The significance of ionic concentrations in the internal media of animals

Evolutionary determinants of normal arterial plasma pH in ectothermic vertebrates

Mean values of normal arterial pH in different species of fish, amphibians and reptiles at 15 and 25°C, taken from the literature, are negatively correlated with arterial PCO2 and plasma [Na+]. At either temperature, the data accord with the hypothesis that extracellular acid–base homeostasis evolved to maintain an optimal pH at particular cell-surface sites that are similar in all species. These hypothetical sites bear fixed negative charges that attract H+, but which are partially screened by Na+; for the surface pH to be constant, the bulk interstitial pH should then vary inversely with [Na+], as is the case. At the same time, the bulk interstitial fluid must be more acid than arterial plasma by an amount that increases with decreasing arterial PCO2. With allowance made for additional screening by Ca2+ and Mg2+, the relevant cell-surface pH is probably approximately 6.2.

The dependence of normal plasma pH on sodium concentration in amphibians, reptiles and man

Normal plasma pH at a given temperature is typically higher in freshwater turtles than in other reptiles, but lower than in amphibians. Normal pH in the different species at a given temperature shows a negative correlation with sodium concentration. Human plasma fits this pattern. What may be more nearly constant from species to species than plasma pH is the pH at particular sites on cell surfaces, and therefore the ionization of proteins located there.

The roles of intracellular buffers and bone mineral in the regulation of acid-base balance in mammals

1. Regulation of intracellular and extracellular pH may conflict in their requirements for movement of acid or base. 2. Cells make a positive or a negative contribution to 'tissue buffering' of extracellular fluid (ECF), depending on their internal buffer value, on the tightness of their internal pH control by membrane mechanisms, and on the nature of the acid-base disturbance. 3. A role is suggested for electrogenic Na-HCO3 co-transport in some of the ion shifts that occur in acid-base disturbances. 4. The time course of 'tissue buffering' in nephrectomized mammals in hypercapnia is variable, and it is far from clear in intact, unanaesthetized mammals. 5. Buffering of ECF by Ca salts of bone mineral in acidosis can only be substantial if accompanied by Ca excretion; the release of HCO3 with Na and K is more significant. 6. The relative importance of cells and of bone mineral in the buffering of ECF is unclear.

The role of imidazole ionization in the control of breathing

Interacting effects of pH and temperature on ventilation in turtles and alligators have suggested that pH is monitored in terms of imidazole ionization, but the view that this is true in alligators and ectotherms generally has been attacked. Published evidence re-interpreted here indicates that this "alphastat control" could indeed operate in turtles, alligators, chickens (i.e. intrapulmonary chemoreceptors) and perhaps cats. Nevertheless, studies on temperature effects cannot distinguish with certainty between imidazole and some amino groups in this context.

Internal reference standards in ionic regulation and the predictability of ionic concentrations in animals

The regulation of ions at similar concentrations in most individuals of a species suggests the existence of internal reference standards. Few have been identified, but many probably relate to cell membrane properties, including potentials, surface charge densities and equilibrium constants of receptor molecules. Solubility may sometimes determine the product [Ca2+] [CO2-3]. Reference standards must generally each relate to more than one ionic species. For some concentrations, including osmolality, there may be no direct reference standard.

Cell composition as assessed from osmolality and concentrations of sodium, potassium and chloride: total contributions of other substances to osmolality and charge balance

From published data for various tissues on intracellular Na, K and Cl were calculated the net anionic charge on all other substances present and also the total contribution of these to osmolality, assuming osmotic equilibrium with extracellular fluid. These parameters were compared for muscle and nerve of animals differing widely in osmolality and also for other mammalian cells. Cell volume regulation in some euryhaline species was considered; it is only partly due to ninhydrin-positive materials. In sheep mammary glands K seems to be sequestered with lactose and anion. Changes in mammalian muscle due to adrenalectomy, hypophysectomy, K deficiency, myotonia, acid-base imbalance and treatment with deoxycorticosterone or insulin were discussed.

Intracellular buffering

In the quantification of buffering the distinct roles of bicarbonate and of other buffers must be taken into account. The determination of the total non-bicarbonate buffer value, betaa, in intact tissues is complicated by active pH regulation and by heterogeneity of cytoplasm with respect to betaa, while heterogeneity with respect to pH in vivo could cause errors in estimates made with homogenates. Available estimates of betaa are discussed, as are the individual contributions of proteins, dipeptides and phosphates. A high betaa is appropriate in cells which sometimes have high rates of glycolysis, or which buffer extracellular fluid, but non-protein buffer concentrations can be well below the limits imposed by osmolarity, perhaps because buffering can upset ionic gradients.

Comparative physiology of cellular ion and volume regulation

1. Intracellular [K+]i in most cells is around 100-150 mOs. This is true in invertebrate or vertebrate muscle or any other tissue, and the only known exceptions being the axons of some invertebrates (Burton, '73). 2. When cell volume is increased due to hypoosmotic swelling, cell volume is regulated back toward normal in almost all cells. In cells with an osmolality of 300-350 mOs, the regulatory volume decrease is caused primarily by a passive efflux of potassium and anions due to increased membrane permeability to potassium. In cells with a higher osmolality, regulatory volume decrease is caused by the passive efflux of small organic molecules. 3. When cell volume is decreased due to hyperosmotic shrinking, volume is not always regulated back toward normal in in vitro experiments carried out for several hours. However, it appears from the in vivo experiments that some cell volume control is always present. The volume control in the flounder red cells took place through increased sodium influx brought about by a relative increase in cell membrane permeability to sodium. This mechanism also appears to be dominant in the mammalian renal papilla, where the [Na+]i increases in proportion to the increase in cellular osmolality with no change in [K+]i. 4. Cells which remain shrunken in vitro, or cells which only partly restore their volume in vivo, do not exhibit increased [K+]i. This finding means that potassium is lost from the cell and, in most cases, exchanged for sodium when the volume is decreased, even though the osmotic concentration in the extracellular fluid is elevated. 5. Finally, it appears that the maintenance of a rather constant [K+]i is important for all cells. This constancy may be due to the fact that certain enzymes are sensitive to the intracellular potassium concentration. Thus, the DNA synthesis is blocked in mammalian kidney cells when [K+]i is increased (Bygrave, '67). Furthermore, Lubin ('64) has shown that low [K+]i concentrations also cause a decrease in DNA, RNA, and protein synthesis. An optimal concentration appears to be necessary for protein synthesis.