Chemical reactions of Acidosis in the cells

Metabolic acidosis is a medical condition whereby a chemical reaction causes the chemical equilibrium of acids and bases inherent in the blood is disrupted. This is as a result of excess production of acid in the blood with no equal amount of base to react equally to the normal amount of acid. As a result, the assimilation process in the body may not work properly. The mild case of metabolic acidosis can be treated straight away, although some cases may be risky. (omo-Aghoja 8).

Acid-base balance equilibrium can be regulated through the inclusion of two types of intercalated cells inherent in the cortical collecting duct (CCD) of the kidney. These intercalated cells are known as the alpha (α) and beta (β) – intercalated cells. Alpha-intercalated cells secrete protons through the apical B1V-ATPase (B1), while β- intercalated cells secrete the bicarbonate ion through a pendrin exchanger and a basolateral B1 (Kampik 29).

In this particular paper, the down regulation of intercalated cells during pregnancy shall be further investigated through the explanation of maternal acidosis and its influence on intercalated cells. Previous studies on acidosis, acid-base balance and intercalated cells shall be taken into consideration in order to explain impact of maternal acidosis on β-intercalated cell differentiation.

Maternal Acidosis

Metabolic acidosis is described as an adverse acid-base disequilibrium which may be influenced by factors such as diarrhea, toxicity, starvation and certain health complications. One of the most common forms of metabolic acidosis is acute metabolic acidosis since its duration of occurrence can last for several days (Kampik 56). An elevation in alveolar ventilation resulting in the release of greater amounts of carbon dioxide is regarded as a rapid response to metabolic acidosis. This is normally attained through the development of respiratory tidal frequency and volume. Nevertheless, it is important that the kidney builds the final plasma correction which is the proportion of the bicarbonate (HCO3-) ion with carbon dioxide (CO2). As a result, an upregulation of hydroxyl (H+) ion and HCO3- ion reabsorption is achieved (Kampik 56; Aronson & Giebisch 1982). 

Acidosis occurs in pregnant women and is referred to as maternal acidosis. This is because fetal growth and development induces greater excretion of urine as well as water retention (Omo-Aghoja 8). Moreover, maternal acidosis is correlated with lower fetal pH as a result of fetal hypoxia. This happens when there are complications in maternal oxygenation. With such complications, there is a decrease or blockage in the transport of oxygenated blood from the placenta to the fetus. Insufficient fetal oxygenation also leads to the malfunction of oxidative metabolism in carbohydrates. In turn, metabolism follows the anaerobic pathway, generating lactic acid and other organic acids that cannot be easily eliminated or metabolized. Maternal acidosis then results from the accumulation of lactic acid which causes the disruption of the buffer system (Omo-Aghoja 8-9).

During pregnancy, the respiratory aspect of the mother’s acid-base system displays a decline in arterial carbon dioxide tension as well as an increase in oxygen tension. At the same time, there is a proportional decrease in plasma bicarbonate. This leads to the occurrence of primary respiratory alkalosis and secondary metabolic acidosis. Metabolic acidosis and normal pH happens on the alkaline portion at a relatively normal range. “Acidosis of pregnancy” does not normally occur, rather, it entails a decrease in the quantity of total acid and total base. The decline in pCO2 is safeguarded through the mechanism of the kidney, which regulates bicarbonate loss or conservation (Omo-Aghoja 8).

The decrease of maternal carbon dioxide tension elevates the transplacental gradient to around 8-10 mmHg, thus allowing it to be carried from the fetus. The fetus undergoes development in an environment where there is carbon dioxide tension almost the same or partially lower than that of the adult (Cecere et al., 1).

During labor, a rise in metabolic acidosis occurs because of maternal fixed acids obtained from metabolism in the uterus and placenta as well as from ketone bodies which occur with stress. During fetal acidosis, increased levels of pyruvic and lactic acid are present in the umbilical artery. This is because the mother and the placenta become large storage vessels for hydrogen in order for the fetus to endure anaerobic metabolism. Nevertheless, molecules of CO2 readily enter the placenta, and evidence shown in numerous experiments proves that there are differences in terms of the degree of permeability to bicarbonate ions. Thus, the fetus becomes independent to the concentration of bicarbonate ions. The mechanism of antacids under a high degree of dissociation as that of sodium bicarbonate can be beneficial in decreasing the occurrence of maternal acidosis. This is also the same for substances with low solubility of liquids and low placental penetration such as tromethamine (Cecere et al., 2-3).

