Acid base and GIT: part 2

The pancreatic secretion of electrolytes

The aqueous component of pancreatic cells is contributed by acinar cells ( isotonic, rich in Na, K, Cl and bicarbonates, stimulated by CCK and Ach; concentration similar to that in plasma), intralobular ductal cells (bicarbonates double and chlorides less of that in plasma, spontaneous secretion) and extralobular ductal secretion ( rich in Bicarbonates and poor in chlorides, stimulus is SECRETIN)

The basolateral transporters are called house keepers as they maintain the normal pH and maintain the cell  membrane volatage.

ref: Kim D, Steward MC. The role of CFTR in bicarbonate secretion by pancreatic duct and airway epithelia. J Med Invest. 2009;56 Suppl:336-42. Review

Secretin secretion from the duodenal S cells is stimulated by gastric distension and acid secretion of stomach  that signals the ductal cells to secrete bicarbonate-rich, clear, watery fluid ( hydraulic secretion).

The ''exocrine'' pancreas is an abdominal “salivary gland” and releases 1.5 l of pancreatic juice daily, with the pH increasing with increased secretion rate. The maximal secretion rate about 20-50 µl/min. The pancreatic juice is a clear fluid, isosmolar with plasma. Bicarbonates in the pancreatic secretion can approximate to the [H⁺] in gastric juice (150 mM). Pancreatic juice (pH 8) thus buffers the extremely acid gastric juice and protects the duodenal mucosa.

At high secretory rate bicarbonates increase and chloride decreases and vice-versa. Thus both share a reciprocal relationship.

This is because at low flow rates, the pancreatic juice which is rich in Na and Cl comes from the acinar and ductal cells whereas at high flow rates, the proportion of ductal cell secretion rises which has got a high HCO₃ concentration.

Thus Bicarbonate secretion can vary from 80 to 120 mEq/l depending on the secretory rate. Na and K secretion (unlike saliva) are similar to plasma and remain unaffected by the secretory rate.

A schema of transporters including the SKOU pump in pancreatic ductal cell

The secretion of HCO₃ being the primary process, can be understood as:

A: Entry of Bicarbonate in to the duct cell    

i.                    Through Na/ HCO₃ cotranspoter (NBC) located on the basolateral membrane.

ii.                  Through the H₂ CO₃  reaction inside the cell through Carbonic anhydrase catalysed reaction where CO₂ + H₂O  combine to form H₂CO₃ and then breaking into H⁺ and HCO₃^-. The H+ are extruded through either an active H+-ATPase pump or by Na/ H exchanger.

B: Release of HCO₃ into the lumen

1.         Depends on Cl- transport across the apical membrane (as seen to be defective in cystic fibrosis).
2.          Secretin opens the Chloride channels which is proved by 20- fold decrease in the resistance of the apical membrane to chloride ions. The chloride channels can be CFTR Cl^- channels or Ca ⁺² sensitive Cl^- channels.
3.    The chloride recycles through the Cl/ HCO₃ exchanger which extrudes the HCO₃ ions into the lumen.
4.     A rise in [Ca²⁺] opens up luminal chloride channels and a basolateral K channel through which these ions can leave the cell.
5.       The fall in intracellular Cl and K activates a basolateral Na/K/Cl cotransporter through which NaCl enters the cell.
6.      It is followed by paracellular transport of Na and accompanied by H₂O (the acinar lumen has  about -6mV potential)
7.     If the luminal [Cl-] concentration falls or there is a reduction in the activity of Cl/HCO₃ exchanger, then HCO₃ can be extruded from the CFTR channels directly into the lumen.

        i.           

C: Holding of HCO₃ in the lumen

Because of electrical gradient, HCO₃ has a tendency to go back in the cell at 145 mM concentration.The IC concentration of HCO₃ is about 10-15 mM and the cell membrane potential is about  -60mV (even at max stimulation??).Another important event which takes place during this instance is the inactivation/ closure of basolateral Cl/ HCO₃ exchanger ( I have not drawn in this figure). This decreases Cl concentration in the lumen and favors HCO₃ extrusion through the CFTR channel which otherwise shows only 25 % conductance for HCO₃ as compared to Cl conductance.

suggested reading:

1. secretin receptors

2. VIP and Secretin

3. Potassium ion in pancreatic secretion

Acid Base and GIT: part 1

GIT and Acid base homeostasis

Acid- base changes in the GIT are actually in sync and infact only to bring about efficient absorption of the dietary component and hence do not play a role in acid-base homeostasis (unlike kidney). The small amount of alkali lost as a by-product of acid-base transport events in the guts is easily regenerated by renal net acid excretion. Abnormal gut function brings about a upheaval in maintenance of acid-base homeostasis.

