The acid–base relevant molecules carbon dioxide (CO2), protons (H+), and bicarbonate
(HCO3−) are substrates and end products of some of the most essential physiological
functions including aerobic and anaerobic respiration, ATP hydrolysis, photosynthesis, and calcification. The structure and function of many enzymes and other macromolecules are highly sensitive to changes in pH, and thus maintaining acid–base homeostasis in the face of metabolic and environmental disturbances is essential for proper cellular function. On the other hand, CO2, H+, and HCO3− have regulatory effects on various proteins and processes, both directly through allosteric modulation and indirectly through signal transduction pathways. Life in aquatic environments presents organisms with distinct acid–base challenges that are not found in terrestrial environments. These include a relatively high CO2 relative to O2 solubility that prevents internal CO2/HCO3
− accumulation to buffer pH, a lower O2 content that may favor anaerobic metabolism, and variable environmental CO2, pH and O2 levels that require dynamic adjustments in acid–base homeostatic mechanisms. Additionally, some aquatic animals purposely create acidic or alkaline microenvironments that drive specialized physiological functions. For example,
acidifying mechanisms can enhance O2 delivery by red blood cells, lead to ammonia
trapping for excretion or buoyancy purposes, or lead to CO2 accumulation to promote
photosynthesis by endosymbiotic algae. On the other hand, alkalinizing mechanisms
can serve to promote calcium carbonate skeletal formation. This nonexhaustive review summarizes some of the distinct acid–base homeostatic mechanisms that have evolved in aquatic organisms to meet the particular challenges of this environment.
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Rapid blood acid–base regulation by European sea bass ( Dicentrarchus labrax ) in response to sudden exposure to high environmental CO2
ABSTRACT Fish in coastal ecosystems can be exposed to acute variations in CO2 of between 0.2 and 1 kPa CO2 (2000–10,000 µatm). Coping with this environmental challenge will depend on the ability to rapidly compensate for the internal acid–base disturbance caused by sudden exposure to high environmental CO2 (blood and tissue acidosis); however, studies about the speed of acid–base regulatory responses in marine fish are scarce. We observed that upon sudden exposure to ∼1 kPa CO2, European sea bass (Dicentrarchus labrax) completely regulate erythrocyte intracellular pH within ∼40 min, thus restoring haemoglobin–O2 affinity to pre-exposure levels. Moreover, blood pH returned to normal levels within ∼2 h, which is one of the fastest acid–base recoveries documented in any fish. This was achieved via a large upregulation of net acid excretion and accumulation of HCO3− in blood, which increased from ∼4 to ∼22 mmol l−1. While the abundance and intracellular localisation of gill Na+/K+-ATPase (NKA) and Na+/H+ exchanger 3 (NHE3) remained unchanged, the apical surface area of acid-excreting gill ionocytes doubled. This constitutes a novel mechanism for rapidly increasing acid excretion during sudden blood acidosis. Rapid acid–base regulation was completely prevented when the same high CO2 exposure occurred in seawater with experimentally reduced HCO3− and pH, probably because reduced environmental pH inhibited gill H+ excretion via NHE3. The rapid and robust acid–base regulatory responses identified will enable European sea bass to maintain physiological performance during large and sudden CO2 fluctuations that naturally occur in coastal environments.
