Active Ca2+ transport by membrane vesicles from pigeon erythrocytes. Stimulation by amino acids, ATP, GTP, Pi and some other cell constituents

Comparison of human and pigeon erythrocyte membrane proteins by one- and two-dimensional gel electrophoresis

Pigeon and human bands 1 and 2 (spectrin) and 5 (actin) are conserved. Band 3 anion porters have similar SDS positions, but the pigeon porter has a higher isoelectric point. Both anion porters are inhibited by similar doses of pyridoxal phosphate. Many differences are apparent in minor bands.

Lysolecithin-induced Ca2+ uptake by pigeon red cells

Lysolecithin (LPC) induced a rapid but transitory increase in 45Ca2+ influx, and an abrupt 45Ca2+ net uptake by pigeon red cells. The effect on uptake was seen with doses of added lysolecithin calculated to be 2.1-8.4% of the cytoplasmic membrane phospholipid, well within the reported physiological range of 1-9%. Lysolecithin effects were not due to prelytic membrane changes nor action as a platelet activating factor agonist. Changes in the amino acids in the medium mimicking human homocysteinuria, an atherosclerosis risk factor, increased the lysolecithin effect. The findings are consistent with a role for lysolecithin in atherosclerosis.

Intracellular calcium distribution in pigeon erythrocytes

Subcellular compartmentation of calcium has been studied in digitonin-treated pigeon erythrocytes. The following calcium pools could be detected: A non-vesicular and tightly bound pool of calcium able to reach values equivalent to 5 mumol calcium/ml cells that required millimolar calcium concentrations. A vesicular calcium pool with high calcium affinity that had properties similar to mitochondrial calcium transport. Extracellularly added ATP was strongly hydrolyzed by intact pigeon erythrocytes in the presence of either magnesium or calcium. The hydrolysis of ATP was not coupled to fluxes of either 45Ca2+ or 86Rb+ and most probably took place inside the cells.

Transport and control of Ca2+ by pigeon erythrocytes. I. Survey of some cell responses to a range of A23187 doses in the presence of Ca2+

Cells were subjected to a range of 45Ca2+ influx loads with A23187. We measured cell 45Ca2+ with time and A23187 dose, and the apparent 45Ca2+ influxes (identical to "J(in,app)") at Ca2+ steady state. We also measured endogeneous exchangeable and total cell Ca2+, which were 50 and 17-220 microM respectively. The effects of A23187 and Ca2+ on cell ATP, swelling, net Cl- permeability, and cell morphology were measured. These were modest and do not affect our conclusions. J(in,app) congruent to 3 X 10(-4) [A23187]2.9 X [Ca2+(o)]mumoles/l X min with 92-552 microM [Ca2+(o)] (identical to external Ca2+ concentration) and 0-7 microM A23187. J(in,app) was increased an order of magnitude by vanadate and is probably much less than the true influx. The least unlikely explanation found for the high [A23187] exponent, 2.9, was that most of the Ca2+ crossing the membrane is expelled by the pump before it can move deeper into the cell. Calcium pumping increased rapidly in response to increased influx, but the steady state cell 45Ca2+ was approximately proportional to J(in,app) rather than approximately constant between 10 and 120 mumoles/l X min with 184 microM [Ca2+(o)]. This is not the result expected from a simple feedback mechanism. At high A23187 doses the pump appears fully activated resulting in a linear relation between cell/medium 45Ca2+ and [A23187]-2. From the plot we calculated alpha identical to free/total exchangeable Ca2+ = 0.38 +/- 0.08 (n = 3) and a maximum pump rate, "Pmax" = 78 mumole/l X min. Pmax is underestimated insofar as J(in,app) is less than the true influx.

Organic ion transport during seven decades. The amino acids

The amino acids are ions of various charge combinations, and one can argue that historically they were the first ions for which the ongoing problem of membrane transport was presented; also that among transported ions these may undergo a highly detailed molecular recognition. Furthermore, the distribution of charge on the amino acid molecule determines by what route or routes it is conducted across the biological membrane, with what directional and structural specificity, and therefore what regulation is imposed, and where. Cases where a presumably charged chemical group behaves as if it were somehow absent from the amino acid have been observed to fall into several categories: Straightforward cases where the pH has been low enough or high enough to remove the charge by protonation or deprotonation, even in free solution. Cases where that protonation or deprotonation is facilitated at the binding site, and perhaps by the total transport process. The cystine molecule can apparently thus be rendered either a tripolar anion or a tripolar cation for transport. Cases where an otherwise co-transported Na+ is omitted to redress charge, or where a Na+ serves as a surrogate for a missing charged group on the amino acid molecule. A case where the protonation occurs reversibly at the receptor site rather than on the amino acid molecule.

Recognition chemistry of anionic amino acids for hepatocyte transport and for neurotransmittory action compared

Comparison of neuronal and non-neuronal membrane transport of, and neuroexcitation by, the dicarboxylic amino acids bring out provocative similarities in structural selectivity, and hence in the strategies for studying them. Parallel anomalies in stereoselectivity show for both phenomena that the recognition sites are indeed chiral, as expected for biological functions, even though both fail in special instances to discriminate between DL pairs. High and low affinity, or Na+-dependence or Na+-independence, are not fully reliable bases for discriminating receptor sites serving one of these functions. Tolerance of N-methylation of the amino acid serves in discriminating families of recognition sites for both phenomena, as does substitution of the sulfonate or sulfinate for the distal carboxylate group, or other structural changes. Analogs in which the functional groups of aspartate or glutamate are presented in restrained arrays serve for both, although they have so far suggested identity neither of recognition sites for transport and excitation, nor of the events consequent to binding for these two phenomena.

Rapid stopping of A23187 action by phosphatidylcholine

The action on pigeon erythrocytes of the Ca2+ ionophore, A23187, can be nearly completely stopped within 30 s by treatment with a phosphatidylcholine dispersion at 39 degrees C. A time-limited 45Ca2+ uptake pulse can be produced by treating cells sequentially with A23187 and lipid, and the time-course of expulsion of this 45Ca2+ uptake pulse can be easily determined.