Sodium, Potassium & Chlorine

Physiological Roles
Maintenance of intra- and extracellular concentrations of Na⁺ and K⁺
Regulation of Na, Cl, and K
Dietary requirements (% of dietary DM)
Sources of Na, Cl, and K
Dietary Cation-Anion Difference (DCAD)
Deficiency
Toxicity

I.  Physiological Roles

1. Sodium
   1. Predominant cation in extracellular fluid.
      1. Osmotic relationships, fluid volume
      2. Acid-base balance, pH
   2. Contributes to hardening of outer layers of bone
   3. Active transport of glucose across the intestinal mucosa requires movement of Na in the same direction
   4. Many enzyme systems function in an Na environment
   5. Maintenance of electrical activity of body cells
   6. Maintenance of muscle and heart contractions
2. Potassium
   1. Principal intracellular electrolyte
      1. Osmotic relationships, fluid volume
      2. Acid-base balance, pH
      Involved in cellular uptake of amino acids
   2. Carbohydrate metabolism
      1. Peripheral uptake of glucose by cells
      2. Some enzymes require K for glycolysis and oxidative phosphorylation
      3. Secretion of insulin and glucagon
   3. Several vital processes are controlled by release of K⁺ from cells through channels
      1. Transmission of nerve impulses
      2. Endocrine and exocrine secretions
3. Chlorine
   1. Predominant anion in extracellular fluid
      1. Osmotic relationships, fluid volume
      2. Acid-base balance, pH
   2. Maintenance of electrical homeostasis
   3. Chloride-bicarbonate shift in O₂ and CO₂ transport by RBC (See figures 1-4.)
   4. Activation of intestinal amalyase
   5. Gastric juice

During Respiration, Two Reactions Occur Simultaneously in the Tissues and in the Lungs

Figure 1.  O2 and CO2 Transport By Red Blood Cells in Tissues In the tissues: CO2 is constantly being produced as the result of oxidation of food materials CO2 dissolves in tissue fluids and diffuses into plasma CO2 from tissues enters the RBC In the RBC, CO2 favors breakdown of oxyhemoglobin O2 is removed from the RBC because of decomposition of oxyhemoglobin Reduced hemoglobin is less acidic than ocyhemoglobin and favors formation of hemoglobin-carbomino thus favoring taking on of CO2

Figure 2.  O2 and CO2 Transport By Red Blood Cells in Lungs In the lungs: CO2 leaves the RBC and is expelled into the air Loss of CO2 from RBC favors formation of oxyhemoglobin RBC accumulates O2 as oxyhemoglobin Oxyhemoglobin is more acidic than reduced hemoglobin and favors breakdown of hemoglobin-carbamino, thus releasing CO2 for removal from RBC Movement of ions across RBC membranes: RBC membrane is permeable to H2O, CO2, HCO3-, Cl and H+ Na+, K+ATPase tends to keep K+ in the cell and Na+ in the plasma Cl ions diffuse into RBC's in the tissues and out of RBC's in the lungs to compensate for movement of HCO3- ions.  This is the chloride shift

Figures 3. & 4.   Chloride-Bicarbonate Shift

In the Lungs Hemoglobin-carbamino breaks down releasing CO2  H+ ions from reduced Hb combine with HCO3 ions  H2CO3 is decomposed by carbonic anhydrase, forming H2O and CO2 CO2 diffuses into plasma and into the lungs where it is exhaled As CO2 is removed from RBC, HCO3- ions concentrations fall so HCO3- ions diffuse into plasma Cl- ions diffuse out of RBC to maintain ionic equilibrium O2 from lungs diffuses into plasma O2 diffuses into RBC O2 combines with Hb- to form oxyhemoglobin

In the Tissues CO2 from oxidation of food materials CO2 diffuses into plasma CO2 diffuses from plasma into RBC Part of the CO2 unites with Hb to form a carbamino  compound Most of the CO2 reacts with H2O to form H2CO3 (catalyzed by carbonic anhydrase) H2CO3 ionizes into H+ and HCO3- ions As HCO3- ion concentration increases in RBC, HCO3- ions diffuse into plasma Cl- ions diffuse into RBC to compensate for HCO3-  ions lost Oxyhemoglobin decomposes O2 is liberated into the plasma and taken up by tissues

II.  Maintenance of intra- and extracellular concentrations of Na⁺ and K⁺

1. Na is pumped out of K+ is pumped into the cell by Na, K-ATPase, and enzyme located in the cell membrane
   1. The enzyme transfers 3 Na⁺ to the extracellular fluid for each ATP molecule hydrolyzed
   2. This occurs in exchange for 2K⁺
   3. Suboptimal Mg⁺² intake decreases Na, K-ATPase activity (J. Nutr. 117:2091, 1987)
2. Mechanism of Na, K-ATPase