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- Award ID(s):
- 1754994
- NSF-PAR ID:
- 10322225
- Date Published:
- Journal Name:
- Journal of Experimental Biology
- Volume:
- 225
- Issue:
- 2
- ISSN:
- 0022-0949
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Optimal function in the brain, especially in hippocampus—an area involved in learning and memory—requires tight regulation of intracellular pH (pHi) within neurons and neuroglial. The Na‐H exchangers (NHEs) are the major family of acid/base proteins involved in regulating pHi in the absence of CO2/HCO3. In the present study, we used the pH‐sensitive dye BCECF to examine the regulation of steady‐state pHi and the recovery of pHi from NH4+ ‐induced intracellular acid loads in HC neurons and astrocytes, co‐cultured from embryonic (E18‐20) Sprague Dawley rats, and studied in CO2/HCO3 −‐free HEPES buffered (“HEPES”) solutions. After at least 14‐days in a CO2/HCO3 – incubator, cells were removed, loaded with BCECF, and placed in a recording chamber with flowing HEPES. At the beginning of each experiment, we measured pHi (checkpoint A) after allowing pHi to stabilize for 5 minutes (checkpoint C), and reported mean “initial pHi”/SEM for neurons as 7.351/0.0597; N=37 (astrocytes: 7.189/0.0118, N=25) the value at checkpoint C = (pHi)C. After using the twin paired NH4+ ‐pulse protocol to acid load cells, we find that—after the pHi recovery from the first acid load—the average neuronal steady‐state pHi (now at checkpoint E; (pHi)E) is 6.953/0.0601(astrocytes: 7.037/0.0081). After the second NH4+ pulse the neuronal steady‐state pHi (now at checkpoint F; (pHi)F) in neurons is 6.937/0.010 (astrocytes: 7.020/0.0062). The recovery from acidosis is fit with a double exponential (DExp) which we replot as dpHi/dt vs pHi. With this traditional approach, dpHi/dt, the fit as it approaches the asymptotic pHi, becomes slightly non‐linear. To exploit the mainly linearity portion of the dpHi/dt vs. pHi plot (from the DExp fit) of the double exponential, we fit these dpHi/dt vs. pHi points with a DExp with a quasi‐ single exponential (SExp) to produce a quasi–single‐exponential rate constant (kqSExp) measured as dpH/dt. This analysis—when transformed to the pHi vs. time domain—generally produces a very good fit to the original pHi vs. time data. The mean kqSExp1 in neurons is 0.0054/ 0.0008 (astrocytes: 0.0107/0.0002) whereas the mean kqSExp2 in neurons is 0.0055/0.0008 (astrocytes: 0.0010/0.0003). We summarize the twin pHi recoveries from individual experiments in which we display as thumbnails the quasi–single‐exponential dpHi/dt line segments that represent the pHi recoveries from the first and second NH3/NH4+ pulses. These new analytical approaches may ultimately provide mechanistic insight into cell‐to‐cell heterogeneity of pHi regulation in the nervous system.
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The inner ear is essential for maintaining balance and hearing predator and prey in the environment. Each inner ear contains three CaCO3 otolith polycrystals, which are calcified within an alkaline, K+-rich endolymph secreted by the surrounding epithelium. However, the underlying cellular mechanisms are poorly understood, especially in marine fish. Here, we investigated the presence and cellular localization of several ion-transporting proteins within the saccular epithelium of the Pacific Chub Mackerel (Scomber japonicus). Western blotting revealed the presence of Na+/K+-ATPase (NKA), carbonic anhydrase (CA), Na+-K+-2Cl--co-transporter (NKCC), vacuolar-type H+-ATPase (VHA), plasma membrane Ca2+ ATPase (PMCA), and soluble adenylyl cyclase (sAC). Immunohistochemistry analysis identified two distinct ionocytes types in the saccular epithelium: Type-I ionocytes were mitochondrion-rich and abundantly expressed NKA and NKCC in their basolateral membrane, indicating a role in secreting K+ into the endolymph. On the other hand, Type-II ionocytes were enriched in cytoplasmic CA and VHA, suggesting they help transport HCO3- into the endolymph and remove H+. In addition, both types of ionocytes expressed cytoplasmic PMCA, which is likely involved in Ca2+ transport and homeostasis, as well as sAC, an evolutionary conserved acid-base sensing enzyme that regulates epithelial ion transport. Furthermore, CA, VHA, and sAC were also expressed within the capillaries that supply blood to the meshwork area, suggesting additional mechanisms that contribute to otolith calcification. This information improves our knowledge about the cellular mechanisms responsible for endolymph ion regulation and otolith formation, and can help understand responses to environmental stressors such as ocean acidification.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1242/jeb.242735
