NAS 161

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NAS 161

• Membrane Physiology Part I
• Joseph S. DiPietro, Ph.D., RRT
• Fall 2004

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FIGURE 3-1. The fluid compartments of a
prototypical adult human weighing 70 kg. Total
body water is divided into four major
compartments intracellular fluid (green),
interstitial fluid (blue), blood plasma (red),
and transcellular water such as synovial fluid
("sand" color). Color codes for each of these
compartments are maintained throughout this book.
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Water movement

• Is critical to life-sustaining processes and
  chemical reactions
• Is facilitated by channels in biological
  membranes
• Is predicated upon the concepts of charge balance
  and osmotic pressures
• Is based upon each cells needs and the concept
  of homeostasis

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FIGURE 3-2. Uncoupled transport of a solute
across a cell membrane. The net passive movement
of a solute (X) depends on both the difference in
concentrations between the inside of the cell
(Xi) and the outside of the cell (Xo) and the
difference in electrical potential between the
inside of the cell Yi) and the outside of the
cell Yo).
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FIGURE 3-3. Three types of passive, non-coupled
transport through integral membrane proteins.
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Na/K Pump
FIGURE 3-8. Model of the sodium pump. A,
Schematic representation of the ? and ? subunits
of the pump. B, The protein cycles through at
least 8 identifiable stages as it moves 3 Na out
of the cell, and 2 K into the cell. ADP,
adenosine diphosphate. ATP, adenosine
triphosphate Pi, inorganic phosphate.
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The actual pump steps
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ATP formation/membrane
FIGURE 3-9. The FoF1 ATPase and its role as the
ATP synthase in the mitochondrial synthesis of
ATP. A, A cartoon of the FoF1 ATPase. The pump
has two functional units, Fo (which historically
stood for oligomycin-sensitive factor) and Fl
(which historically stood for factor 1). Fo is
the transmembrane portion that contains the ion
channel through which the H passes. The F1 is
the ATPase. In one complete cycle, the downhill
movement of ten H causes the c subunits of Fo
and the axle formed by the ?? subunits of F1 to
rotate 360 degrees and causes the ? and ?
subunits to sequentially synthesize and release 3
ATP molecules. B, Complexes I, III, and IV of the
respiratory chain use the energy of NADH to pump
a total of ten H out of the mitochondrial
matrix. The resulting H gradient causes the
mitochondrial FoF1 ATPase to run as an ATP
synthase. The mitochondrion appears to synthesize
approximately three ATPs for each NADH and
approximately two ATPs for each
FADH2. Abbreviation meanings (ADP, adenosine
diphosphate ATP, adenosine triphosphate ATPase,
adenosine triphosphatase NAD, nicotinamide
adenine dinucleotide NADH, reduced NAD Pi,
inorganic phosphate.)
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Cellular channels for ion transfer
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Explanation for Fig 3-12, 1-9

• 1, sodium Na gradient is maintained by the
  sodium extrusion (pushed outside the cell) via
  the Na/K pump.
• 2, potassium, on the other hand is very low in
  extracellular fluid (ECF), but is at a
  concentration 25 to 30 times higher in
  intracellular fluiddue to the Na/K pump (2
  potassium's in, while three sodium's leave)if
  membrane potential is inappropriate, potassium
  leaves via this channel.
• 3, amiloride sensitive channels (ENaC) exist
  at luminal cell surfaces preventing constant
  cycling of sodium across epithelial cells,
  thereby preventing potassium (K) loss
• 4, Excitable cell sodium movement is passive
  through voltage-dependent channels, providing for
  action potential, critical in muscle, nerve and
  cardiac function.
• 5, sodium/solute co-transporter allows
  electrochemical gradients to move nutrients and
  ions across membranes
• 6, voltage-gated calcium channels allows for
  short burst entry of calcium into cells when
  needed to maintaining electrical gradients across
  membranes
• 7, Calcium pumps (ATPases) are available within
  certain organelles such as the endoplasmic
  reticulum---stores intracellular calcium to later
  be released for aiding signal transmission during
  cell depolarization for example.
• 8, Plasma membrane calcium pump (PMCA) instead
  of calcium leaving the cell when inner cell
  concentrations are elevated, calcium binds with
  calmodulin, which lowers the Michaelis Constant
  Km(solute concentration that gives 1/2 the
  maximal rate of transport), allowing calcium to
  leave the cell.
• 9, Sodium-Calcium Exchanger (NCX) allows calcium
  to leave the cell when calcium concentrations are
  substantially higher than normal.

