Research Paper

Embryogenesis of Longer-Lived Mammalian Species Occurs in a More Severe Hypoxic-Hypercapnic Environment

Authors

Khachik K. Muradian,^1,2,* Vadim E. Fraifeld,^1,2,*

¹Department of the Biology of Aging and Life Span Extension, State Institute of Gerontology of National Academy of Medical Sciences of Ukraine, Kiev, Ukraine

²The Shraga Segal Department of Microbiology, Immunology and Genetics, Center for Multidisciplinary Research on Aging, Ben-Gurion University of the Negev, Beer-Sheva, Israel

^*Corresponding authors: muradian1944@gmail.com; vadim.fraifeld@gmail.com

DOI:https://doi.org/10.59368/agingbio.20230018

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Abstract

In this study, we analyzed for the first time the relationships between species-specific longevity (maximum lifespan [MLS]) or gestation period (GP) and partial pressure of O₂ (Po₂), partial pressure of CO₂ (Pco₂), and pH in the blood of umbilical arteries (UAs) and veins in mammals. The results obtained allow to make four suggestions: (a) Embryonic development of placental mammals occurs in hypoxic-hypercapnic environment (HHE); (b) longer-lived and slower-developing species start their lives in more severe HHE; (c) mammalian longevity is associated with more efficient exchange of fetal blood CO₂ in the placenta; and (d) UA blood Pco₂ and pH, but not Po₂, display strong correlative links with MLS and GP, thus underpinning a special role of hypercapnia in development and longevity.

Introduction

It has long been known that embryonic development in multicellular animals occurs in low O₂ and high CO₂ environments^1,2. In mammals, the partial pressure of O₂ (Po₂) in fetal blood and tissues is lower, whereas the partial pressure of CO₂ (Pco₂) is higher than in adult organisms. For example, in pregnant sheep, llamas, horses, and humans, umbilical artery (UA) blood Po₂ is around twice as low as in the uterine artery and several fold lower than in the maternal systemic arterial Po₂^3–6.

The occasional decline of Po₂ from the normal values is a frequent and hazardous event, often with clinical complications for the fetus and mother^7,8. Natural selection should allegedly establish a wide safety window between the normal “hypoxic” values of Po₂ and critical levels of hypoxia. Nevertheless, Po₂ in a mammalian fetus is usually kept on the brink of life-threatening low levels. Suffice to mention that in humans, UA Po₂ is around five times lower than in the maternal artery^6,9. Yet, it is believed that many late-onset diseases originate from the latent impairment generated during embryogenesis but manifested in the advanced ages. The list of such diseases includes cardiovascular disorders, hypertension, obesity, type II diabetes, neurological and cognitive disorders, and so on^10–12. However, the question of how and why the hypoxic-hypercapnic environment (HHE) has become indispensable in embryogenesis and to what extent fetal hypoxia may impact gestation period (GP) and longevity has not yet been explored. Equally enigmatic and even less explored are the effects of elevated Pco₂ and associated hypercapnia on embryogenesis and longevity. The role of Pco₂ could be even more multifocal than Po₂ due to its direct effects not only on metabolic processes but also on the ubiquitous acid-base balance and pH through the well-known chain: CO₂ + H₂O ↔ H₂CO₃ ↔ HCO₃⁻ + H⁺. In this comparative study, we demonstrate for the first time the strong correlative links between species-specific longevity, GP, and fetal blood Pco₂ and pH, but not Po₂, in placental mammals.

Materials and Methods

Data on Po₂, Pco₂, and pH in the blood of UAs and veins of mammalian species were collected from publications in the PubMed, PMC, and Google Scholar databases. Over a thousand articles were extracted and manually curated. After rigorous curation, the data for random sampling of all mammals available in the literature (10 species, mostly animal husbandry or model organisms), with a clear description of the subjects and methods of measurement, were included in the analysis. To somewhat compensate for the relative scarcity of species, the data of different experiments (n) on the same species were used. The following species were included in the analysis: humans (n = 9), rhesus monkey (n = 9), horse (n = 5), cow (n = 3), llama (n = 3), pig (n = 4), sheep (n = 5), goat (n = 6), dog (n = 6), and guinea pig (n = 3), totaling 51 entries (Supplemental Table). The GP, maximum lifespan (MLS), and adult body mass (BM) records for these species were taken from the AnAge database¹³. Three-dimensional (3D) relationships between MLS, Po₂, and Pco₂ of UA or umbilical vein (UV) blood were analyzed by the quadratic 3D contour plots. Pairwise correlative relations were assessed by Spearman’s nonparametric coefficient of correlation or Pearson’s parametric coefficient of correlation. The parametric and nonparametric estimates brought about nonconflicting results. To evaluate a possible impact of BM on correlations of umbilical blood parameters with MLS, we used partial correlation analysis, which allows for eliminating the covariation effects. To ensure linear relationships, the MLS and BW values were log-transformed. Only coefficients with p < 0.05 were considered statistically significant and taken into discussion. All statistical analyses and figure drawing were performed by the Microsoft “Statistica-64” program.

