Biomedical subjects

G K Snyder

Publications and source records attributed to G K Snyder.

At least 19 recordsLinked to original sources

PubMed1999

Oxygen transport and acid-base balance during exercise in the tammar wallaby.

Rates of oxygen consumption (V(O)2), body temperatures and pulmonary blood temperatures, blood gases and blood pH were measured for seven 4.9+/-0.8 (SE) kg tammar wallabies (Macropus eugenii) during rest and during treadmill hopping. For animals resting on the treadmill V(O)2 averaged 0.030+/-0.003 L min(-1). During hopping V(O)2 increased linearly with speed up to 2.5 m sec(-1). Above 2.5 m sec(-1) V(O)2 was independent of hopping speed and averaged 0.340+/-0.004 L min(-1). At rest, rectal temperatures and pulmonary blood temperatures averaged 36 degrees C. During treadmill hopping, rectal temperatures and pulmonary blood temperatures increased similarly, to 39 degrees C. The Pv(CO)2 increased and pHv decreased in proportion to the increased V(O)2. The Pa(CO)2 and pHa were not significantly changed from values for animals resting on the treadmill. Cardiac output (Vb) averaged 0.97+/-0.04 L min(-1) when the wallabies were at rest on the treadmill and increased linearly with treadmill speeds up to 2.5 m sec(-1). Above 2.5 m sec(-1) Vb was independent of hopping speed and averaged 2.9+/-0.04 L min(-1). When data for all speeds were combined, Vb increased linearly with V(O)2. Thus, in spite of their unique mode of locomotion wallabies have maintained relationships between pulmonary ventilation and V(O)2 and between Vb and V(O)2 that are similar to those reported for eutherian mammals.

Acid-Base Equilibrium↗

PubMed1995

Gas exchange in the insect tracheal system.

The role of the insect tracheal system in gas exchange during cyclic ventilation is investigated using mathematical models. Three models are presented, one to describe the important elements of gas exchange during each of the three phases of cyclic ventilation. The effects of normobaric hypoxia on gas exchange are then examined, first assuming the initial parameter values set for the tracheal system and, second, assuming conditions of tracheal hypertrophy produced by an increase in the cross-sectional area of the tubes in the tracheal system. An increase in tracheal tube cross-sectional area is an important adaptation to normobaric hypoxia, but only if the tracheae themselves are the primary sites of resistance to gas exchange. Under conditions where the spiracles are the sites of resistance to gas exchange, volume expansion of the tracheae, not an increased cross-sectional area per se, is the important adaptation to normobaric hypoxia.

Adaptation, Physiological↗

PubMed1995

Capillary growth in chick skeletal muscle with normal maturation and hypertrophy.

Tissue capillarity and tissue enzyme activity (citrate synthase and lactate dehydrogenase) were determined for two red muscles and two white muscles from the domestic chicken during normal maturation and, for one red muscle, during muscle hypertrophy. Muscle fiber cross sectional area increased with muscle mass during normal maturation and with the additional increase in muscle mass following hypertrophy. Normal maturation and hypertrophy did not affect lactate dehydrogenase activity or citrate synthase activity for the muscles with anaerobic fiber types. Citrate synthase activity per unit muscle mass was positively correlated with muscle capillary density for the muscles with aerobic fiber types. Capillary to fiber ratios increased with fiber size and were significantly higher in muscles with aerobic fibers than in muscles with nonaerobic fibers. However, capillary densities decreased with maturation and with fiber hypertrophy. For each of the muscles sampled, the number of capillaries per unit linear distance of muscle fiber perimeter was independent of muscle fiber growth during normal maturation and during hypertrophy. The results from the present work are consistent with the hypothesis that muscle fiber type and muscle fiber surface area may be the primary determinants of capillary growth during normal maturation and during fiber hypertrophy.

Aerobiosis↗

PubMed1995

Intracellular pH in lizards after hypercapnia.