Normal Physiology of Gut Fluid and Electrolyte Transport

During the course of each day, secretion as well as absorption of fluid and electrolytes occurs along the gastrointestinal tract.

Normally 7 to 8 L of fluid is secreted each day, far exceeding dietary consumption, and almost all of these secretions, as well as any ingested fluid, are absorbed by the end of the colon.

Saliva

Via parotid, submandibular, and sublingual glands; in addition, there are many very small buccal glands. The parotid glands secrete almost entirely the serous type of secretion, while the submandibular and sublingual glands secrete both serous secretion and mucus. The buccal glands secrete only mucus. The submandibular glands contribute about two thirds of resting salivary secretion, the parotid glands about one fourth, and the sublingual glands the remainder. Saliva has a pH between 6.0 and 7.0, a favorable range for the digestive action of ptyalin.

At rest salivary secretion is low, amounting to about 30 mL/hr. Stimulation increases the rate of salivary secretion, most notably in the parotid glands, up to 400 mL/hr. The most potent stimuli for salivary secretion are acidic-tasting substances ( citric acid),  smell of food and chewing. Anxiety, fear, dehydration, and medications (e.g., antihistamines) inhibit secretion.

Primary secretion (from the acini)  is isotonic as plasma as if formed as a result of ultrafiltration. But actually as the final secretion (ductular) occurs, Cl^- and Na⁺  are reabsorbed and and K⁺ and HCO₃^-  are secreted (via CFTR channel??). The changes in the ductular cells result in a hypotonic secretion. When salivary flow is rapid, there is less time for ionic composition to change in the ducts.  Aldosterone increases the K⁺ concentration and reduces the Na⁺ concentration of saliva in an action analogous to its action on the kidneys.

The hypotonicity facilitates taste sensitivity and results in various organic compounds form a protective coating on the oral mucosa. Salivary bicarbonate , calcium and phosphate act as buffer and  neutralize acids that would otherwise destroy tooth minerals 

For details readers are advised to read an excellent paper by Melvin JE (1999)

Gastric secretion of acid

Acid is secreted by parietal cells. Various transport proteins are present in the parietal cells.

1.       H⁺/ K⁺ ATPase

2.       K⁺ selective channel

3.       Cl^- channel

4.       Cl^-/ HCO₃^- exchanger  :   At the serosal surface

5.       Na⁺/ K⁺ ATPase   :     At the baso- lateral surface  

The pH and volume of the gastric secretion is regulated primarily by gastrin so maximum acid secretion occurs only after a meal [ pH= 1, volume= 7 ml/ min].

A fall in pH also increases the amount of HCO₃ absorbed [ alkaline tide]. Eventually HCO₃^- secreted back into GI tract by pancreas.

It is believed that K⁺ is merely recycled at the canalicular membrane on the luminal side otherwise intracellular K⁺ levels may rise to dangerous levels.

         

Rest of  the details  will be discussed in the next post.

Poorvi

Ammonia buffer

Ammonia buffer

Most ammonia (60%) is synthesized in proximal tubule cells by deamidation and deamination of the amino acid glutamine [rest comes from glycine and alanine].

Ammonia is secreted into the urine by two mechanisms.

1.   As NH₃ (because of its lipid soluble nature), it diffuses into the tubular urine; as NH₄ ⁺, it substitutes for H⁺ on the Na⁺/H⁺ exchanger. In the lumen, NH₃ combines with secreted H⁺ to form NH₄⁺, which is excreted [because of its polar nature and therefore can not diffuse back into the tubular epithelium].

For each mEq of H⁺ excreted as NH₄⁺, one mEq of new HCO₃^_ is added to the blood. The hydration of CO₂ in the tubule cell produces H⁺ and HCO₃ ^_.

2.   Two H⁺s are consumed when the anion α₂-ketoglutarate^2_ is converted into CO₂ and water or into glucose in the cell. The new HCO₃^_ returns to the blood along with Na⁺.

If excess acid is added to the body, urinary ammonia excretion is increased for two reasons.

·        More acidic urine traps more ammonia (as NH₄⁺) in the urine.