III.  Regulation of Na, Cl, and K

1. Close regulation of Na, Cl, K concentrations is necessary for health
2. Regulation of concentrations of these elements involves two main processes:
   1. Control of intake
   2. Control of loss from the body
3. Absorption of these elements from GI tract is essentially complete (not a control)
4. Conservation and excretion of excess Na, K, and Cl is controlled primarily by the kidney
   1. Rennin is secreted in response to a decrease in interstitial fluid volume or blood pressure, both of which are sensitive to decreased Na
   2. Rennin cleaves a leucine-leucine bond on a circulating a_z globulin to liberate angiotensin I
   3. The highly active vasopressor angiotensin II is formed from angiotensin I by angiotensin converting enzyme which is zinc dependent (Biochem. Pharmacal 20:1637, 1971; J Nutr. 116:128, 1986)
      Angiotensin II stimulates adrenocortical secretion, primarily aldosterone
   4. Effects of aldosterone:
      1. Increased retention of Na, Cl, and H₂0
         1. Increased kidney resorption
         2. Increased intestinal absorption
         3. Reduced loss in sweat
      2. Increased excretion of K
      3. Decreased Na⁺ uptake by cells
   5. Aldosterone is also stimulated by excess K intake
5. Excess K can be transferred from extracellular to cellular spaces where it is less harmful

IV.  Dietary requirements (% of dietary DM)

V.  Sources of Na, Cl, and K (J Dairy Sci 69:595, 1986)

VI.  Dietary Cation-Anion Difference (DCAD)

1. Acid-base balance is affected by intake and excretion of fixed cations (Na⁺ and K⁺) and anions (Cl^-, SO₄^-)
2. DCAD = (meq Na + meq K) – (meq Cl + meq S)
   – 100 g DM
3. Mongin ratio = (meq Na + meq K) – (meq Cl)  is frequently used with poultry
   – 100 g DM
4. Acid-base balance, as reflected by DCAD of Mongin ratio, appears to be related to performance
   1. With dairy cows, maximal DM intake and milk yield occurred with +38 mEq/100 g DM
   2. Magnitude of response differences was small, between +25 and +50
   3. A Mongin ratio near +40 appears to be optimum for feed intake and growth of poultry
5. It may be desirable to lower DCAD to a minus value for dairy cows for a 2-3 weeks before calving to reduce incidence of milk fever
6. Calculation of DCAD using NRC recommended intakes of Na, K, Cl, and S in the diet
   

   Na K Cl S Atomic Wt (g) Equivalent wt (g) mEq MEq/g ( 1/mEq) % element in diet g element/100 g diet mEq/g element mEq element/100 g diet 22.991 22.991 0.022991 43.495 0.18 0.18 43.495 7.8 39.1 39.1 0.0391 25.575 0.9 0.9 25.575 23.0 35.457 35.457 0.035457 28.203 0.25 .025 28.203 7.05 32.066 16.033 0.016033 62.371 0.2 0.2 62.371 12.47 DCAD = (7.8 + 23.0) – (7.05 + 12.47) = 11.28

7. Sources of minerals to alter DCA
   

   Cations	Anions
   NaHCO3 (27.37% Na) Na2CO3 (43.38% Na) KCHO3 (39.05% K)	CaCl2 • 2H2O (48.23% Cl) MgCl2 • 6H2O (34.88% Cl) MgSO4 (26.64% S) CaSO4 (23.55% S) CH4Cl (66.28% Cl)

VII.  Deficiency

1. Symptoms of deficiency
   1. Sodium
      1. Salt craving
      2. Appetite loss
      3. Decreased gain or weight loss
      4. Unthrifty appearance
      5. Reduced milk yield
   2. Chlorine
      1. Appetite loss
      2. Decreased performance
      3. Lethargy
      4. Dehydration
         1. Polydipsia
         2. Polyurea
      5. Reduced respiration rate
      6. Cardiovascular depression
      7. Alkalosis
   3. Potassium
      1. Appetite loss
      2. Decreased water intake and urinary output
      3. Weight loss
      4. Reduced milk yield
      5. General weakness
2. Factors increasing Na requirement (could contribute to development of an Na deficiency)
   1. Inadequate intake
   2. Excessive sweating
   3. Heavy lactation
   4. Excessive gastrointestinal losses during diarrhea or vomiting
   5. Excessive urinary loss due to reduced Na resorption or renal insufficiency
   6. Adrenocortical failure
3. Conditions which could contribute to Cl deficiency
   1. Failure of normal kidney function to conserve body Cl
   2. Excessive loss in vomiting
   3. Ingestion of diuretics
   4. Sequestration of HCl in a displaced abomassum (dairy cow)
   5. High grain supplemented with excess NaHCO₃ (dairy cow)
      1. Concentrate feeds are usually low in Cl
      2. Feeding of high concentrates to ruminants is often accompanied by feeding of bicarbonate buffers
      3. Excess bicarbonate could increase blood bicarbonate sufficiently to produce alkalosis
   6. Potassium deficiency may be caused by:
      1. Inadequate K intake
      2. High salt consumption
      3. GI losses
      4. Stressful conditions

VIII.  Toxicity

1. Sodium and Chloride
   1. Effects of large excess of salt consumed in water
      1. Hypertonicity of extracellular fluid
      2. Increase in body fluid volume
      3. Intracellular dehydration
      4. Death (concentration by kidney < concentration in fluid ingested)
   2. Cattle can tolerate a comparatively high amount of salt in dry feed
2. Potassium
   1. Ruminants can tolerate high dietary K without immediate effects as long as plasma K⁺ concentration can be kept within normal limits by excretory and other mechanisms.
      1. Cows can consume >500 g of K daily as alfalfa hay
      2. Organic acids in fresh grass reduce tolerance of cows to K
      3. High K content of early spring pasture can reduce Mg absorption and utilization
      4. High K intake can contribute to edema and milk fever in dairy cows
   2. Nonruminants are likely to vomit before ingesting amounts of K too great to permit regulation by storage and excretion

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