The role of calcium in the regulation of sugar transport in the pigeon red blood cell

Cell swelling, blebbing, and death are dependent on ATP depletion and independent of calcium during chemical hypoxia in a glial cell line (ROC-1)

The morphological and biochemical changes that occur during chemical hypoxic injury in a neural cell line were studied in the presence and absence of calcium. Oligodendroglial-glioma hybrid cells (ROC-1) were subjected to inhibitors of glycolytic and oxidative ATP synthesis (chemical hypoxia). Complete respiratory inhibition depleted [ATP] to less than 5% of control by 4 min. Blebs appeared on the cell surfaces and cells began to swell within a few minutes of ATP depletion. A 200% increase in cell volume and bleb coalescence preceded irreversible cell injury (lactate dehydrogenase release) which began at approximately 20 min with 50% cell death by 40 min. In energized cells an equivalent degree of osmotic swelling induced by ouabain inhibition of the Na+, K(+)-ATPase pump did not produce blebbing or cell death. Partial inhibition of respiration decreased [ATP] to approximately 10% of control by 40 min. Blebbing and swelling began at 40 min and bleb coalescence preceded plasma membrane disruption which began at approximately 55 min. ATP depletion, blebbing, swelling, and death followed similar time courses in the presence or absence of extracellular calcium ([Ca2+]e). Intracellular calcium ([Ca2+]i) was measured using fura-2. In calcium-containing medium metabolic inhibition caused a transient increase in resting [Ca2+]i (100 +/- 17 nM) followed by a low steady-state level preceding plasma membrane disruption. Following deenergization in calcium-free medium, [Ca2+]i remained below 60 nM throughout injury and death. These data suggest that decreased ATP initiates a sequence of events including bleb formation and cell swelling that lead to irreversible cell injury in the absence of large increases in [Ca2+]i.

Calcium and pH in anoxic and toxic injury

The critical events that lead to the transition from reversible to irreversible injury remain unclear. Studies are reviewed that have suggested that a rise in cytosolic free Ca2+ initiates plasma membrane bleb formation and a sequence of events that leads ultimately to cell death. In recent studies, we have measured changes in cytosolic free Ca2+, mitochondrial membrane potential, cytosolic pH, and cell surface blebbing in relation to the onset of irreversible injury and cell death following anoxic and toxic injury to single hepatocytes utilizing multiparameter digitized video microscopy (MDVM). MDVM is an emerging new technology that permits single living cells to be labeled with multiple probes whose fluorescence is responsive to specific cellular parameters of interest. Fluorescence images specific for each probe are collected over time, and then digitized and stored. Image analysis and processing then permits quantitation of the spatial distribution of the various parameters within the single living cells. Our results indicate the following: (1) formation of plasma membrane blebs accompanies all types of injury in hepatocytes; (2) cell death is a rapid event, initiated by rupture of a plasma membrane bleb, and is coincident with the onset of irreversible injury; (3) an increase of cytosolic free Ca2+ is not the stimulus for bleb formation or the final common pathway leading to cell death; (4) a decrease of mitochondrial membrane potential precedes loss of cell viability; (5) cytosolic pH falls by more than 1 pH unit during chemical hypoxia. This acidosis protects against the onset of cell death.

Adenosine triphosphate depletion induces a rise in cytosolic free calcium in canine renal epithelial cells

An elevation in cytosolic free calcium (Cai) produced by cellular ATP depletion may contribute to the initiation of cytotoxic events in renal ischemia. To evaluate whether ATP depletion results in a rise in Cai we examined the effect of cyanide and 2-deoxy-D-glucose on the Cai of Madin-Darby canine kidney cells. Exposure to the metabolic inhibitors resulted in a rise in Cai from 112 +/- 11 to 649 +/- 99 nM in 15 min. This combination of metabolic inhibitors also resulted in a decrement of cell ATP to 11 +/- 2% of control by 15 min. Experiments that were performed with other metabolic inhibitors confirm that the increment in Cai is due to inhibition of ATP synthesis. With the removal of cyanide and 2-deoxy-D-glucose, Cai recovered to 101 +/- 16 nM. In the absence of extracellular calcium activity (Ca0), Cai declined from 127 +/- 7 to 38 +/- 6 nM, whereas with cyanide plus 2-deoxy-D-glucose in the absence of Ca0 the Cai rose from 108 +/- 21 to 151 +/- 28 nM. Because the rise in Cai produced by ATP depletion in the absence of Ca0 is significantly less than that which occurs in the presence of Ca0, influx of Ca0 is necessary for the maximal rise of Cai. The rise in Cai that occurred in the absence of Ca0 suggests that the release of calcium from intracellular stores contributes to the increment in Cai seen with ATP depletion. TMB-8, an inhibitor of calcium release from intracellular stores, blunted the rise in Cai by nearly 50%. Neither verapamil nor nifedipine inhibited the rise in Cai. This study demonstrates that ATP depletion induced by the metabolic inhibitors cyanide and 2-deoxy-D-glucose is associated with a rapid and reversible increase in Cai. Both Ca0 influx and Cai redistribution contribute to this rise.

Characterization of sugar transport in the pigeon red blood cell

Sugar transport in pigeon red blood cells is mediated by two pathways. One is saturable, shows competition between sugars, is inhibited by phloretin and cytochalasin B, and shows many of the properties of 'carrier-mediated' transport, characterized in the human red blood cell. The other is not saturable, and shows no competition between sugars. The saturable pathway is virtually absent from freshly drawn cells, but may be stimulated by pre-incubation with 2 mM-NaCN, or by Ca and the ionophore A 23187. The non-saturating pathway is stimulated only slightly by CN, but considerably by Ca and A 23187. The inhibition of sugar transport by cytochalasin B is antagonized competitively by sugars acting at the inner surface of the membrane. External sugars have no effect, as in the human red blood cell (Widdas, 1980). The binding of cytochalasin B to the cells shows a limited number of high-affinity sites. These are unrelated to inhibition of sugar transport as binding, but not transport, is prevented by the presence of cytochalasin E.