We used the transmembrane distribution of 5,5-[2-14C]dimethyloxazolidine-2,4-dione ([14C]DMO) and 31P magnetic resonance spectroscopy (NMR) to investigate the effects of hypercapnia on intracellular pH (pHi) in brain and skeletal muscle of two lizard species: Anolis equestris and Dipsosaurus dorsalis. In control animals (normocapnic), plasma PCO2 (3.3 +/- 0.1 kPa) and plasma pH (7.52 +/- 0.01) for D. dorsalis were not significantly different from the values for A. equestris (2.8 +/- 0.2 kPa and 7.59 +/- 0.02, respectively). Furthermore 60 min of 5% CO2 increased plasma PCO2 and decreased plasma pH by the same amounts in both species. Brain pHi values determined with the DMO method were not significantly different from values determined with NMR. Control values of brain pHi (DMO, 7.16 +/- 0.01; NMR, 7.11 +/- 0.02) and muscle pHi were significantly higher for D. dorsalis (DMO, 7.15 +/- 0.03) than for A. equestris (DMO, 6.99 +/- 0.03; NMR, 7.02 +/- 0.02 for brain; DMO, 6.97 +/- 0.03 for muscle). In addition, changes in tissue pHi after 60 min of 5% CO2 were significantly different for the two species. In D. dorsalis muscle and brain pHi decreased significantly after hypercapnia, whereas in A. equestris muscle pHi decreased significantly but brain pHi was unchanged. Our findings were independent of the methods used to determine pHi. The smaller change in brain and muscle pHi than in plasma pH for A. equestris is consistent with the view that pHi regulation involves active processes such as transmembrane ion transport.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1994

Acid-base status of a pulmonate land snail (Helix aspersa) and a prosobranch amphibious snail (Pomacea bridgesi) during dormancy.

Changes in metabolism and acid-base status were compared during dormancy in the pulmonate land snail Helix aspersa and a prosobranch amphibious snail Pomacea bridgesi. The typical condition of higher blood PCO2 and bicarbonate levels for air-breathing versus water-breathing vertebrates was shown for the two snail species. When exposed to dry air for 24 hr, both species depressed oxygen uptake by about 65%. In Pomacea, hypercapnia (increase in hemolymph PCO2 from 5.5 to 18 torr) resulting from dormancy produced no significant change in pH due to large increases in bicarbonate (over 17 mmol/l). In Helix, on the other hand, hypercapnia (increase in hemolymph PCO2 from 13 to 18 torr) resulting from dormancy produced a significant decrease in pH and a less than 7 mmol/l increase in bicarbonate. Pre-existing high levels of bicarbonate in Helix may prevent compensation of hypercapnia resulting from dormancy, similar to the case described for air-breathing vertebrates. Complete compensation of respiratory acidosis during the first 24 hr of dormancy in Pomacea suggests that metabolic rate suppression is independent of pH.

Acid-Base Equilibrium↗

PubMed1993

An improved method for studying microvascular geometry using fluorescent dyes: preventing dye extravasation, preserving dye integrity and enhancing tissue morphometry.

A procedure for stabilizing fluorescent markers used to study the microvascular geometry and morphometry of muscle tissue is described. The procedure involves fluorescent labeling of plasma, fixation of muscle tissue in 10% buffered formalin, and quick freezing. This procedure prevents extravasation of the fluorescent dyes out of the capillaries as frequently seen in other muscle microvascular techniques, thereby greatly increasing the time that capillaries are visible. We found that formalin may actually increase the rate of fluorochrome bleaching by photo-oxidation, but the increased rate of bleaching is more than offset by the greater concentration of dye trapped in the capillaries. Further, formalin fixation results in little distortion of the muscle fibers themselves, making this approach ideal for morphometric studies.

Animals↗

PubMed1992

Adaptations in skeletal muscle capillarity following changes in oxygen supply and changes in oxygen demands.

The effects of changes in oxygen supply and oxygen demands on fiber cross-sectional areas, capillary densities and capillary to fiber ratios were determined in three skeletal muscles of rat. The muscles examined were the vastus lateralis, soleus, and diaphragm. Reduced oxygen supply was produced by subjecting rats to ambient hypoxia, and increased oxygen demands were produced by subjecting rats to low ambient temperatures or treatment with thyroxin. Capillaries were visualized by injecting fluorescent dyes into the circulation. Muscles were quick frozen at resting lengths to preserve normal fiber geometry and were subsequently sectioned on a cryostat. All of the muscles sampled from animals in the experimental groups had elevated capillary densities. However, capillary to fiber ratios were not increased significantly in any muscle, for any experimental condition. Thus, all of the observed differences in capillarity were due to changes in the intrinsic rate of muscle fiber growth. Further, the relations of capillary density and capillary to fiber ratio to fiber area were the same as those obtained during normal maturation, suggesting that capillary growth is closely linked to the intrinsic rate of fiber growth.

Adaptation, Physiological↗

PubMed1992

Capillary perfusion in skeletal muscle.

Timed perfusions of capillaries in soleus, vastus lateralis, and diaphragm of rats were determined by injecting a fluorescent dye into the aorta or right atrium. At preselected time intervals after dye injection, the circulation was interrupted, skeletal muscles were excised and frozen, and numerical counts of capillaries were made from transverse sections of the muscles. When fluorescent dye was injected into the right atrium or injected relatively slowly into the aorta, 100% of the capillaries contained dye by 3 s in vastus lateralis, 5 s in diaphragm, and 7 s in soleus. Rapid injection into the aorta delayed capillary filling. The rapid appearance of dye in skeletal muscle microvasculature suggests that either all capillaries are open to plasma flow all of the time or that the time constant for reopening of closed capillaries is very short. Furthermore, the patterns of capillary filling in skeletal muscle appear to be independent of muscle fiber type.