·        Renal ammonia synthesis from glutamine increases over several days which is a lifesaving adaptation because it allows the kidneys to remove large H⁺ excesses and add more new HCO₃^_ to the blood. With severe metabolic acidosis, ammonia excretion may increase almost 10-fold.

Renal ammonia synthesis is increased through an increase in the glutaminase activity.

Phosphate buffer and protein buffers

Phosphate buffer

The pKa for phosphate, H₂PO₄ , is 6.8, close to the desired blood pH of 7.4, so phosphate is a good buffer.

 In the ECF, phosphate is present as inorganic phosphate in a very low concentration  (about 1 – 2 mmol/L), so it plays a minor role in extracellular buffering.

Phosphate is an important intracellular buffer  for the reasons:

1.    Cells contain large amounts of phosphate in such organic compounds as adenosine triphosphate (ATP), adenosine diphosphate (ADP), and creatine phosphate. Although these compounds primarily function in energy metabolism, they also act as pH buffers.

2.    Intracellular pH is generally lower than the pH of ECF and is closer to the pKa of phosphate which is 6.8 (e.g.The cytosol of skeletal muscle has a pH of 6.9.).

Phosphoric acid is triprotic weak acid and has a pKa value for each of the three dissociations:

	pKa1 = 2		pKa2 = 6.8		pKa3 = 12
H3PO4	<= = = >	H+ + H2PO4-	<= = =>	H+ + HPO4-2	< = = = >	PO4-3 + H+

The three pKa values are sufficiently different so that at any one pH only the members of a single conjugate pair are present in significant concentrations.

The pKa2 is 6.8 and this makes the closed phosphate buffer system a good buffer intracellularly and in urine. The pH of glomerular ultrafiltrate is 7.4 and therefore phosphate is predominantly in the monohydrogen form so as to  combine with more H⁺ in the renal tubules efficiently. This makes the phosphate buffer more effective in buffering against a drop in pH than a rise in pH.

Protein buffer

Proteins act as efficient buffers because of their amino acid structure which  have a central carbon with four groups off of it.

The carboxyl and amino groups are what enable proteins to act as buffers.

Protein buffers in blood include haemoglobin (150g/l) and plasma proteins (70g/l). Buffering is by the imidazole group of the histidine residues which has a pKa of about 6.8. This is suitable for effective buffering at physiological pH (7.4).

Haemoglobin is quantitatively about 6 times more important than the plasma proteins as it is present in about twice the concentration and contains about three times the number of histidine residues per molecule. For example if blood pH changed from 7.5 to 6.5, haemoglobin would buffer 27.5 mmol/l of H⁺ and total plasma protein buffering would account for only 4.2 mmol/l of H⁺.

Deoxyhaemoglobin is a more effective buffer than oxyhaemoglobin and this change in buffer capacity contributes about 30% of the Haldane effect. The major factor accounting for the Haldane effect in CO₂ transport is the much greater ability of deoxyhaemoglobin to form carbamino compounds.

Haemoglobin has a special place in the pHstabilizing mechanisms of blood because

(a)  Haemoglobin and oxyhaemoglobin have different iso-electric points and different ionization constants so that at the pH of blood, absorption of about 0.7 g. of hydrogen ion [from carbonic acid mainly] (and the release of an equivalent amount of potassium ion in exchange) takes place during liberation of oxygen from 1 g molecule of oxyhemoglobin.

(b)  (b) Haemoglobin, more easily than other proteins, reacts reversibly with carbonic acid to form a still weaker carbaminoacid; and an increasing accumulation of carbamino-haemoglobin as the blood passes through the capillaries and a sharp reversal when, in the lungs, the equilibrium is disturbed, carbon dioxide being excreted, carbonic acid converted to carbon dioxide under the influence of carbonic anhydrase, and, consequently, carbaminohaemoglobin  is decomposed.

(Frazer &Stewart, J. clin. Path. (1959), 12, 195-206)

For more details readers are advised to go through the above mentioned paper and then directed to the link http://www.madsci.org/posts

major buffer systems in the body

Major chemical buffers in the body

Bicarbonate buffer

A normal adult produces about 300 L of CO₂ daily from metabolism. CO₂ from tissues enters the capillary blood, where it reacts with water to form H₂CO₃, which dissociates instantly to yield H⁺ and HCO₃ ^–.

Blood pH would rapidly fall to lethal levels if the H₂CO₃ formed from CO₂ were allowed to accumulate in the body.

