What will likely happen to a cell if the concentration of electrolytes inside the cell is higher than in the extracellular environment?

C Intracellular Fluid Volume

The ICF volume represents the fluid content within the body's cells. This volume cannot be measured directly but is calculated as the difference between the measured TBW and the measured ECF volume. Potassium provides the osmotic skeleton for the ICF in much the same way that sodium provides the osmotic skeleton for the ECF. Because water is freely diffusible into and out of the cell, changes in the tonicity of the ECF are rapidly reflected by similar changes in ICF tonicity (Saxton and Seldin, 1986). This is largely the result of the movement of water across the cell membrane with resultant changes of ICF volume. Thus, whereas plasma sodium concentration decreases in response to water retention, ICF volume increases (Humes, 1986). On the other hand, with water depletion resulting in hypernatremia, ICF volume decreases (Humes, 1986). Relatively little is known about the organization of intracellular water into the various subcellular compartments and organelles.

9.07.5.3 Intracellular Fluids

Intracellular fluids (also called the cytosol) are quite different compositionally from plasma and interstitial fluids (Table 4, Figures 4 and 5). The internal pH of many cells is maintained near 6.9–7.0 through various membrane transport mechanisms such as Na+/H+ and Cl−/HCO3− exchangers, various phosphate and protein buffers, and adjustment of arterial CO2 pressure. In contrast to the plasma, the intracellular fluids have substantially lower concentrations of sodium, calcium, chloride, and bicarbonate, and higher to substantially higher concentrations of potassium, magnesium, sulfate, inorganic and organic phosphates, proteins, and other organic species. The cells maintain their high K/Na via the sodium pump, also known as the Na+/K+ ATPase pump; the free energy derived from the hydrolysis of ATP to ADP is used to drive the transport of sodium out of the cell and potassium into the cell.

The redox potential within the cells is “substantially lower” than in the plasma (May and Williams, 1980), and may vary depending upon particular cellular biochemical activities and a myriad of potential redox couples such as reduced and oxidized glutathione species. In general, greater levels of reduced glutathione in the intracellular fluids than in the plasma may provide an indication of an overall lower oxidation condition within the cells. However, it is interesting to note that intracellular fluids have relatively high concentrations of dissolved sulfate (Table 4), in spite of the more reduced conditions inferred to be present.

The deleterious effects of toxicants and processes that cause shifts in the redox balance within the cells are thought to lead to tissue damage and disease such as cancer. For example, generation of ROS such as superoxide and hydroxyl radicals by the intracellular reduction of metals such as Fe(iii) (when present in excess over the amounts needed for proper cellular function) and Cr(vi) is thought to lead to cell membrane damage, destruction of enzymes and other proteins (through oxidation of HS groups), and induction of breaks in DNA strands (Kawanishi, 1995; Rhoades and Pflanzer, 1992; Aust and Lund, 1990). Chemical species such as glutathione, ascorbic acid, and selenium and enzyme systems such as catalase are examples of the chemicals that the body mobilizes to scavenge free radicals, but adverse effects occur when the free radicals are generated in excess and these defense mechanisms are overwhelmed (Kawanishi, 1995).

Intracellular fluid analysis

A high PNa indicates that there is a water deficit in the ICF compartment with rare exceptions when hypernatremia is caused by a shift of water into the ICF of muscle (e.g., a patient with hypernatremia following a seizure or rhabdomyolysis). The amount of water needed to restore the ICF tonicity and volume can be estimated using the following calculation. For this calculation, we have made two assumptions: first, the normal ICF volume is two-thirds of the total body water; second, the number of particles in the ICF compartment does not change appreciably.

ICF H2O deficit = Normal ICF volume − Current ICF volume

Current ICF volume = Normal PNa (140 mmol/L) × Normal ICF volume (20 L)/Current PNa (154 mmol/L)

Based on this calculation, there is a water deficit in the ICF in each of these two patients of 2 L.

Why did the PGlucose and the PEffective osm fall in the second 100 minutes?