Results and Discussion

The MLS values of the analyzed species varied within an order of magnitude (from 12 y in guinea pigs to 122.5 y in humans), and GP varied around 5 times (from 68 days in guinea pigs to 337 days in horses). As seen in Figure 1A, UA Pco₂ highly correlates with logMLS or GP, so the higher the UA Pco₂, the longer the MLS or GP. It should be taken into account that these correlations could be driven by BM rather than MLS. For example, Lorenzini et al. (2005) showed that the replicative capacity of cultured fibroblasts derived from 11 mammalian species correlates primarily with their BM but not longevity¹⁴. Similarly, as was shown in 15 rodent species, telomerase activity coevolves with BM but not MLS^15,16. In our dataset, the UA Pco₂ significantly correlates not only with logMLS (Fig. 1A) but also, almost to the same degree, with logBM (r = 0.69; p = 0.029). Yet, the partial correlation analysis revealed that controlling for logBM did not significantly affect the correlation between logMLS and UA Pco₂ (r = 0.69; p < 0.05). A similar effect was observed for logBM and UA Pco₂ when logMLS was controlled (r = 0.68; p < 0.05). The results obtained suggest that the impact of UA Pco₂ on mammalian longevity is independent of BM.

Figure 1.

The CO₂ clearance from CO₂-enriched blood of UA occurs in the placenta via the UA-capillary-UV system. Then, CO₂-depleted blood returns back to the fetus via UV. Thus, the difference in Pco₂ in UA and UV could obviously be indicative of the efficiency of fetal CO₂ diffusion and removal. Fairly good positive correlations of ΔPco₂ (UA Pco₂ − UV Pco₂) with MLS or GP suggest that longer-lived and slower-developing mammalian species possess a more efficient system of fetal trans-capillary CO₂ outflow (Fig. 1B).

Highly significant correlations of UA blood pH with MLS or GP were also observed (Fig. 1C). Because of the inverse relations between Pco₂ and pH, the negative correlations of pH with MLS and GP could at least partly be the consequences of the higher Pco₂ in longer-lived and slower-developing species.

In contrast to the significant correlations of UA Pco₂ and pH with MLS or GP, the corresponding correlations for Po₂ were insignificant (data not shown). Moreover, the same trend in adult mammalians has previously been reported by us, indicating that MLS correlates with systemic arterial blood Pco₂ and related HCO₃⁻ and pH, but not with Po₂¹⁷.

On the whole, the results of pairwise correlative analysis were further strengthened by the 3D contour plots of the MLS dependence on Pco₂ and Po₂ in UA. It should, however, be stressed that the 3D analysis highlights the complicated relationships between biological variables more adequately than linear models. Indeed, as seen in Figure 2, not just a high UA Pco₂, but rather the combination of higher UA Pco₂ with lower UA Po₂ is typical for the longer-lived mammals (red area). Specifically, in our sampling, the longer-lived mammals have UA Pco₂ higher than 50 mm Hg and UA Po₂ lower than 25 mm Hg. Accordingly, the shorter-lived species (green area) have UA Pco₂ lower than 50 mm Hg and UA Po₂ covering the entire range, including values that are higher than 25 mm Hg. It means that fetuses of long-lived species develop more severe HHE.

Figure 2.

All in all, placental mammals, humans included, start their existence and develop in HHE. Whatever the case, HHE effects are apparently not just the sum of reactions to hypoxia and hypercapnia but rather the result of intricate confrontation and/or cooperation, presumably between hypoxia-inducible factor-1 (HIF-1) and hypothetical hypercapnia-inducible factor (HcIF)^18,19. Acting mostly through HIF-1, lower oxygenation in hypoxia may decrease the fetal metabolic rate and thereby extend GP^20,21. The same hypometabolic effect could be expected for hypercapnia and hypothetical HcIF^19,22. Moreover, although one should be careful in the interpretation of correlations, positive links between Pco₂ and MLS could underpin the special role of hypercapnia in longevity.