Animals↗

PubMed1992

Energetic cost of locomotion in the tammar wallaby.

Rates of oxygen consumption and blood lactate levels were measured in tammar wallabies (Macropus eugenii) trained to hop on a treadmill. In addition, the work required to overcome wind resistance during forward locomotion was measured in a wind tunnel. Up to approximately 2.0 m/s, rates of oxygen consumption increased linearly with speed and were not significantly different from rates of oxygen consumption for a quadruped of similar body mass. Between 2.0 and 9.4 m/s, rates of oxygen consumption were independent of hopping speed, and between 3.9 and 7.9 m/s, the range over which samples were obtained, blood lactate levels were low (0.83 +/- 0.13 mmol.min-1.kg-1) and did not increase with hopping speed. The work necessary to overcome drag increased exponentially with speed but increased the energy cost of locomotion by only 10% at the average speed attained by our fast hoppers. Thus, during hopping, the energy cost of locomotion is effectively independent of speed. At rates of travel observed in the field, the estimated energy cost of transport in large macropods is less than one-third the cost for a quadruped of equivalent body mass. The energetic savings associated with this unique form of locomotion may have been an important physiological adaptation, enabling large macropods to efficiently cover the distances necessary to forage in the semiarid landscapes of Australia.

Aerobiosis↗

PubMed1992

Capillarity in rat skeletal muscle: effects of rapid freezing versus perfusion fixation.

We determined the effects of rapid freezing and perfusion fixation on fiber geometry and capillarity in rat skeletal muscle. Fiber areas were significantly decreased, and capillary densities significantly increased, in perfusion-fixed versus quick-frozen muscle. Significant differences in capillary-to-fiber ratios were not observed, suggesting that differences in fiber geometry, not the methods of quantifying capillaries, accounted for the differences in capillary density. We conclude that estimates of fiber geometry, capillarity, and diffusive gas conductances obtained from perfusion-fixed muscles are subject to significant error due to shrinkage.

Animals↗

PubMed1991

Intracellular pH in the toad Bufo marinus following hypercapnia.

We investigated the effects of hypercapnia on intracellular acid-base regulation in brain and liver of the toad Bufo marinus L. After 1 h at 5% CO2, arterial PCO2 increased significantly, from 1.6 +/- 0.04 to 5.7 +/- 0.23 kPa, while brain and liver intracellular pH (pHi) decreased significantly. Reductions in pHi of both tissues were partially compensated by increased levels of bicarbonate. Surprisingly, however, compensation was lower than expected in brain and higher than expected in liver. We suggest that compensation in brain may be limited by secondary effects of bicarbonate loading in this tissue.

Acid-Base Equilibrium↗

PubMed1990

Capillarity and diffusion distances in skeletal muscles in birds.

Tissue capillarity and diffusion distances were determined for red and white skeletal muscles of adult birds ranging in mass from 10.8 to 6200 g. In addition, literature values for capillarity and diffusion distances in skeletal muscles of mammals were incorporated into the data set. Muscle mass was closely coupled to body mass. However, no significant allometric relations were found for any of the other variables measured. Number of capillaries per fiber was not correlated with cross sectional area of individual muscle fibers. Thus, capillary density decreased in a hyperbolic manner against fiber area and diffusion distance decreased in a hyperbolic manner against the number of capillaries per muscle fiber. Red muscles had significantly higher numbers of capillaries per fiber and significantly shorter diffusion distances than did white muscles. The patterns for tissue capillarity and diffusion distances in avian muscle reported here are similar to values reported previously for mammalian muscles. In both taxonomic groups capillarity and diffusion distances are independent of body mass. In addition, diffusion distances are characteristic of capillaries distributed in random arrays through the muscle cross section.

Animals↗

PubMed1990

Relationships between body temperature, thermal conductance, Q10 and energy metabolism during daily torpor and hibernation in rodents.

