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Bautista NM, Malte H, Natarajan C, Wang T, Storz JF, Fago A. New insights into the allosteric effects of CO2 and bicarbonate on crocodilian hemoglobin. J Exp Biol 2021; 224:271141. [PMID: 34338300 DOI: 10.1242/jeb.242615] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/22/2021] [Accepted: 06/28/2021] [Indexed: 01/01/2023]
Abstract
Crocodilians are unique among vertebrates in that their hemoglobin (Hb) O2 binding is allosterically regulated by bicarbonate, which forms in red blood cells upon hydration of CO2. Although known for decades, this remarkable mode of allosteric control has not yet been experimentally verified with direct evidence of bicarbonate binding to crocodilian Hb, probably because of confounding CO2-mediated effects. Here, we provide the first quantitative analysis of the separate allosteric effects of CO2 and bicarbonate on purified Hb of the spectacled caiman (Caiman crocodilus). Using thin-layer gas diffusion chamber and Tucker chamber techniques, we demonstrate that both CO2 and bicarbonate bind to Hb with high affinity and strongly decrease O2 saturation of Hb. We propose that both effectors bind to an unidentified positively charged site containing a reactive amino group in the low-O2 affinity T conformation of Hb. These results provide the first experimental evidence that bicarbonate binds directly to crocodilian Hb and promotes O2 delivery independently of CO2. Using the gas diffusion chamber, we observed similar effects in Hbs of a phylogenetically diverse set of other caiman, alligator and crocodile species, suggesting that the unique mode of allosteric regulation by CO2 and bicarbonate evolved >80-100 million years ago in the common ancestor of crocodilians. Our results show a tight and unusual linkage between O2 and CO2 transport in the blood of crocodilians, where the build-up of erytrocytic CO2 and bicarbonate ions during breath-hold diving or digestion facilitates O2 delivery, while Hb desaturation facilitates CO2 transport as protein-bound CO2 and bicarbonate.
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Affiliation(s)
- Naim M Bautista
- Department of Biology, Aarhus University, 8000 Aarhus C, Denmark
| | - Hans Malte
- Department of Biology, Aarhus University, 8000 Aarhus C, Denmark
| | | | - Tobias Wang
- Department of Biology, Aarhus University, 8000 Aarhus C, Denmark
| | - Jay F Storz
- School of Biological Sciences , University of Nebraska, Lincoln, NE 68588, USA
| | - Angela Fago
- Department of Biology, Aarhus University, 8000 Aarhus C, Denmark
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Williams CJA, Malte CL, Malte H, Bertelsen MF, Wang T. Ectothermy and cardiac shunts profoundly slow the equilibration of inhaled anaesthetics in a multi-compartment model. Sci Rep 2020; 10:17157. [PMID: 33051496 PMCID: PMC7555730 DOI: 10.1038/s41598-020-74014-y] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/01/2020] [Accepted: 09/21/2020] [Indexed: 01/08/2023] Open
Abstract
The use of inhalational anaesthesia is ubiquitous in terrestrial vertebrates. Given the dependence of these agents on delivery by the cardiorespiratory system, we developed a new computational model predicting equilibration of inhaled anaesthetics in mammalian and ectotherm conditions including the ability of reptiles to maintain vascular shunts. A multi-compartment model was constructed from simultaneously-solved equations, verified by comparison to the literature for endo and ectotherm physiology. The time to 90% equilibration of anaesthetic in arterial blood (t90) is predicted and used to compare anaesthetics and physiologies. The five to tenfold lower cardiac output and minute ventilation of ectothermic vertebrates is predicted to slow equilibration times by five to ten times leading to 90% equilibration in ectotherm arterial blood of over 200 min, compounded by reduction in body temperature, and the extent of right-to-left vascular shunts. The impact of these findings is also influenced by the solubility coefficient of the anaesthetic, such that at net right-to-left shunt fractions of over 0.8, sevoflurane loses the advantage of faster equilibration, in comparison with isoflurane. We explore clinical strategies to regulate anaesthetic uptake in ectotherms by managing convectional flow especially by supportive ventilation and reduction of the right-to-left shunt.
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Affiliation(s)
- Catherine J A Williams
- Section of Zoophysiology, Department of Biology, Aarhus University, 8000, Aarhus C, Denmark. .,Center for Zoo and Wild Animal Health, Copenhagen Zoo, Roskildevej 38, 2000, Frederiksberg, Denmark. .,Ontario Veterinary College, University of Guelph, 50 Stone Road E, Guelph, ON, N1G 2W1, Canada.