It is also during labor that the mother exerts forced hyperventilation as that of being under anesthesia during surgery. At this point the lungs may be overly inflated which can be attributed to forced hyperventilation. Such situations may result to fetal acidosis and newborn depression. Nevertheless, it should be taken into account that hyperventilation, when done moderately can be advantageous in order to develop feto-maternal gradients in placental areas. But this may rapidly become correlated with severe maternal and fetal complications as it progresses. Generally, when a maternal pCO2 of 15 mmHG and pH of 7.65 is reached, there is a decline in the oxygen saturation of fetal blood. Consequently, maternal muscle cramps and disruption in fetal heart rate happens. Numerous studies involving animals have proven that excessive hyperventilation results to placental vessel spasms and gradual weakening of umbilical circulation accompanied by fetal acidosis (Omo-Aghoja 9-10).

Maternal acidosis is also correlated with relatively low Apgar score. This refers to the basic test given to the newborn right after birth. The goal of the Apgar score is to assess the physical condition and immediate needs of the baby. Evaluation of the baby’s condition is based on five criteria. Each criterion uses the 0-2 scale for scoring, with 2 being the highest score. This is the reason why maternal acidosis is correlated with low Apgar scores. Babies who were reported to have low Apgar scores were found to be more acidotic during a mother’s labor and delivery, as well as during the first six months of a baby’s life (Omo-Aghoja 9).

Distal renal tubular acidosis (dRTA) refers to the kidney’s incapacity to induce acidification of the urine. This condition normally results to metabolic acidosis accompanied by certain complications such as hypercalciuria, hypocitraturia and hypokalemia (Oguejiofor et al., 1). Impaired acid secretion through α- ICs is genetically acquired and it becomes the apparent defect in this particular case. Insufficient urine acidification may lead to systemic metabolic acidosis or metabolic acidosis after a forced acid load (Kampik 57).

A type 1 dRTA also results from a reduction of H+ secretion in the kidney’s collecting tubule which eventually leads to a urinary pH greater than 5.5.  This severe urine alkalinity is related to periods of nephrocalcinosis and hypokalemia in some patients (Oguejiofor et al., 1).

In the study of Roy and her co-workers on Collecting Duct Intercalated Cell Function and Regulation, the occurrence of chronic metabolic acidosis leads to a high proportion of α- intercalated cells while metabolic alkalosis results in a high proportion of β-intercalated cells. During the experimentation process, it was found that metabolic acidosis occurred in mice having a hensin defect in intercalated cells. In this particular experiment, intercalated cells located in the cortex had a characteristic β-phenotype (4). It was also found that certain genetic complications such as carbonic anhydrase II deficiency results to proximal tubular dysfunction and impaired intercalated cell function. Thus, renal tubular acidosis is also known as mixed renal tubular acidosis, and this is specifically common on patients experiencing calcification of the cerebrum and osteoporosis (12).

One of the functions of the kidney is acid-base homeostasis through the recovery of bicarbonate in its proximal tubule. Bicarbonate that is produced from intercalated cells is used up through nonvolatile acid titration. Metabolic acidosis is linked with intercalated cell malfunction. Acidemia, which may affect the bones, lungs and proximal kidney tubules, can be prevented through the action of intercalated cells. Nevertheless, an increase in intercalated cell surface area can be observed in animals that have undergone acid loading due to dietary changes (Roy et al., 1; Welsh-Bacic et al., 1).

Intercalated Cells

Intercalated cells refer to epithelial cells which are directly linked with acid-base homeostasis in the distal area of the kidney tubule. These specific cells function in the transport of ammonia and potassium and are also responsible for normal immune function. Intercalated cells are important in an organism’s response to acid-base balance since such cells help in the elimination of non-volatile acids that cannot be expelled through the lungs (Roy et al., 1).

Intercalated cells not only function in the urinary regulation of bicarbonate excretion but also in the reabsorption of transcellular sodium and chloride ions. Such findings suggest that some patients with dRTA experience loss of sodium and chloride ions in the urine which is retained even upon resolution of acidosis (Almomani et al., 76).

Generally, there are three distinct types of intercalated cells according to cell structure and localization. These are the type A (α-intercalated cells), type B (β-intercalated cells) and non-A, non-B intercalated cells (Roy et al., 2). All types of intercalated cells have the capacity to express carbonic anhydrase, a metalloenzyme that is involved in the catalysis of CO2 hydration leading to HCO3- formation (Almomani et al., 73).

Type A or α-intercalated cells combine their roles with urinary acidification as well as their inability to express pendrin, which belongs to a superfamily of Cl- anion exchangers located in the kidneys, epithelial cells of the inner ear and the thyroid gland (Wagner et al., 2109). Alpha-intercalated cells have HT1-ATPase in their apical membranes and widely express AE1 in the basolateral membrane (Almomani et al., 73; Aronson & Giebisch 1986).

Non-A, non-B intercalated cells, on the other hand, are situated in the connecting tubules of the kidneys. They have the ability to express H1-ATPase and pendrin in their apical membranes (Roy et al., 4).