The first step is to examine the balance for water. The patient was given 1 L of water with 150 mmol NaCl (isotonic saline) and excreted 1 L of urine. Thus, there appears to be external balance for water. The fall in the PGlucose was consistent with the amount of glucose excreted in the urine, and this suggests that there was little glucose absorbed from the gastrointestinal tract in the second 100 minutes. Notwithstanding, the PNa should have risen because there was a positive balance of 100 mmol of Na+ ions (input of 150 mmol minus output of 50 mmol in the urine) and zero balance for water. The PNa actually fell by 2 mmol/L, and hence there must be a gain of water that prevented the rise in the PNa. Recall that the patient changed her intake from fruit juice to water in the emergency department.

18.3.5 Potassium-40

Potassium is found mostly in the intracellular fluid, and is used to estimate body cell mass. Body cell mass is the fat-free intracellular space and the most metabolically active part of the body.35 It consists of the intracellular fluids and a smaller proportion of intracellular solids of the organs and muscles, and excludes extracellular fluids and solids (such as bone mineral and collagen). A constant ratio of intracellular fluid to body cell mass is assumed, so measurement of total body potassium can be used to estimate body cell mass (body cell mass = total body K (mmol) × 0.0083). Potassium (40K) is a naturally occurring stable isotope found in human tissue. 40K emits a strong gamma ray which can be counted in a lead-shielded room (40K counter) with a gamma ray detector for determination of the whole-body content of 40K. 40K occurs as a very small percentage of the non-radioactive 39K also present in the body, and total body potassium occurs in the ratio of 40K/0.0118%. Since potassium is within the intracellular space, 40K also can be used in combination with TBW to estimate the intracellular and extracellular fluid compartments of the body.

Cellular Fluid Exchange

Osmotic pressure differences between ECF and ICF are responsible for fluid movement between these compartments. Because the plasma membrane of cells contains water channels (aquaporins [AQPs]), water can easily cross the membrane. Thus a change in the osmolality of either ICF or ECF results in rapid movement (i.e., in minutes) of water between these compartments. Thus, except for transient changes, the ICF and ECF compartments are in osmotic equilibrium.

AT THE CELLULAR LEVEL

Water movement across the plasma membrane of cells occurs through a class of integral membrane proteins called aquaporins (AQPs). Although water can cross the membrane through other transporters (e.g., an Na+-glucose symporter), AQPs are the main route of water movement into and out of the cell. To date, 13 AQPs have been identified. These AQPs can be divided into two subgroups. One group, which includes the AQP involved in the regulation of water movement across the apical membrane of renal collecting duct cells by arginine vasopressin (AQP-2) (see Chapter 5), is permeable only to water. The second group is permeable not only to water but also to low-molecular-weight substances, including gases and metalloids. Because glycerol can cross the membrane via this group of aquaporins, they are termed aquaglyceroporins. AQPs exist in the plasma membrane as a homotetramer, with each monomer functioning as a water channel (see Chapter 4).

In contrast to the movement of water, the movement of ions across cell membranes is more variable from cell to cell and depends on the presence of specific membrane transport proteins. Consequently, as a first approximation, fluid exchange between the ICF and ECF under pathophysiologic conditions can be analyzed by assuming that appreciable shifts of ions between the compartments do not occur.

A useful approach for understanding the movement of fluids between the ICF and the ECF is outlined in Box 1-1. To illustrate this approach, consider what happens when solutions containing various amounts of NaCl are added to the ECF.∗

4.10 Changes in Body Fluid Compartments

There are two fluid compartments in the body. Intracellular fluid (ICF) is the cytosol within the cell. Extracellular fluid (ECF) surrounds the cells serves as a circulating reservoir. The ECF is divided into the interstitial fluid which bathes the outside of the cells and intravascular fluid (i.e., plasma, lymph, and cerebral spinal fluid). In the adult 70 kg male, approximately 60% of body weight is water that is about 42 L. Under normal conditions, ICF is 28 L. ECF is 14 L, in which interstitial fluid is 10.5 L and intravascular fluid is 3.5 L. Since capillaries that separate the interstitial and intravascular fluid are leaky, the composition of these compartments is identical. ECF has high concentrations of Na+, Ca2 +, and Cl− than those of ICF.

