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Section III.Disturbance of Potassium Balance. Potassium Balance (1) Content and distribution (2) Function of potassium (3) Regulation of K + balance Hypokalemia Hyperkalemia. 1. Potassium Balance (1) Content and distribution.

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Section III.Disturbance of Potassium Balance

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Section III.Disturbance of Potassium Balance

Potassium Balance

(1) Content and distribution

(2) Function of potassium

(3) Regulation of K+ balance

Hypokalemia

Hyperkalemia


1. Potassium Balance(1) Content and distribution

  • The adult body contains about 45 mmol/Kg of BW.

  • About 98% of potassium is within the cells,

  • [K +] i= 140~160 mmol/L.

  • About 2% of K + is in the ECF,

  • [K +]e = 3.5~5.5 mmol/L.

  • 3/4 of the intracellular K+ is in muscle cells.


  • There are two forms of existence:

  • (1) free state of potassium, K+,

  • (2) the K combining to protein and glycogen.

  • Both are exchangeable.

  • (Na+)


(2) Function of potassium

  • 1) Metabolism

  • 2) Membrane potential

  • 3) Regulation of pH

  • 4) Osmotic pressure of ICF


1) Metabolism

  • (a) K+ is required for the activity of some intracellular enzymes e.g. the enzyme for ATP production.

  • (b) K+ is involved in anabolism.

  • 1 g of glycogen contents 0.33~0.45 mmol,

  • The K+ moves into cells with glucose, during the synthesis of glycogen.

  • 1 g of protein needs 30 mmol of K+.


2) Membrane potential

  • The ratio of the intracellular to the extracellular potassium concentration

  • ([K+]i∕[K+]e) constitutes the cellular resting membrane potential.

  • Minimal change of [K+]e may affect the membrane potential obviously.

  • K+ is important for normal neuromuscular irritability.


3) Regulation of pH

  • Exchange of K+ and H+ crossing the cell membrane is important for acid-base balance.

  • Changes of K+ concentration will lead to the changes of pH.

  • When K+ moves out of the cells, H+ will move into the cells as an exchange for electrical neutrality. More H+ will lead to acidosis.


4) Osmotic pressure

  • Potassium ion is the major intracellular cation, so K+ is important in the formation of osmotic pressure in the cell.

  • To keep the volume of ICF.


(3) Regulation of K+ balance

  • 1) Equilibrium of K+ in ICF and ECF

  • 2) Balance of intake and excretion

  • K excretion in kidney

  • K excretion in colon


1) Equilibrium of K+ in ICF and ECF (transcellular potassium movement)

  • Equilibrium means to keep

  • [K+]i= 140~160 mmol/L;

  • [K+]e = 3.5~5.5 mmol/L


Significance :

After intake a bottle of orange juice (40 mmol/L of K+),

if all K+ stays in ECF, the [K+]e=4.5+2.4=6.9 mmol/L, which will lead to abnormal ECG. Abnormal ECG means the heart muscle is injured.

If all K+ is transported into ICF, the [K+]i=160+1.2=161.2 mmol/L.

Then the excessive K+ will be eliminated within several hours in the urine.


Mechanism to keep the equilibrium:

The basic mechanism to the balance is “leak and pump”.

(Integrity of cell membrane and function of Na+-K+ pump)


钾的跨细胞转移调节

细胞内液[K+]

140-160mmo1/L

Na+ - K+泵(泵)

K+

K+

细胞外液[K+]

4.2mmol/L

Na+

Na+

K+

K+

K+通道(漏)

K+细胞内外移动的泵一漏机制

(Pump-leak mechanism)


钾代谢紊乱

【影响钾的跨细胞转移的主要因素】

可直接刺激Na+-K+泵活性,影响钾转移主要激素。

胰岛素

肾上腺素通过cAMP机制激活Na+-K+泵活性,肾上腺能神经激活是促K+自细胞内移出。

儿茶酚胺

ECF [H+]↑, H+入细胞内,细胞内K+外移。ECF每0.1 pH变化大约引起0.6mmol/L血清钾变化

ECF [K+]

ICF [K+]

140-160mmo1/L

酸碱平衡状态

渗透压

ECF 渗透压↑ ↑ ,使细胞内K+外移。

运动

ECF [K+] 4.2mmol/L


(a) Changes of pH

A decrease in pH of 0.1 units usually elevates the serum [K+] by about 0.6 mmol/L.

(alkalosis: pH ↑ 0.1, [K+] ↓ 0.6 mmol/L)


(b) Extracellular K+ concentration

  • A high [K+]e will stimulate the activity of Na+-K+ pump.

  • A low [K+]e will do in the converse way. 


(c) Total K quantity in the body

  • When the total quantity of K in the body reduces, the loss of intracellular K+ is more than extracellular K+, but the ratio of extracellular K+ loss is more than intracellular K+ loss.

  • When the total quantity of K in the body increases , the increase of intracellular K+ is more than extracellular K+, but the ratio of extracellular K+ increase is more than intracellular K+ increase.