Mammalian clades differ in types of placenta, which presumably may determine the level of fetal HHE. For example, primates have a hemochorial placenta that deeply penetrates into the uterine endometrium, while ungulates have a shallow placenta of epitheliochorial type²³. This, however, could not explain quite strong correlations between UA Pco₂, exchange of fetal blood CO₂, UA pH, and longevity. Indeed, the shortest-lived species in our set, guinea pigs, have the same placental type as humans and rhesus monkeys, the first- and third-longest-lived species in our set. Along with this, horses, the second-longest-lived species, have another type of placenta, an epitheliochorial one (Supplemental Table).

It remains poorly understood when and why HHE became an indispensable attribute of fetal development. We do not exclude that fetal HHE is a remnant of the adaptive reactions in response to the lower atmospheric O₂ and higher CO₂ at the time of life or multicellularity origin²⁴. Such adaptive traits could be fixed in the genome and preserved in the program of embryogenesis, as some other primordial features do.

Concluding Remarks

Here, we analyzed the links of MLS or GS with Po₂, Pco₂, and pH in the blood of UAs and UVs in mammals. The results of this pilot study allow to make four suggestions: (a) Embryonic development of mammals occurs in HHE; (b) longer-lived and slower-developing species start their lives in more severe HHE; (c) longer-lived species are characterized by more efficient fetal blood O₂/CO₂ exchange in the placenta; and (d) UA blood Pco₂ and pH, but not Po₂, display strong correlative links with MLS and GP. To our knowledge, this is the first study of that kind and therefore cannot answer all rising issues. It rather provokes new questions and explores new ideas. There are no satisfactory explanations why HHE is needed for embryogenesis at all. Are there HHE regimes that could be more optimal for embryogenesis than natural ones? Is it possible to extend the health span by maintaining pregnant or adult animals in artificial HHE? Despite the long history of the problem and obvious successes in embryology, these and many other related issues remain completely unexplored. Definitely, further research is necessary to fill in the gaps and create a stronger and wider basis for further findings.

Acknowledgments

K.K.M. was awarded Emergency Fellowships for Ukrainian researchers from The Israel Academy of Sciences and Humanities. The authors appreciate the assistance of Dr. E. David and Dr. E. Rudnitsky in preparing this article.

Conflict of Interest

No conflict of interest is declared.

Supplementary Materials

Supplemental information can be found online at https://doi.org/10.59368/agingbio.20230018.

References

1. Baron D.H., & Meschia G. (1954). A comparative study of the exchange of the respiratory gases across the placenta. Cold Spring Harb. Symp. Quant. Biol. 19, 93–101. PMID: 13291191; doi: 10.1101/sqb.1954.019.01.013.

2. Coleman A.J. (1967). Absence of harmful effect of maternal hypocapnia in babies delivered at caesarean section. Lancet 289(7494), 813–814. PMID: 4164298; doi: 10.1016/S0140-6736(67)92778-X.

3. Comline R.S., & Silver M. (1970). PO2, PCO2 and pH levels in the umbilical and uterine blood of the mare and ewe. J. Physiol. 209(3), 587–608. PMID: 5499798; doi: 10.1113/jphysiol.1970.sp009181.

4. Wilkening R.B., & Meschia G. (1992). Current topic: Comparative physiology of placental oxygen transport. Placenta 13(1), 1–15. PMID: 1502133; doi: 10.1016/0143-4004(92)90002-B.

5. Riquelme R.A., Sánchez G., Liberona L., Sanhueza E.M., Giussani D.A., Blanco C.E., … Llanos A.J. (2002). Nitric oxide plays a role in the regulation of adrenal blood flow and adrenocorticomedullary functions in the llama fetus. J. Physiol. 544(1), 267–276. PMID: 12356897; doi: 10.1113/jphysiol.2002.018325.

6. Kotaska K., Urinovska R., Klapkova E., Prusa R., Rob L., & Binder T. (2010). Re-evaluation of cord blood arterial and venous reference ranges for pH, pO2, pCO2, according to spontaneous or cesarean delivery. J. Clin. Lab. Anal. 24(5), 300–304. PMID: 20872563; doi: 10.1002/jcla.20405.

7. Siragher E., & Sferruzzi-Perri A.N. (2021). Placental hypoxia: What have we learnt from small animal models? Placenta 113, 29–47. PMID: 34074553; doi: 10.1016/j.placenta.2021.03.018.

8. Sebastiani G., Navarro-Tapia E., Almeida-Toledano L., Serra-Delgado M., Paltrinieri A.L., García-Algar Ó., & Andreu-Fernández V. (2022). Effects of antioxidant intake on fetal development and maternal/neonatal health during pregnancy. Antioxidants (Basel) 11(4), 648. PMID: 35453333; doi: 10.3390/antiox11040648.