The two fluid compartments of the body, intracellular fluid and extracellular fluid, are in osmotic equilibrium. Water moves by facilitated diffusion through aquaporin channels across cell membranes. Nonpermeable solutes such as Na+ and Ca2 + are called effective solutes. Cellular volume is dependent on the concentration gradient of effective solutes and water across the cell membrane. Cells shrink if the concentration of Na+ in the extracellular fluid compartment is higher. Cells swell if the concentration of Na+ in the extracellular fluid compartment is lower. Intake of isotonic NaCl increases total body water and ECF volume but brings no effects on ICF volume and ECF osmolality. Isotonic saline loss caused by diarrhea decreases total body water and ECF volume but no effects on ICF volume and ECF osmolality. Excessive intake of NaCl has no effects on total body water but increase ECF volume and decrease ICF volume because of moving water from ICF to ECF to balance osmosis. Water loss from excessive sweating causes decreases in total body water and ICF volume because of moving water from ICF to ECF. (See Table 4.1.)

Table 4.1. Changes in body compartments

Potassium and acid-base balance

The distribution of potassium ions between intracellular fluid and ECF may be affected by acid-base disorders. When HCl was infused acutely into nephrectomized dogs, approximately 50% of the H+ load was buffered intracellularly.55,65 Intracellular sodium and potassium ions entered ECF in exchange for the H+ entering cells, and serum potassium concentration increased. These early animal studies and observations in a small number of human patients10 led to the prediction that metabolic acidosis would be associated with a 0.6-mEq/L increase in serum potassium concentration for each 0.1-U decrease in pH. A review of animal studies demonstrated that the change in serum potassium concentration observed during acute metabolic acidosis caused by mineral acids (e.g., HCl, NH4Cl) was variable.3 Furthermore, an increase in serum potassium concentration does not occur in acute metabolic acidosis caused by organic acids (e.g., lactic acid, ketoacids).* Acute infusion of β-hydroxybutyrate in normal dogs caused an increase in insulin in portal venous blood and hypokalemia, presumably as a result of potassium uptake by cells.2 Acute infusion of HCl led to hyperkalemia and increased portal vein glucagon concentration.2 These acute changes in serum potassium concentration are not the result of changes in renal excretion of potassium.2,49

The hyperkalemia associated with acute metabolic acidosis caused by mineral acids is transient. In a study of acute and chronic metabolic acidosis induced in dogs by administration of HCl or NH4Cl, hyperkalemia was observed after acute infusion of HCl, but hypokalemia developed after 3 to 5 days of NH4Cl administration.41 The observed hypokalemia was associated with inappropriately high urinary excretion of potassium and increased plasma aldosterone concentration.41 Similar findings in rats with chronic metabolic acidosis induced by NH4Cl have been reported.53 Acute metabolic acidosis induced by administration of mineral acid decreases renal proximal tubular reabsorption of sodium, leading to volume contraction and increased distal delivery of sodium. Increased Na+-H+ and Na+-K+ exchange then occurs in the distal nephron, mediated by increased distal tubular fluid flow and hyperaldosteronism. These findings suggest that mild hypokalemia and potassium depletion are likely to develop during chronic metabolic acidosis caused by administration of a mineral acid. The observation of hyperkalemia during chronic metabolic acidosis should prompt consideration of impaired renal potassium excretion or some other cause of hyperkalemia (see Chapter 5).

The resting cell membrane potential

The normal relationship between ECF and ICF potassium concentrations is maintained by sodium, potassium-adenosinetriphosphatase (Na+, K+-ATPase) in cell membranes. This enzyme pumps sodium ions out of, and potassium ions into, the cell in a 3:2 Na/K ratio so that the intracellular concentration of potassium is much higher than its extracellular concentration. As a result, K+ ions diffuse out of the cell down their concentration gradient. However, the cell membrane is impermeable to most intracellular anions (e.g., proteins and organic phosphates). Therefore, a net negative charge develops within the cell as K+ ions diffuse out, and a net positive charge accumulates outside the cell. Consequently, a potential difference is generated across the cell membrane.

