowledge this was done for the first time in the cartilage of a reptiles. The tracier was mostly localized in the apical-intermediate cartilage and one hour postinjection it was mostly concentrated in the ergastoplasm around the Golgi apparatus and its secretory vesicles. Three hours after the injection of H3-proline some of the tracier was found in the cytoplasm of chondrocytes and in the extracellular matrix.

Melatonin deacetylase activity in the pineal gland and brain of the lizards Anolis carolinensis and Sceloporus jarrovi

Melatonin modulates a variety of rhythmic processes in vertebrates, and is synthesized in both the retina and pineal gland. We have shown previously that retinal melatonin is deacetylated generating 5-methoxytryptamine, which is then deaminated by monoamine oxidase, producing 5-methoxyindoleacetic acid and 5-methoxytryptophol. This process occurs within the eyes of a variety of vertebrates including the iguanid lizard Anolis carolinensis. To determine whether melatonin deacetylase activity also occurs in the pineal organ or in other parts of the lizard brain, pineals and brains of Anolis carolinensis and Sceloporus jarrovi were cultured in the presence of [3H-methoxy]-melatonin. High-performance liquid chromatography of the resulting culture media and tissues revealed the generation of radiolabeled 5-methoxytryptamine and 5-methoxyindoleacetic acid. These two methoxyindoles were the only radiolabeled metabolites detectable, and together accounted for all melatonin lost. Both the loss of melatonin and the production of melatonin metabolites were inhibited by inclusion of 100 microM eserine, an inhibitor of the melatonin deacetylase. Pargyline, a monoamine oxidase inhibitor, reduced the production of 5-methoxyindoleacetic acid and increased the production of 5-methoxytryptamine relative to control incubations. Similar effects of eserine and pargyline were seen in eyecup, brain and pineal gland, but the specific activity of melatonin deacetylation in cultured pineal glands was much greater than in either brains or eyecups. These results indicate that pineal glands of both Anolis carolinensis and Sceloporus jarrovi can rapidly catabolize melatonin by a mechanism very similar to that in the eye, that the melatonin deacetylation pathway exists elsewhere in the iguanid brain, and also extend our previous observations of ocular melatonin deacetylation to an additional species.

Plasma catecholamine and corticosterone and their in vitro effects on lizard skeletal muscle lactate metabolism

Lizard skeletal muscles utilize primarily lactate as a gluconeogenic substrate for glycogen replenishment following exercise. To understand the influence of selected hormones on this process, we measured changes in plasma catecholamines and corticosterone resulting from exercise in the lizard Dipsosaurus dorsalis and then investigated the physiological effects of those hormones on skeletal muscle lactate and glucose metabolism in vitro. Plasma epinephrine (Epi), norepinephrine, and corticosterone (Cort) increased 5.8, 10.2, and 2.2 times, respectively, after 5 min of exhaustive exercise. Epi and Cort levels remained elevated after 2 h of recovery. Skeletal muscle fiber bundles isolated from the red and white regions of the iliofibularis muscle were incubated 2 h at 40 degrees C in the presence of postexercise concentrations of [14C]lactate (15 mM) and glucose (8.5 mM) in the presence and absence of Epi or Cort. Red muscle oxidized both substrates at 2-3 times the rate of white muscle, and both red and white fibers oxidized lactate at 5-10 times the rate of glucose oxidation. Epi had a stimulatory effect on lactate oxidation by white muscle. Lactate incorporation into glycogen proceeded at 2-3 times the rate of glucose incorporation in both muscle types, with rates in red muscle again 2-3 times that for white muscle. Epi stimulated lactate carbon incorporation into glycogen by 50-140% in both red and white muscle but had no effect on glucose incorporation into glycogen in either tissue. We interpret these data as evidence that epinephrine stimulates lactate removal by skeletal muscle. Cort had no effect on lactate metabolism in either muscle type.(ABSTRACT TRUNCATED AT 250 WORDS)

Aerobic and anaerobic metabolism of paired male lizards (Anolis carolinensis)

The associations of physical activity, skin color, body mass difference and conspecific olfactory cues with short-term aerobic and anaerobic metabolism in paired male Anolis carolinesis were examined. Control measurements for mild laboratory manipulation (such as the movement of a metabolic chamber) yielded significant increases in the rate of oxygen consumption, but not in the lactate concentration of these animals; stronger manipulation increased both. A possible influence of conspecific olfactory cues on the metabolism of lizards introduced into empty chambers was undetected. Anolis that turned from green to brown with handling showed an increase in oxygen uptake, but an association between this color shift and total body lactate level was not detected. Elevated rates of oxygen use and glycolysis were found in pairs of males in the absence of physical activity. Lactate levels of the individuals of a pair were positively correlated with one another; the lactate concentrations of lizards placed into occupied metabolic chambers were correlated with the difference in body mass of the pair.

What transport adaptations enable mammals to absorb sugars and amino acids faster than reptiles?