(d) Regulation of hormone

Insulin promotes the movement of K+ into the liver cells and skeletal muscle cells by increasing sodium-potassium ATPase activity.

The β-adrenergic agonists also elevate Na+ -K+ pump activity to enhance K+ entry.

The α-adrenergic agents enhance the K+ transport out of cells.

Epinephrine stimulates α-recepter first,then β-recepter.

(K+ )


(e) Metabolism(anabolism; catabolism)

Increased anabolism (AA protein, glucose glucogen) elevates the [K+]i.

Increased catabolism (protein AA, glucogen glucose) reduces the [K+]i.


f) Increased smotic pressure of ECF leads to increased [K+]e.

  • Increased osmolality of extracellular fluid draws the water out of cells with K+.

    Decreased water in the cells elevates the [K+]i. The difference [K+]iand [K+]e increases, which leads to the shift of K+ out of cells increases.


(g) Exercise

  • Exercise can promote K shift out of cells through:

    (1) opening of ATP-dependent K+ channels

    与电压依赖型的钾离子通道不同,也与依赖钙离子的钾离子通道不同,,KATP通道主要受细胞内的ATP浓度调节。在生理条件下细胞内ATP浓度约为3-4 mmol.L-1, KATP通道基本处于关闭状态。只有当心肌细胞发生缺血缺氧,能量耗竭,胞内ATP浓度低于0.2 mmol.L-1时通道开放,K+外流,

    游离ATP是KATP通道最强而有效的内源性阻断剂,其主要功能有:(1)舒张血管,包括外周血管和冠状动脉。主要由于KATP激活,K+外流,细胞复极化加速,使动作电位时程缩短,Ca2+内流减少,血管舒张。 (2)Ca2+内流减少,使心肌收缩力减弱,降低心肌氧耗,产生心脏保护作用。

    (2) decrease Na+ -K+ ATPase activity due to ATP depletion.


(h) Integrity of cellular membrane

  • “Leak” indicates the moving of K+ out of the cell according to the gradient of [K+ ] between ICF and ECF, without expending ATP.

  • Leaking leads to the tendency to reduce the [K+]i. When the cell membrane is injured, the permeability of cell membrane to K+ is increased. More K+ move from cells into ECF.


2) Balance of intake and excretion

  • (a) Intake: The common foods, like lean meat, milk and fruits content a lot of potassium.

  • The average diet contains 60~100 mmol of potassium per day, which is enough for the daily body requirement. 90% of potassium in food is absorbed in small intestine.


(b) Excretion

  • ① Via kidney

  • About 90% or more potassium is eliminated from kidney. (12字)

  • The more K we eat, the more K is eliminated from kidneys. When the intake of potassium is decreased, the elimination from urine is decreased.

  • If no potassium intake, the kidneys will still secrete small amount of potassium (20~15 mmol/day). (Na+?)


  • Potassium is freely filtered at the glomerulus. Almost all the potassium filtered is reabsorbed in proximal tubules via active transport. In loop of Henle: via Na+-K+-2Cl-contransporter.

  • Most of the potassium in the urine is secreted from distal tubules and collecting ducts.

  • No decrease of K+ filtration except severe reduction of GFR.


  • a) aldosterone

  • Aldosterone activates pump (Na+ /K+ pump) in basolateral membrane, the K+ transport from peritubular interstitial fluid into renal tubular cells will increase.

  • Aldosterone increases the permeability of lumenal membrane, the K+ transport from renal tubular cells into tubules (urine) will increase.

  • (C)Regulation of renal loss in renal distal tubules and collecting ducts,


b) High [K+]e

High [K+]e activates pump (Na+ /K+ pump) in basolateral membrane, the K+ transport from peritubular interstitial fluid into renal tubular cells will increase.

High [K+]e increases the permeability of lumenal membrane, the K+ transport from renal tubular cells into tubules (urine) will increase.

High [K+]edecreases the [K+] difference between renal tubular cells and peritubular interstitial fluid, then decrease the back-flow of K+ from tubule.


c) Volume and flow rate of urine in distal tubules and collecting ducts

Increased volume and flow rate of urine in distal tubules and collecting ductsreduce the [K+], increase the difference between the [K+] in urine and in tubule cells, increase the excretion of K.


d) Acid-base balance

  • In acute acidosis, increased [H+] suppresses Na+-K+-ATPase, and the excretion of K+ decreases.

  • In alkalosis the excretion of K+ increases.

  • In chronic acidosis, the dominant effect is that increased [H+] suppresses the reabsorption of water and sodium in proximal tubule cells, urine volume increases, excretion of K+ increases.


e) Electric field

If more negative charges in tubular fluid, more K+ will be excreted.


② Via intestinal tract K excretion in colon

  • 10% of K is excreted through the colon. The epithelial cells are just as the principal cells in the collecting duct, and affected by aldosterone.

  • K+ excretion via colon will increase in renal failure.

  • GFR↓↓↓ K excretion in colon to 1/3 of intake .


  • ③ Via sweating

  • Generally speaking, the loss of K+ with sweat is neglectful (5~10 mmol/L). This kind of loss may be significant some time (in plenty of sweat).


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