The principal extracellular cation is sodium, and it enters the cell relatively slowly down its concentration and electrical gradients, because the permeability of the cell membrane to potassium is 100-fold greater than its permeability to sodium. Diffusion of K+ ions from the cell continues until the ECF acquires sufficient positive charge to prevent further diffusion of K+ ions out of the cell. The ratio of the intracellular to extracellular concentrations of potassium ([K+]I/[K+]O) is the major determinant of the resting cell membrane potential as described by the Nernst equation:

Em=−61 log10[K+]I[K+]O

The Goldman-Hodgkin-Katz equation is a modification of the Nernst equation that allows prediction of Em based on the ionic permeability characteristics of the cell membrane to sodium and potassium and the concentrations of these ions inside and outside the cell:

E m=−61log10rPk[K+]I+ PNa[Na+]IrPk[K+ ]O+PNa[Na+]O

where PNa and PK are the membrane permeabilities for sodium and potassium. The term r is included in the equation to account for the effect of the electrogenic Na+, K+-ATPase pump under steady-state conditions. This term is assigned the Na/K transport ratio of 3:2 so that r = 1.5. If the membrane permeability for potassium is assigned a value of 1.0 and the cell membrane is 100 times more permeable to potassium than sodium:

Em=−61log101.5[K+]I +0.01[Na+]I1.5[K+]O+0.01[Na+]O

For example, using the hypothetical ECF and ICF concentrations of sodium and potassium given at the beginning of this chapter:

Em=−61log101.5[140]+0.01[10]1.5[4]+0.01[140]

Em=−61log10(28.4)=−89mV

In one study of dogs with potassium deficiency, the predicted Em was −86.6 mV and the measured Em in skeletal muscle of control animals was −90.1 mV.20 The resting cell membrane potential plays a vital role in the normal function of skeletal and cardiac muscle, nerves, and transporting epithelia.

6.1 Role of Peptidomics as in Cancer Biomarker Discovery

Body fluids, including extracellular and intracellular fluids, are an accurate and comprehensive source of potential biomarkers due to their ability to actively exchange factors with intracellular and extracellular fluids such as the lymphatic system, tissue interstitial fluid, and blood plasma [112]. Such factors and signals that promote tumor proliferation and development, and changes to the immediate surroundings of the tumor microenvironment, are reflected in perturbed protein profiles of such body fluids. These perturbations are potentially derived from peptide and protein cleavage fragments, secreted and leakage components, in addition to factors that emanate from the tumor directly, or its microenvironment. Characterizing these “signatures” of disease is currently a major importance in understanding the functional interaction between tumor cells and their microenvironment. In addition to their diagnostic potential, many body fluids can be attained noninvasively, avoiding risks of invasive tissue sampling through biopsies, and have the ability for high-throughput, large-scale, prognostic/diagnostic tests. However, it should be noted that these “signatures” of disease (biomarkers), typically, are low abundant (e.g., PSA ~ 3–4 ng/mL) in body fluids such as plasma.

The tumor microenvironment consists of a variety of components from the stroma, vasculature, and the ECM, which regulate such signaling networks through the secretion and release of growth factors, cytokines, proteases, and other bioactive molecules and their receptors. Due to their diversified involvement in key regulatory processes and biological processes, peptides and low-Mr protein/peptide fragments (Mr < 20 K) possess unique characteristics which make them important and feasible targets for current cancer biomarker discovery efforts [142,149,150]. The diagnostic potential of peptides has been explored for diagnosis of Alzheimer's disease [151] and for diagnosis of ovarian carcinoma [116]. These peptides are either (i) intact small factors, such as hormones, cytokines, and growth factors; (ii) peptides that are secreted from various tissues; (iii) peptides released from larger protein precursors during protein processing; or (iv) represent degradation products during proteolytic activities. In the context of biomarker discovery, peptides exhibit far greater tumor and vascular permeability due to their molecular size range [152–154]. Yuan et al. [153] concluded that the permeability and diffusion of different molecules in solid tumors is directly dependent on their molecular weight. This observation suggests the increased likelihood of identifying endogenous peptides and proteolytic fragments of tissue-secreted proteins in the tumor microenvironment, and to a lesser degree, proximal fluids and blood plasma.