What digestive adaptations enable mammals to process much more food in much less time with equal or higher digestive efficiency than reptiles and thus to sustain much higher metabolic rates? To answer this question, we measured glucose and proline uptake in small intestinal sleeves of three mammal and three reptile species of similar body size and natural diet. All species exhibit saturable, stereospecific uptake of D-glucose and Na+-dependent L-proline uptake. Passive permeability to glucose is high in hamsters and low in the other species. Uptake increases with temperature up to a maximum around 45-50 degrees C. This temperature dependence may help explain why reptiles bask after meals and why their digestion is impaired if basking is prevented. The total uptake capacity of the small intestine for glucose and proline is seven times higher in mammals than similar-sized reptiles, mainly because the area of mammalian intestine is 4-5.5 times greater. Minor reasons for the higher uptake capacity of mammals are that the transport activity of mammal intestine normalized to quantity of tissue is up to twofold higher and that reptile intestine operates at a lower temperature at night. Vmax for glucose transport varies 10-fold among species, but apparent differences in Km values may be unstirred-layer artifacts. Carrier-mediated uptake of glucose and proline is measurable in the colon of at least three species, but the uptake capacity of the colon is less than 10% of that of the small intestine. An appendix presents a method for measuring the microscopic area of intestines with ridges rather than villi, applies this method to desert iguana intestine, and measures area amplification due to villi in wood rat intestine.

The energetic cost of the fever response in three species of ectothermic vertebrates

Three species of ectothermic vertebrates: goldfish (Carassius auratus), green tree frogs (Hyla cinerea), and desert iguanas (Dipsosaurus dorsalis) were used in this study. Metabolic rates for each species were determined at the normal afebrile preferred body temperature and at the febrile preferred body temperature or other higher body temperatures. The febrile metabolic rate (or higher rate) was significantly greater than the afebrile metabolic rate (or lower rate) in each species. The average increase in energetic cost for goldfish and desert iguana was 64.5% while the increase for the green tree frog was between 24 and 70%.

Glycogen synthesis from lactate in skeletal muscle of the lizard Dipsosaurus dorsalis

The capacity of skeletal muscle to synthesize glycogen from lactate was tested in the iliofibularis muscle of the desert iguana, Dipsosaurus dorsalis. Like other reptiles, Dipsosaurus accumulates significant lactic acid concentrations following vigorous exercise. After 5 min of progressively faster treadmill running at 35 degrees C (final speed = 2.2 km/h), blood lactate concentration increased over 14 mM, which decreased 11 mM after 2 h of recovery. Blood glucose concentration remained unchanged throughout at 8.6 +/- 0.46 mM. The role that muscle gluconeogenesis might play in the removal of post-exercise lactate was evaluated. Animals were run to exhaustion at 1.5 km/h on a treadmill thermostatted at 35 degrees C. Animals (n = 43) ran 6.9 +/- 0.75 min prior to exhaustion. Animals were sacrificed and iliofibularis muscles of both hindlimbs removed and stimulated at 2 Hz for 5 min, reducing twitch tension to 6% of prestimulus tension. Fatigued muscles were then split into red and white fiber bundles and incubated 2 h or 5 h at 35 degrees C in Ringer solution or in Ringer plus 20 mM lactate. In muscles tested in August, red fiber bundles incubated in lactate demonstrated a rate of glycogen synthesis of approximately 1 mg/(g muscle . h). In muscles tested in December, red fiber bundles synthesized glycogen at a reduced rate that was not statistically different than in fiber bundles incubated in Ringer solution without lactate. Glycogen synthesis from lactate was not evident in white fiber bundles in either August or December. The period of peak gluconeogenic capacity coincides with the field active season of Dipsosaurus.(ABSTRACT TRUNCATED AT 250 WORDS)

Lysine derived cross-links are present in a non-elastin, proline-rich protein fraction of Iguana iguana eggshell

1. Eggshell of Iguana iguana contains a variety of lysine-derived cross-linking amino acids apparently identical with some that occur in elastin. 2. Quantification of desmosine by amino acid analysis and radioimmunoassay gave estimates of 217 ng per mg and 109 ng per mg (dry weights), respectively. 3. [3H]NaBH4 reduction of eggshell protein established presence of lysine-derived aldehydes, allysine and allysine-aldol. 4. Enzymatic and chemical treatments indicate these cross-links occur in a non-elastin eggshell protein. 5. Iguana eggshell protein is unusually proline-rich, containing almost 440 proline residues per thousand.

A phylogenetic study of the role of cyclic AMP in lipid synthesis in vertebrates

The effect of cyclic AMP on the incorporation of acetate-2-14C into sterols and fatty acid in vitro in slices of liver and intestine was examined in various representatives of the vertebrate group. In no instance was an effect on lipid synthesis noted in intestine. Cyclic AMP exerted no significant effects on hepatic lipogenesis in lower vertebrates, including the nurse shark, catfish, toad, or iguana. However, the nucleotide strongly inhibited the incorporation of acetate-2-14C into fatty acid by the chicken liver. Similar inhibition of fatty acid synthesis was also noted in rat liver, but in this mammalian species hepatic sterol synthesis was also strikingly suppressed by cyclic AMP. Interruption of the enterohepatic circuit in the rat, while enhancing rates of sterol synthesis in both liver and intestine, neither enhanced nor diminished hepatic susceptibility to suppressed sterologenesis by cyclic AMP, nor did it confer on the intestine any newfound capacity for cyclic AMP-regulated lipid synthesis.