The effect of metabolic inhibition on ion contents and sodium exchange in human lymphocytes

A cooperative transition theory applied to the kinetics of ionic exchanges in cells

Manifestations of a cooperative interaction between ion-adsorbing sites in cells include steep, sigmoidal equilibrium adsorption isotherms of K+ and Na+, critical temperature transitions of net exchanges of Na+ for K+, and the allosteric nature of the effects of ligands on cellular K+ and Na+. Cooperative ionic adsorption is described by a one-dimensional Ising model. The experimentally-determined equilibrium parameters permit prediction of the kinetics of exchange of K+ for Na+ (the approach to equilibrium) by stochastic or hydrodynamic solutions of a time-dependent Ising model. Studies of the rates of self-exchange of adsorbed ions reveal properties of the cooperatively interacting adsorption sites and their dependence on temperature and chemical potential. High rates of isotopic exchanges of K+ and Na+ occur near the transition point. This is explained by the hypothesis of an increase in susceptibility of the ensemble to slight variations of microK or microNA near the phase transition, which leads to an increase in microscopic fluctuations within the ensemble. It is suggested that the isotopic ion exchange experiment may be a means to explore the microscopic states of the ensemble and their transition probabilities.

Effect of elastin peptides on ion fluxes in mononuclear cells, fibroblasts, and smooth muscle cells

Elastin peptides prepared by alcoholic potassium hydroxide degradation of highly purified fibrous elastin from bovine ligamentum nuchae (kappa-elastin) were shown to act on the ion channels of human monocytes, aorta smooth muscle cells, and skin fibroblasts. In small amounts (between 0.1 and 1 microgram/ml), elastin peptides strongly increased calcium influx and inhibited calcium efflux by an apparently calmodulin-dependent mechanism. They also were shown to increase sodium influx and to decrease rubidium influx in monocyte preparations obtained from human blood. Only the ouabain-sensitive portion of rubidium influx was inhibited. The action of elastin peptides is strongly concentration-dependent; the maximal activity observed in the above reactions was less than 1 microgram/ml. These results suggest that elastin peptides may play a role in the regulation of the biological activity of mesenchymal cells, in the proximity of which they are released by the action of elastase-type enzymes. Such enzymes were demonstrated in aorta smooth muscle cells (membrane-bound serine protease) and in fibroblasts (metalloprotease). Monocytes and polymorphonuclear leukocytes were also shown to carry elastase-type enzymes. The release of peptides from elastin by elastase-type enzymes and the action of such peptides on the ion fluxes through the cell membrane may well be involved in mechanisms of the modulation of the phenotype of mesenchymal cells during aging as well as in the development of age-dependent pathologies such as arterioclerosis.

Proteolysis of poly(ADPribose) polymerase by a pyrophosphate- and nucleotide-stimulated system dependent on two different classes of proteinase

We have identified a system in human lymphocytes which proteolytically cleaves poly(ADPribose) polymerase to specific fragments of molecular weight 96 000, 79 000 and 62 000-60 000. This proteolytic processing is dependent on two different classes of proteinase. One of these proteinases is a serine proteinase, since the processing is inhibited by phenylmethylsulfonyl fluoride, antipain, soybean trypsin inhibitor and diisopropylfluorophosphate, the other is a cathepsin D-like proteinase, since processing is also inhibited by pepstatin A. The processing that occurs in permeabilized cells can be simulated in vitro by treating purified poly(ADPribose) polymerase with trypsin, but not by treating the polymerase with cathepsin D. Since processing at the cellular level is blocked by inhibitors of either of the two proteinases, but only trypsin could cleave the purified polymerase, this suggests that in the cell the action of the cathepsin D-like proteinase is a prerequisite for cleavage of poly(ADPribose) polymerase by the serine proteinase. Thus, a pathway involving sequential action of these proteinases may exist. Proteolysis in permeabilized human lymphocytes is stimulated by nucleotides containing a pyrophosphate group, such as 5',5'''-P1,P4-tetraphosphate and ATP, or by pyrophosphate itself. In contrast, nucleotides containing only a single phosphate, such as AMP and cyclic AMP, or inorganic sodium phosphate, do not show this stimulation of proteolysis. These results suggest that a pyrophosphate linkage is the minimum molecular requirement for stimulation of proteolytic processing of poly(ADPribose) polymerase. Proteolytic processing of poly(ADPribose) polymerase is independent of ADPribosylation. Following proteolysis, specific fragments of the polymerase, particularly the 62 000-60 000 molecular weight fragment(s), are still capable of being ADPribosylated.

Self-exchange of sodium in human lymphocytes

Self-exchanges of Na and K in human lymphocytes were measured by isotopic efflux techniques. In washed cells, K exchanged in a single slow exponential fraction, but the Na exchange had a marked curvature. It was shown that the curvature was not caused by simple bulk-phase diffusion, and it was resolved into three major fractions: fast (F) (half-time, t1/2 = 2-4 min), intermediate (I) (t1/2 = 12 min), and slow (S) (t1/2 = 125 min). Each of these appeared to follow an exponential function. The I fraction contained approximately 10 mmol Na/kg cells (25-30% of normal cellular Na), was not affected by manipulations that cause lymphocytes to gain Na, and had little or no temperature dependence. The S fraction of Na in normal cells (S1) contained approximately 10 mmol Na/kg cells, had only a slight temperature dependence, and the amount and rate of S1 were independent of external K concentration (Kex). Another slow fraction (S2) appeared when the cells underwent a net gain of Na in exchange for K, and was characterized by a steep temperature dependence and a peak rate around the transition point (the point at which half of cellular K is replaced by Na) at 0.4 mM Kex. The results are discussed within context of a theory that assigns the exchange of the major part of K in its slow exponential fraction and the Na exchange in S2 to interactions of these ions with fixed anionic sites, on intracellular macromolecules, which have been shown previously to interact cooperatively in their association with K and Na.

pH homeostasis in human lymphocytes: modulation by ions and mitogen

Quiescent human peripheral blood lymphocytes have been shown to maintain a relatively constant intracellular pH of 7.0-7.2 over an extracellular pH range of 6.9-7.4. Two methods of measuring intracellular pH were used in these studies, 19F nuclear magnetic resonance and [14C]5,5-dimethyloxazolidine-2,4-dione (DMO) equilibrium distributions. When ATP levels were decreased in these cells, actively maintained pH regulation was abolished and cells exhibited a constant pH gradient of 0.2 pH unit (acid inside relative to outside). Possible mechanisms for pH regulation are discussed. The effects of the Na+ and K+ composition of the medium on pH regulation showed no correlation with their effects on mitogen-induced proliferative response, which we have previously determined (Deutsch, C., and M. Price, 1982, J. Cell. Physiol., 111:73-79). In low-Na+ mannitol medium, pH regulation was similar to that observed for lymphocytes in normal medium, whereas mitogen-induced proliferation was severely inhibited in low-Na+ mannitol. In contrast, high-K+, low Na+ medium caused loss of pH homeostasis, whereas it restored the proliferative response. Loss of pH homeostasis was also observed on prolonged exposure of lymphocytes to mitogen (greater than 6 h in culture). However, mitogen stimulation led to little or no change in intracellular pH in the first few hours of cell culture. Therefore, a shift in intracellular pH is not a necessary or general event in mitogen-stimulated proliferation of lymphocytes.