9. Baena-García L., Coll-Risco I., Ocón-Hernández O., Romero-Gallardo L., Acosta-Manzano P., May L., & Aparicio V.A. (2020). Association of objectively measured physical fitness during pregnancy with maternal and neonatal outcomes. The GESTAFIT Project. PLoS One 15(2), e0229079. PMID: 32069319; doi: 10.1371/journal.pone.0229079.

10. Barker D.J. (1993). Fetal origins of coronary heart disease. Br. Heart J. 69(3), 195–196. PMID: 8461215; doi: 10.1136/hrt.69.3.195.

11. Bateson P., Barker D., Clutton-Brock T., Deb D., D’Udine B., Foley R.A, … Sultan S.E. (2004). Developmental plasticity and human health. Nature 430(6998), 419–421. PMID: 15269759; doi: 10.1038/nature02725.

12. Gluckman P.D., Hanson M.A., Cooper C., & Thornburg K.L. (2008). Effect of in utero and early-life conditions on adult health and disease. N. Engl J. Med. 359(1), 61–73. PMID: 18596274; doi: 10.1056/NEJMra0708473.

13. Tacutu R., Thornton D., Johnson E., Budovsky A., Barardo D., Craig T., … de Magalhães J.P. (2018). Human ageing genomic resources: New and updated databases. Nucleic Acids Res. 46, D1083–D1090. PMID: 29121237; doi: 10.1093/nar/gkx1042.

14. Lorenzini A., Tresini M., Austad S.N., & Cristofalo V.J. (2005). Cellular replicative capacity correlates primarily with species body mass not longevity. Mech. Ageing Dev. 126(10), 1130–1133. PMID: 15993927; doi: 10.1016/j.mad.2005.05.004.

15. Seluanov A., Chen Z., Hine C., Sasahara T.H., Ribeiro A.A., Catania K.C., … Gorbunova V. (2007). Telomerase activity coevolves with body mass not lifespan. Aging Cell 6(1), 45–52. PMID: 17173545; doi: 10.1111/j.1474-9726.2006.00262.x.

16. Gorbunova V., & Seluanov A. (2009). Coevolution of telomerase activity and body mass in mammals: From mice to beavers. Mech. Ageing Dev. 130(1–2), 3–9. PMID: 18387652; doi: 10.1016/j.mad.2008.02.008.

17. Muradian K. (2013). “Pull and push back” concepts of longevity and life span extension. Biogerontology 14(6), 687–691. PMID: 24114506; doi: 10.1007/s10522-013-9472-1.

18. Fraifeld V.E., & Muradian Kh.K. (2021). Editorial. Hypoxic-hypercapnic environment as a model of hypometabolism, calorie restriction, hypoglycemia and non-medicamentous treatment of related pathology. Diabetes Res. Metab. 2, 110. doi: 10.36879/DRM.20.000110.

19. Muradian Kh.K., & Fraifeld V.E. (2021). Hypercapnia inducible factor: A hypothesis. Aging Longevity (A&L) 2(3), 27–31. doi: 10.47855/jal9020-2021-3-4.

20. Fajersztajn L., & Veras M.M. (2017). Hypoxia: From placental development to fetal programming. Birth Defects Res. 109(17), 1377–1385. PMID: 29105382; doi: 10.1002/bdr2.1142.

21. Soares M.J., Iqbal K., & Kozai K. (2017). Hypoxia and placental development. Birth Defects Res. 109(17), 1309–1329. PMID: 29105383; doi: 10.1002/bdr2.1135.

22. Tolstun D.A., Knyazer A., Tushynska T.V., Dubiley T.A., Bezrukov V.V., Fraifeld V.E., & Muradian K.K. (2020). Metabolic remodelling of mice by hypoxic-hypercapnic environment: Imitating the naked mole-rat. Biogerontology 21(2), 143–153. PMID: 31667660; doi: 10.1007/s10522-019-09848-9.

23. Furukawa S., Kuroda Y., & Sugiyama A. (2014). A comparison of the histological structure of the placenta in experimental animals. J. Toxicol. Pathol. 27(1), 11–18. PMID: 24791062; doi: 10.1293/tox.2013-0060.

24. Catling D.C., & Zahnle K.J. (2020). The Archean atmosphere. Sci Adv. 6(9), eaax1420. PMID: 32133393; doi: 10.1126/sciadv.aax1420.