Evidence that calcium acts as an intracellular messenger for adrenergic responses in human erythrocytes

Control of malarial invasion by phosphorylation of the host cell membrane cytoskeleton

It has been shown that the entry of the malaria parasite into the red blood cell requires the presence of ATP in the host cell cytoplasm. In red blood cell ghosts that contain no ATP the receptor on the extracellular surface remains in place and parasites will bind to the membrane, but will not enter. ATP is thus necessary for one of the steps in the invasion sequence that follows recognition and attachment. The process of entry appears to involve the active participation of the host cell membrane cytoskeleton. We have suggested that the function of the intracellular ATP may be to regulate phosphorylation of the cytoskeleton. We now present evidence that the activity of the membrane-associated cyclic AMP-independent kinase of the red blood cell is inseparable from invasion; the active substrate may be spectrin.

Cholinergic agonists stimulate calcium uptake and cGMP formation in human erythrocytes

Human erythrocytes possess a muscarinic cholinergic receptor sensitive to cholinergic agonists which stimulate transient increases in calcium uptake and subsequent cyclic GMP formation. These phenomena can be blocked by atropine and EGTA. The cholinergic stimulation of cyclic GMP formation depends on Ca2+ uptake from external media. The effects of cholinergic agonists on the erythrocyte resemble their effect on calcium channels in nervous tissue. The cholinergic stimulation of Ca2+ uptake in erythrocytes may affect the calcium-sensitive mechanism involved in the shape, permeability and rigidity of these cells.

Calcium and ionophore A23187 induce the sickle cell membrane phosphorylation pattern in normal erythrocytes

Pre-treatment of normal erythrocytes with micromolar Ca2+ and ionophore A23187 induces abnormal phosphorylation of membrane polypeptides, as determined by labeling with exogenous 32Pi. The Ca2+ -induced effects, which include increased incorporation of 32P into acid-stable linkages and increased labeling in the Band 3 and 4.5-4.9 regions of SDS gels, are similar to those seen in untreated sickle erythrocytes. Part of the abnormal phosphorylation of sickle cells may be caused by their elevated intracellular Ca2+ levels.

Effects of divalent metal ions on the calcium pump and membrane phosphorylation in human red cells

In inside-out red cell membrane vesicles ATP-dependent calcium transport is activated by the divalent metal ions Mg2+, Mn2+, Co2+, Ni2+ and Fe2+. This activation is based on the formation of Me2+ -ATP complexes which can serve as energy-donor substrates for the calcium pump, and probably, satisfy the requirement for free Me2+ in this transport process. Higher Me2+ concentrations inhibit calcium transport with various efficiencies. Mn2+ directly competes with Ca2+ at the transport site, while other divalent metal ions investigated have no such effect. The formation of the hydroxylamine-sensitive phosphorylated intermediate (EP) of the red cell membrane calcium pump from [gamma-32P]ATP is induced by Ca2+ while rapid dephosphorylation requires the presence of Mg2+. At higher concentrations Mn2+ and Ni2+ inhibit predominantly the formation of EP, while Co2+ and Fe2+ block dephosphorylation. The possible sites and nature of the divalent metal interactions with the red cell calcium pump are discussed. Hydroxylamine-insensitive membrane phosphorylation in inside-out vesicles from [gamma-32P]ATP is significantly stimulated by Mn2+ and Co2+, as compared to that produced by Mg2+, Fe2+ and Ni2+. Part of this labelling is found in phospholipids, especially in phosphatidylinositol. The results presented for the metal dependency of protein and lipid phosphorylation in red cell membranes may help in the characterization of ATP consumptions directly related to the calcium pump and those involved in various regulatory processes.

Erythrocytic calmodulin correlates with red cell age

Red blood cells from normal persons and from patients with reticulocytosis were tested for their calmodulin content. Elevated calmodulin correlated with high reticulocyte counts, rather than with any specific disorder. The correlation between reticulocyte content and elevated calmodulin was further tested by fractionating the erythrocytes from 12 patients into reticulocyte-rich and reticulocyte-poor fractions. In the samples from each patient, erythrocytic calmodulin increased with increasing reticulocyte count.

Calcium does not mediate the shape change that follows ATP depletion in human erythrocytes

Crenation, the shape change that follows ATP depletion in human erythrocytes, also follows ionphore-mediated Ca2+-loading. Experiments designed to test whether Ca2+ mediates metabolic crenation showed that: (1) an influx of extracellular Ca2+ is not required for metabolic crenation; (2) metabolic crenation is accompanied by a 70% increase in 86Rb+ permeability, a change much smaller than the increase expected if crenating concentrations of Ca2+ were released from bound intracellular pools; (3) A23187 plus EGTA, a treatment that depletes intracellular Ca2+ and stops Ca2+ crenation, does not affect metabolic crenation; (4) calmodulin inhibitors do not slow metabolic crenation. We conclude that Ca2+ does not mediate metabolic crenation. Albumin washes reverse Ca2+ crenation and metabolic crenation involve the accumulation of some amphiphilic species (e.g., lysolipid or diacylglycerol) in the cell membrane outer monolayer, and that ATP depletion induces a second crenating process which might be a reorganization of the cytoskeleton.

Calmodulin-dependent spectrin kinase activity in resealed human erythrocyte ghosts

Membrane protein phosphorylation has been studied in resealed human erythrocyte ghosts by measuring the incorporation of 32P into spectrin and band 3. Norepinephrine- and Ca2+-stimulated phosphate incorporation was diminished in ghosts depleted of calmodulin. Ghosts prepared with endogenous calmodulin showed Ca2+- and norepinephrine-stimulated protein phosphorylation only when the ghosts had been resealed in the presence of (gamma-32P)ATP. Ghosts resealed with or without calmodulin in the presence of unlabeled ATP showed no net gain or loss of 32P when exposed to norepinephrine or a Ca2+-specific ionophore. These observations suggest that Ca2+ and norepinephrine stimulation of membrane protein phosphorylation is mediated by calmodulin-dependent spectrin kinase activity, and not by increased turnover of spectrin ATPase or by inhibition of phosphospectrin phosphatase.