| | - Christian Lind Malte
- Section of Zoophysiology, Department of Biology, Aarhus University, 8000, Aarhus C, Denmark
| | - Hans Malte
- Section of Zoophysiology, Department of Biology, Aarhus University, 8000, Aarhus C, Denmark
| | - Mads F Bertelsen
- Center for Zoo and Wild Animal Health, Copenhagen Zoo, Roskildevej 38, 2000, Frederiksberg, Denmark
| | - Tobias Wang
- Section of Zoophysiology, Department of Biology, Aarhus University, 8000, Aarhus C, Denmark.,Aarhus Institute of Advanced Sciences, Aarhus University, 8000, Aarhus C, Denmark
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Burggren W, Filogonio R, Wang T. Cardiovascular shunting in vertebrates: a practical integration of competing hypotheses. Biol Rev Camb Philos Soc 2019; 95:449-471. [PMID: 31859458 DOI: 10.1111/brv.12572] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/05/2019] [Revised: 10/30/2019] [Accepted: 11/07/2019] [Indexed: 12/20/2022]
Abstract
This review explores the long-standing question: 'Why do cardiovascular shunts occur?' An historical perspective is provided on previous research into cardiac shunts in vertebrates that continues to shape current views. Cardiac shunts and when they occur is then described for vertebrates. Nearly 20 different functional reasons have been proposed as specific causes of shunts, ranging from energy conservation to improved gas exchange, and including a plethora of functions related to thermoregulation, digestion and haemodynamics. It has even been suggested that shunts are merely an evolutionary or developmental relic. Having considered the various hypotheses involving cardiovascular shunting in vertebrates, this review then takes a non-traditional approach. Rather than attempting to identify the single 'correct' reason for the occurrence of shunts, we advance a more holistic, integrative approach that embraces multiple, non-exclusive suites of proposed causes for shunts, and indicates how these varied functions might at least co-exist, if not actually support each other as shunts serve multiple, concurrent physiological functions. It is argued that deposing the 'monolithic' view of shunting leads to a more nuanced view of vertebrate cardiovascular systems. This review concludes by suggesting new paradigms for testing the function(s) of shunts, including experimentally placing organ systems into conflict in terms of their perfusion needs, reducing sources of variation in physiological experiments, measuring possible compensatory responses to shunt ablation, moving experiments from the laboratory to the field, and using cladistics-related approaches in the choice of experimental animals.
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Affiliation(s)
- Warren Burggren
- Department of Biological Sciences, Developmental Integrative Biology Cluster, University of North Texas, Denton, TX, 76203-5220, U.S.A
| | - Renato Filogonio
- Department of Physiological Sciences, Federal University of São Carlos, São Carlos, SP, 13565-905, Brazil
| | - Tobias Wang
- Zoophysiology, Department of Bioscience, Aarhus University, Aarhus C, 8000, Denmark.,Aarhus Institute of Advanced Sciences (AIAS), Aarhus University, Aarhus C, 8000, Denmark
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Conner JL, Crossley JL, Elsey R, Nelson D, Wang T, Crossley DA. Does the left aorta provide proton-rich blood to the gut when crocodilians digest a meal? ACTA ACUST UNITED AC 2019; 222:jeb.201079. [PMID: 30787137 DOI: 10.1242/jeb.201079] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/04/2019] [Accepted: 02/15/2019] [Indexed: 11/20/2022]
Abstract
Reptiles have the capacity to differentially perfuse the systemic and pulmonary vascular circuits via autonomic regulation of the heart and the vascular trees. While this aptitude is widely recognized, the role of 'shunting' as a homeostatic mechanism to match convective transport with tissue demand remains unknown. In crocodilians, it has been hypothesized that a pulmonary vascular bypass of systemic venous blood - a right-to-left (R-L) shunt - serves to deliver CO2-rich blood with protons needed for gastric acid secretion during digestion. This hypothesis is partially based on the unique crocodilian vascular anatomy where a left aorta (LAo) arises from the right ventricle, and appears to preferentially supply the gastrointestinal system, whereas the right aorta emerges from the left ventricle. Recent theoretical considerations imply that a R-L shunt would have minuscule effects on P CO2 , but direct measurements of blood gases in both the right and left aortae or both the right and left atria in fed animals have not been conducted. For this reason, we measured blood parameters including P O2 , P CO2 , pHe and [HCO3 -] in the right and left aortae and atria following ingestion of a gavage-fed standardized meal (5% body mass). Blood samples were taken at 3, 6, 12, 24, 36 and 48 h into the digestive period to encompass the period of maximal gastric acid secretion. At no point did P CO2 or pH differ between the left and right aortae, whereas P O2 was significantly lower in the left aorta at several time points during digestion. Our findings do not support the hypothesis that a R-L shunt serves to deliver CO2 for the gastrointestinal system after feeding in crocodilians.
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Affiliation(s)
- Justin L Conner
- Department of Biological Sciences, University of North Texas, Denton, TX 76203, USA
| | - Janna L Crossley
- Department of Biological Sciences, University of North Texas, Denton, TX 76203, USA
| | - Ruth Elsey
- Department of Wildlife and Fisheries, Grand Chenier, LA 70603, USA
| | - Derek Nelson
- Department of Biological Sciences, University of North Texas, Denton, TX 76203, USA
| | - Tobias Wang
- Zoophysiology, Department of Bioscience, Aarhus University, 8000 Aarhus C, Denmark
| | - Dane A Crossley
- Department of Biological Sciences, University of North Texas, Denton, TX 76203, USA
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Findsen A, Crossley DA, Wang T. Feeding alters blood flow patterns in the American alligator (Alligator mississippiensis). Comp Biochem Physiol A Mol Integr Physiol 2018; 215:1-5. [DOI: 10.1016/j.cbpa.2017.09.001] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/04/2016] [Revised: 02/07/2017] [Accepted: 09/05/2017] [Indexed: 10/18/2022]
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