Type B or β- intercalated cells are characterized by an apical cell pole which is narrow and filled with short microvilli. The basal pole is broad and contains deep membrane infoldings. Beta-intercalated cells possess an impenetrable area of vesicle-free cytoplasm containing filaments on the apical membrane. The lateral membrane is aligned with mitochondria and the cytoplasm contains numerous vesicles (Kampik 29-30; Fejes-Toth 5487). These cells also possess large Golgi bodies and a sufficient quantity of smooth endoplasmic reticulum. Compared to α-intercalated cells, β-intercalated cells contain more lysosomes, microtubules, phagolysosomes and polyribosomes (Al-Awqati & Bao 266).

Beta-intercalated cells, which are of opposite polarity with that of α-intercalated cells, participate in the secretion of the HCO3- and in the expression of the Cl-/ HCO3- exchanger, pendrin in their apical membrane. The reabsorption of protons is facilitated through the metalloenzyme H1-ATPase, which is expressed in the basolateral pole of type B-intercalated cells. In turn, HCO3- is released into the urine through pendrin (Roy et al., 4; Almomani et al., 75).

Since one of the main functions of β-cells is bicarbonate secretion, it is also important to understand the processes of activation and inactivation facilitated by these cells. Bicarbonate secretion is activated during metabolic alkalosis and inactivated during metabolic acidosis. The secretion of HCO3- in β-intercalated cells entails the participation of pendrin (Wagner et al., 2115; Fejes-Toth 5487).

The gene SLC26A4 PDS encodes pendrin. This protein enables the transport of anions such as Cl2, HCO3- and iodide. Pendrin functions as a chloride-bicarbonate exchanger specifically in the epithelial cells of the inner ear and in kidney intercalated cells. In terms of the thyroid gland, it is responsible for the mediation of iodide transport (Roy et al., 8; Almomani et al., 73-74).

Pendrin mutation in humans results to Pendred syndrome, which involves goiter and deafness. In previous studies, pendrin was found to be situated in the apical side of non-A intercalated cells of the kidney’s CCD. Decrease bicarbonate secretion was also observed in a pendrin-treated mouse model. In the study of Wagner and his co-workers on the Regulation of the expression of the Cl-/anion exchanger pendrin in mouse kidney by acid-base status, the potential function of pendrin in the modulation of acid-base transport in the CCD was investigated. To achieve optimum results, the regulation of pendrin expression through the acid-base balance of the mouse kidney was observed and assessed (2109).

In one study done on a rabbit model, the conversion of β-intercalated cells to α-intercalated cells accounts for the adaptation of rabbits to acidosis. This transformation between the two types of intercalated cells is dependent on the basolateral placement of extracellular proteins such as galectin 3 and hensin. The physical appearance of medullary epithelial cells was altered in mice that are deficient with hensin. Altered medullary cells possess the morphology of β-intercalated cells located in the cortex yet these cells do not express pendrin compared to normal β-intercalated cells (Roy et al., 4).

In order to prove that β-intercalated cells come from α-intercalated cells, hensin-lacking mice in one study were produced from the intercalated cell lineage. The loss of hensin did not affect the intercalated cell percentage from the total number of cells in the CCD of the kidney. Nevertheless, previous studies done have shown that all intercalated cells in the CCD of mutant mice were β-intercalated cells, with apical pendrin and basolateral H1-ATPase vacuolar expression comparable to about 30% of intercalated as of the β-type in wild mice. A new type of cell was determined in the medulla of knockout mice, and this was similar to the cortical β-intercalated cells, yet it did not express pendrin. The existence of β-intercalated cells and the absence of α- intercalated cells in the CCD of the hensin-treated mice led to constant secretion of bicarbonate ions, which eventually results to metabolic acidosis and complete dRTA (Almomani et al., 77).

Objective of the Study

The goal of this study is to determine whether maternal acidosis has an impact on β-intercalated cell differentiation. This is based on the hypothesis that acidosis stimulates modifications in transport proteins that are expressed in β-intercalated cells. Acidosis was also found to cause down-regulation of pendrin in the apical membrane of these cells (Roy et al., 9).

Methodology

Based from a previous study, pregnant rabbits were fed with an alkaline ash diet with alterations of acid-loading of ammonium chloride. This was done during their fourth and final weeks of gestation. Afterwards, fresh kidneys were obtained from the litter of rabbits at one week (1 kit) and three weeks (2 kits). Kidney tissues were embedded in paraffin, cut into sections of 4-6μm thickness and placed in slides. Intercalated cell (IC) types were identified through immunofluorescence staining. This form of staining will help determine the presence of pendrin, AE1 and B1. Rabbit serial slides were stained and labeled: one for pendrin only and one labeling B1 and AE1. The quantity of α- intercalated cells and β-intercalated cells was determined by means of cell counting. B1 cells were assigned as the total number of intercalated cells, AE1 as the number of α- intercalated cells and pendrin cells as the number of β-intercalated cells. Cell counts were eventually compared to ordinary adult rabbits (Riley; Wagner et al., 2110).