In the present paper we examine the ability of rodents to maintain body temperature (TB) following the marked reductions in metabolic heat production associated with torpor. Previously published values for metabolic rate (M), TB and ambient temperature (TA) were used to calculate thermal conductances (C') during normothermy and torpor in rodents capable of daily torpor (11 species) and hibernation (18 species). Values of C' for torpid animals are uniformly lower than C' in normothermic animals. In addition, C' of normothermic and torpid rodents decreases with increasing body mass (BM). However, the slope of the relationship between C' and BM is almost 4-fold greater for normothermic than for torpid animals. Thus, the ability of torpid rodents to conserve body heat by reducing C' decreases with increasing mass. Rodents that use daily torpor tend to be small and they tend to maintain TB well above TA during torpor. Hibernators tend to be larger and regulate TB relatively close to TA. Thus, the reductions in C' appear to be closely correlated with the level of TB regulation during torpor. We suggest that the changes in C' represent a suite of physiological adaptations that have played a central role in the evolution of torpor, enabling rodents to regulate TB above TA during periods of very low heat production. Based on the approach used here we address the controversy of whether reductions in M during torpor are due entirely to temperature effects or whether metabolic inhibition in addition to temperature effects may be important. We suggest that the controversy has been confused by using Q10 to evaluate the relationship of M and TB in endotherms.(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptation, Physiological↗

PubMed1989

Functional organization of cerebral microvasculature in a marsupial, the northern quoll (Dasyurus hallucatus).

Capillaries within the central nervous system (CNS) of eutherian mammals form meshworks with numerous anastomoses, whereas capillaries in the CNS of marsupials consist entirely of hairpin-like loops, without anastomotic interconnections. Counter-current blood flow in capillary loops may have been important in the evolutionary development of a cerebral vascular supply. However, loops are not found in eutherian mammals, perhaps because of a limited benefit to the diffusive conductance of gases.

Animals↗

PubMed1989

Microvascular development in chick anterior latissimus dorsi following hypertrophy.

Right (hypertrophied) and left (control) anterior latissimus dorsi muscles (ALD) were injected with India ink to visualise microvascular geometry in these muscles. The ALD muscles have multiple feeding arterioles that branch into meshworks of arcade arterioles. Transverse arterioles arise from the arcade system and give rise to capillaries. Hypertrophy is attended by an increase in the combined length of arterioles in the arcade system due to an increase in the number of arcade segments. Morphometric characteristics of each segment remained constant. Hypertrophy produced a significant increase in the numbers of capillaries per individual muscle fibre due to an increase in number of transverse arterioles and an increase in number of capillaries per transverse arteriole. However, the increased numbers of capillaries per fibre just matched fibre hypertrophy, so that the number of exchange vessels per cross sectional area of muscle remained constant. These results are consistent with the argument that muscle growth is the primary determinant of capillarity.

Animals↗

PubMed1988

Model analyses of capillary growth and tissue oxygenation during hypoxia.

Theoretical analyses were used to determine whether capillary growth is an adaptive response to hypoxia. Parameter values were obtained from models of transverse sections of muscles in which individual fibers were distributed in square-ordered arrays and capillaries were added to the perimeters of individual fibers in the arrays. Increasing the number of capillaries up to 2.0 per fiber increased hypoxic tolerance by 157% above that expected for a Krogh cylinder. However, increasing the number of capillaries from 2.0 to 4.0 per fiber increased hypoxic tolerance by only 18% and, assuming the entire perimeter of each fiber was perfused with blood, increased hypoxic tolerance by only 11% over the value obtained when capillary-to-fiber ratio was 4.0. Capillary growth during normal maturation may result in capillary-to-fiber ratios around 2.0, near the upper limit for producing marked changes in hypoxic tolerance. Therefore, capillary growth may not be an adaptive response to ambient hypoxia because there is little or no gas transport benefit derived from the additional capillaries.

Adaptation, Physiological↗

PubMed1988

Anatomical organisation of the microvasculature in the anterior and posterior latissimus dorsi muscles of the chicken.

Anterior and posterior latissimus dorsi muscles of twelve chickens were injected with India ink to visualise their microvasculature. Both muscles have multiple feeding arterioles that branch into a meshwork of arcade arterioles. Transverse arterioles arise from the arcade system and these, in turn, give rise to capillaries. The major differences in the microvasculatures of the two muscles are in the numbers of vessels in the arcade meshworks and the dimensions of these vessels. The differences in geometry and topology of the arcade vessels suggest that the microvasculature in the anterior latissimus dorsi may be able to support levels of oxidative metabolism almost 8 times higher than those in the posterior latissimus dorsi.

Animals↗

PubMed1987

Capillary growth and diffusion distances in muscle.

1. Tissue capillarity in muscle was modelled as square-ordered arrays with capillary-to-fiber ratios (C/F) from 0.5 to 'infinity'. 2. C/F up to two had marked effects on diffusion distances, but C/F above had only slight effects on average distances and almost no effect on maximal distances. 3. Capillary growth during normal maturation results in C/F around two. Thus, capillary growth in adult muscle may not be an adaptive mechanism for reducing diffusion distances.

Aging↗