Amiloride is a potassium-sparing diuretic
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Cellular channel for ion transfer 2
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Explanation for Part II of membrane channels

• 10, Anion (negatively charged ions)-selective
  channels allow passive movement of chloride (Cl-)
  can move passively, since intracellular chloride
  concentrations are lower than extracellular
  levels.
• 11, Cl--HCO3- exchanger, allows and balances
  chloride efflux, allowing chloride to go into the
  cell to balance charges. This occurs in most cell
  types other than skeletal muscle. Known as uphill
  transport.
• 12, Na/K/Ca Cotransporter, another uphill
  transport mechanism, similar to the Cl--HCO3-
  exchanger, functions when chloride concentrations
  are low.
• 13, K/Cl- Cotransporter, tend to move potassium
  and chloride out of the cell, so the potassium
  gradient promotes chloride to leave the cell,
  supporting negative voltage on the inside of the
  cell.
• 14 15, Cation channels and Na-driven HCO3-
  transporters, allow for the balance of CO2,
  HCO3-, and Hall of which are based upon the
  carbon dioxide equation CO2 H2O ? H2CO3 ? H
  HCO3-. This is how pH is maintained within the
  cells, at least in part.
• 16 17, Na/H and Na-driven Cl-HCO3- exchangers,
  are pH sensitive, stimulated when acids are
  increased and inhibited when acids are low.
• 18, V-type H pumps or H-K pumps, are designed
  to push acids out of the cells. Can be found in
  the renal collecting ducts and the stomach, as
  well as intracellular organelles such as the
  lysosomes and Golgi.

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Some definitions you need

• Influx---in this case, into the cell
• Efflux---leaving the cell
• Amiloride--- potassium-sparing diuretic
• Membrane channels--- pores through which
  ions/molecules move
• Extrusion---ion movement (as being pushed out
  of the cell

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Ouabain

• Ouabain is a glycoside poison that binds to and
  inhibits the action of the Na/K pump in the
  cell membrane. The Na/K pump is essential for
  maintaining the balance of these ions across cell
  walls. Used as a cardiac glycoside increasing
  calcium, thereby increasing cardiac output. Used
  in CHF. Another common cardiac glycoside is
  digitalis. A glycoside is a compound comprised of
  a known carbohydrate and a non-carbohydrate such
  as an alcohol.

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The effects of Ouabain
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Serum Osmolality

• Definition measures the amount of chemicals
  dissolved in the fluid portion of blood (serum).
  Chemicals that affect serum osmolality include
  sodium, chloride, bicarbonate, proteins, and
  sugar (glucose). A serum osmolality test is done
  to evaluate electrolyte and water balance.
• Osmolality (2 x (Na K)) (BUN / 2.8)
  (glucose / 18)
• Normal concentration 275-295 milliosmoles/Kg
  (mOSm/Kg)

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Things effecting osmolality

• ADH---antidiuretic hormone or vasopressin
• Manufactured by the hypothalamus
• Released into the blood by the pituitary gland

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Elevated serum osmolality

• Diabetes
• Hypernatremia (elevated sodium)
• Dehydration
• Mannitol use (a plasma expander)
• Uremia
• Poisons
• Methanol
• Ethanol
• Ethylene glycol (antifreeze)

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Decreased serum osmolality

• Over hydration
• Decreased glucose levels
• Hyponatremia (low sodium)
• SAIDH (Isovolemic Hypoosmolar Hyponatremia where
  extracellular volume is normal w/low sodium and
  low serum osmolalityDx by exclusion
• Causes include
• Water intoxication
• Malignancy
• Neurologic disorders
• MS
• Guillain-Barre Syndrome
• Certain meds like Tegretol Thorazine

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Cell response to ? osmolality
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The result of adding urea to ECF (extracellular
fluid)
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Cellular response of adding 1.5 Liters 145 mM NaCl
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Cellular response by adding water or pure NaCl to
ECF
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Model of "isotonic" water transport in a leaky
epithelium. Na-K pumps present on the lateral and
basolateral membrane pump Na into two restricted
spaces the lateral intercellular space and
restricted spaces formed by infoldings of the
basal membrane. The locally high osmolality in
the lateral intercellular space pulls water from
the lumen and the cell. Similarly, the locally
high osmolality in the restricted basal spaces
pulls water from the cell. The solution that
emerges from these two restricted spaces-and
enters the interstitial space-is only slightly
hypertonic (virtually "isotonic") compared with
the luminal solution.
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Global Summary

• Channels in cell membranes allow for ion, charged
  molecules and water movement.
• Different channels exist for different cell
  situations
• Water moves across compartments due to osmosis,
  and cellular needs, and charge differentials
  between intra- and extracellular environments
• Ionic particle movement is similar to water
• Different meds, solutes or solvents added to our
  physiologic system alters our ability to handle
  these either through swelling or diuresis or
  both.
• The endocrine glands like the hypothalamus and
  pituitary are intimately involved in fluid and
  electrolyte balance