Results and Discussion

Based from the study of Ashley Riley on the same topic, pendrin staining displayed a lesser number of β-ICs in 1 week-old kits compared with an adult normal rabbit with 4.3±0.2 β-IC’s/100 μm in the CCD in 1 week-old kits versus 7.4 in the adult and 6.3±0.05 in the 3 week-old kits. The AE1/B1 staining showed similar variations as well. There were 6.3±0.2 total IC’s/100 μm at 1 week kits versus 7.8 in the adult and 9.1±0.1 in 3 week-old kits. Although neonatal collecting ducts have α-IC’s in numbers similar to the adult and 3 week-old kits, with 2.8±0.1 α-IC’s/100 μm at 1 week, 3.1±0.2 at 3 weeks, and 2.4 in the adult, β-IC’s were lower in young rabbits compared with adults and 3 week-old kits with 3.5±0.1 β-IC’s/100 μm at 1 week versus 5.4 in the adult and 6.0±0.3 in the 3 week-old kits. As a result, the proportion of β-IC’s is lower in kits from an acidotic young compared with a normal adult. The ratios of α:β cells were 44:56 at 1 week , 34:66 at 3 weeks, and 31:69 in the adult. This information suggests that there is incomplete IC differentiation in the neonate.

Conclusion

 Based from previous data, it has been found that the number of α- and β-ICs per tubule length is similar between young and adult rabbits. The low quantity of β-IC’s in young rabbits from acid-loaded pregnant rabbits can be attributed to the down-regulation of β-IC differentiation, and not as an upregulation in α-IC differentiation (Riley). It can be therefore concluded that maternal acidosis causes the delay of β-IC differentiation in progeny.

Works Cited

Almomani, Ensaf Y., et al. “Intercalated Cells: More than pH Regulation.” Diseases, Vol. 2, 2014. pp. 71-92. doi:10.3390/diseases2020071

Al-Awqati, Qais, & Gao, Xaio Bo. “Differentiation of Intercalated Cells in the Kidney." Physiology, Vol. 26, 2011. pp. 266-272. doi:10.1152/physiol.00008.2011

Aronson, Peter S., & Giebisch, Gerhard. “Effects of pH on Potassium: New Explanations for Old Observations”. Journal of the American Society of Nephrology, Vol. 22, 2011. 1981-1969. doi: 10.1681/ASN.2011040414

Cecere, Nicolas., et al. “Extreme Maternal Metabolic Acidosis Leading to Fetal Distress and Emergency Caesarian Section”. Case Reports in Obstetrics and Gynecology, Vol. 2012, Article ID 847942, 2013. pp. 1-3. http://dx.doi.org/10.1155/2013/847942

Fejes-Toth, Giza & Fejes-Toth, Aniko Naray. “Differentiation of renal β-intercalated cells to α-intercalated and principal cells in culture”. Proc. Nati. Acad. Sci. Vol. 89, 1992. pp. 5487- 5491

Kampik, Nicole Beate. “New insights into the regulated differentiation of intercalated cells in mouse kidney from Ae1 deficient mice.” Dissertation, University of Zurich. 2012, pp. 1-197

Oguejiofor, P., et al. “Successful Management of Refractory Type 1 Renal Tubular Acidosis with Amiloride.” Case Reports in Nephrology, Vol. 2017, Article ID 8596169, 2017. pp. 1-3. https://doi.org/10.1155/2017/8596169

Omo-Aghoja, L. “Maternal and Fetal Acid-Base Chemistry: A Major Determinant of Perinatal Outcome.” Annals of Medical and Health Sciences Research, Vol. 4, No. 1, 2014. pp. 8-17. doi:  10.4103/2141-9248.126602

Riley, Ashley. “Maternal acidosis down-regulates beta-intercalated cell differentiation in progeny.” Strong Children’s Research Center. 2015

Roy, Ankita, et al. “Collecting Duct Intercalated Cell Function and Regulation.” Clinical Journal of the American Society of Nephrology, Vol. 10, 2015. pp. 1-15

Wagner, Carsten A., et al. “Regulation of the expression of the Cl-/anion exchanger pendrin in mouse kidney by acid-base status.” Kidney International, Vol.62, 2002. pp. 2109-2117

Welsh-Bacic, Desa., et al. “Proliferation of Acid-Secretory Cells in the Kidney during Adaptive Remodelling of the Collecting Duct.” PLOS One, Vol. 6, Issue 10, 2011. pp. 1-13