K+ in the Kidney

K+ in the Kidney

in a normal diet we consume far to much potassium
1. body has to deal with this in some way = excretion
the nephron
1. low potassium - absorption of K+ into bones
2. normal or high potassium - more secretion of potassium
conc. ICF = 150mM and ECF = 4mM
1. maintained in a very close range - above 5mM = hyperkalemia and below 3.5mM = hypokalemia
  1. body tries to maintain a net balance of 0
why is the conc of K+ important?
1. High ICF conc.
  1. maintain cell volume - pump action
  2. regulation of pH
  3. control of enzyme function
  4. control DNA and protein synthesis
  5. control of growth and cell proliferation
2. Low ECF conc.
  1. maintain steep K+ gradient across membrane
  2. maintain potential of cells
  3. Low levels of K+ prevents problems with excitation and contraction
    1. Action potential firing
    2. Muscle contraction
    3. Cardiac rhythmicity
Physiological role of K+
1. cell volume maintenance
  1. Low K+ = cell shrinkage
  2. High K+ = cell swelling
2. intracellular pH regulation
  1. low K+ = cell acidosis
  2. high K+ = cell alkalosis
3. enzyme function
4. DNA/ protein synthesis
  1. lack of K+ results in reduction of protein synthesis = stunted growth
roles of transmembrane K+ ratio
1. resting membrane potential
  1. reduced K+ inside to outside - depolarization
  2. increased K+ inside to outside - hyperpolarization
2. neuromuscular activity
  1. low plasma K+ - muscle weakness, paralysis, vasodilation and respiratory failure
  2. high plasma K+ - conduction disturbances, arrhythmia and fibrillation
3. cardiac activity
  1. low K+ - slow conduction of pacemaker cells, arrhythmia
  2. High K+ - conduction disturbances, arrhythmia and fibrillation
4. Vascular resistance
  1. Low K+ - vaso-constriction
  2. high K+ - vaso-dilation
K+ and the heart
1. high conc. = heart fibrillation and death
2. low conc. = low T wave, high U wave
overall K+ homeostasis - daily intake = excreted, net = 0
1. most K+ excreted via urine
  1. when intake is greater than excreted amount we have a + balance
  2. when intake is less than excreted we have a - balance
2. how does body respond to changes in K+
  1. extracellular renal function - increase K+ uptake into cells
  2. increase pump function and secrete more K+
  3. intracellular renal function - regulation of reabsorption and secretion of K+ along nephron - occurs after several hours
  4. extra renal affects of EPO,insulin, aldosterone
    1. more sodium out, more potassium in
    2. EPO released from chromatin cells form the adrenal medulla increases Na+ K+ ATPase activity
    3. insulin released from beta cells
    4. aldosterone released from zone glomerulosa cells from adrenal cortex
intra-renal effects - GFR x K+ plasma = daily filtered load
1. depends on diet - increase in K+ in diet, increase in K+ filtered load
2. low diet of K+ reabsorption
  1. proximal tubule
    1. paracellular pathway
    2. apical K+ channels
    3. NKCC2
      1. sets up voltage across membrane
    4. 67% reabsorbed
      1. minimal secretion to lumen
  2. thick ascending limb
    1. NKCC2
    2. 20% reabsorbed
    3. paracellular pathway
  3. distal tubule
  4. collecting duct
    1. reabsorption: intercalated cells (30%)
      1. K+/H+ exchanger
      2. pH important
    2. Na+ reabsorption and K+ section principle cells (70%)
      1. aldosterone is important
      2. ENaC
      3. K+ Cl- co transporter
  5. low K+ = low flow rate, low/no secretion
3. normal or increased K+
  1. secretion
    1. distal tubule
    2. collecting duct
    3. excreted in urine
    4. secretion- high plasma K+, action of aldosterone
      1. when we have high K+ we see different things occurring in all areas of the body - lungs, liver, adrenal glands, heart and kidney
      2. if we increase plasma K+ we stimulate the adrenal cortex to release aldosterone
        Aldosterone
        Late distal tubule and collecting duct
        ROMK1 is important
        ENaC activity - cell more positive and lumen more negative
        increase in Na+ K+ ATPase activity on BL membrane
        entry of Na+ makes cell potential more positive = driving force for K+ exit across apical membrane (secretion)
        increase pump function and therefore increase K+
        Increase Na+ K+ Activity
        change in electrochemical gradient - aldosterone is secreted
        K+ permeability increases and K+ is secreted
      3. Activation of Na+ K+ ATPase - increases intracellular K+
        increases K+ secreted across apical membrane - collecting duct and late distal tubule
  2. reabsorption
    1. proximal tubule
    2. thick ascending limb
  3. secretion is high compared to single flow rate
Summary
1. K+ homeostasis is crucial for survival - ICF = 150mM and ECF = 4mM
2. Hormones effect tubular flow rate in K+ secretion
3. various segments of nephron involved in reabsorption and secretion
4. K+ can be transported into tissues

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	Created by Lucy Clapcott about 9 years ago