Fortunately, H₂CO₃ produced from metabolic CO₂ is only formed transiently in the transport of CO₂ by the blood and does not normally accumulate. Instead, it is converted to CO₂ and water in the pulmonary capillaries and the CO₂ is expired. As long as CO₂ is expired as fast as it is produced, arterial blood CO₂ tension, H₂CO₃ concentration, and pH do not change.

pK of the HCO₃^-/CO₂ is 6.1.

pH of plasma is 7.4.

 According to Henderson-Hasselbalch equation

7.4 = 6.1 + log [HCO₃^-]/ [CO₂]

1.3= log [HCO₃^-]/ [CO₂]

20= [HCO₃^-]/ [CO₂]

i.e. At pH 7.4, concentration of bicarbonate should be 20 times that of Carbon dioxide.

As described in the previous post, it is the salt portion that neutralizes external acids, so a higher concentration of bicarbonate should suffice. This explains the relative importance of bicarbonate/CO2 buffer over other buffers because most metabolic end products are acidic in nature.

Concentration of HCO3 ^-   in plasma or ECF normally averages 24 mmol/L.  CO₂ is measured from PCO₂ which is about 40 mm Hg and that amounts to 1.2mmol/L of CO₂ (0.03* 40) {The solubility coefficient for CO2 in plasma at 37°C is 0.03 mmol CO2/L per mm Hg PCO2 }.  Although the concentration of dissolved CO2 is lower, metabolism provides a nearly limitless supply. Hence [HCO₃-]/ [CO₂] = 24/1.2=20 supports the theoretical calculations performed above using Henderson-Hasselbalch equation.

The Henderson-Hasselbalch Equation for HCO₃^_/CO₂

_{In aqueous solutions:}

CO₂(d)   +    H₂O      give       H₂CO₃

_(dissolved)

At equilibrium, CO₂(d) is greatly favored; at body temperature,  [CO₂(d)] : [H₂CO₃] is about 400:1 [If [CO₂(d)] is 1.2 mmol/L, then [H₂CO₃] equals 3 μmol/L].

H₂CO₃     instantaneously breaks into H⁺ + HCO₃^-

The Henderson-Hasselbalch Equation for the above equation is:

pH= 3.5 +  log [HCO₃^-]/ [H₂CO₃]

H₂CO₃ is a fairly strong acid (pKa = 3.5). Its low concentration in body fluids lessens its impact on acidity. Because [H₂CO₃] is so low and hard to measure so [CO_2(d)] is used instead.

pH= 3.5 +  log [HCO₃^-]/ [CO_2(d)]/400

pH= 3.5+ log 400 + log [HCO₃^-]/ [CO_2(d)]

pH= 6.1+ log [HCO₃^-]/ [CO_2(d)]

pH= 6.1+ log [HCO₃^-]/ 0.03*pCO₂

pH= 6.1  + log [24]/[1.2]

pH=7.4

Physiological buffers: a brief introduction

Physiological Buffers

A "buffer system," which minimizes pH changes on addition of acid or base, consists of a solution containing a weak acid together with one of its soluble salts e.g. H₂CO₃ and NaHCO₃.

The acid, being weak, is slightly ionized but the soluble salt is ionized to a large extent. The mixture thus provides a reservoir of base (anions) which can combine with added H⁺ (neutralize added acid) and a reservoir of acid (undissociated acid molecules) which can donate hydrogen ions to neutralize added base.

Efficacy of the buffer depends on the pH of the buffer i.e. the concentration of weak acid and base added to constitute the buffer. This relationship is expressed by the Henderson-Hasselbalch equation.

pH= pK+ log [salt]/[acid]

A buffer is most effective when the concentration of its salt and acid are equal. pK of a buffer is that pH at which its salt and acid forms are in equal concentration. Hence a buffer is most effective in a solution the pH of which is equal to the pK of the buffer. The most effective physiological buffers are those whose pK is around 7.4.

In a solution which has various buffer systems then they should be in mutual equilibrium. If the acid:base is known for any one of the buffer systems, the pH of the mixture can be known. This can be described in terms of isohydric principle:

[H⁺] = K1*[acid 1]/[salt 1] = K2*[acid 2]/[salt 2]= K3*[acid 3]/[salt 3]

In plasma, H₂CO₃-NaHCO₃ buffer is the easiest to measure and most important quantitatively. Therefore total CO₂ content of plasma (sum of HCO₃^- and H₂CO₃ + CO₂) can represent the total buffering capacity.