Uptake of neurotransmitters and precursors by clonal lines of astrocytoma and neuroblastoma: III. Transport of choline

The cation receptor subsite of the choline transporter in preimplantation mouse conceptuses resembles a cation receptor subsite of several amino acid transporters

Mediated choline transport in preimplantation mouse conceptuses was inhibited competitively by Na+ and other cationic osmolites. Uptake of choline by conceptuses was also inhibited relatively strongly by ethanolamine, hemicholinium-3, harmaline, harmalol and harmine. The Ki values for inhibition of choline transport by most of the latter inhibitors were of the same order of magnitude as the Km value for choline transport (approximately 100 microM). To our knowledge, we are the first to show that mediated 'Na(+)-independent' choline transport is, nevertheless, inhibited strongly by the Na(+)-site inhibitor, harmaline. Inhibitions by harmaline, Na+ and other cations have been used to draw a parallel between the substrate receptor sites of amino acid transport systems y+ and bo.+. We suggest that the latter parallel should be extended to include the Na(+)-independent mammalian choline transporter. In addition, the choline transport activity in conceptuses increased by more than 100-fold between the 2-cell and blastocyst stages of development. Mouse blastocysts probably utilize choline for the synthesis of membrane phospholipids during cellular differentiation and when they begin to grow about ten hours prior to implantation. Since we show here that mouse conceptuses develop the capacity to transport choline prior to the onset of growth, some of the choline utilized for growth could come from an exogenous source.

Comparative studies on the isoenzymes of glutathione S-transferase of rat brain and other tissues

Six forms of glutathione S-transferase (GST) designated as GST 9.3, GST 7.5, GST 6.6, GST 6.1, GST 5.7 and GST 4.9 have been purified to homogeneity from rat brain. All GST isoenzymes of rat brain are apparent homodimers of one of the three type subunits, Ya, Yb, or Yc. More than 60% of total GST activity of rat brain GST activity is associated with the isoenzymes containing only the Yb type of subunits. In these respects brain GST isoenzymes differ from those of lung and liver. The Ya, Yb, and Yc type subunits of brain GST are immunologically similar to the corresponding subunits of liver and lung GST. The isoelectric points and kinetic properties of the Yb type subunit dimers in brain are strikingly different from those of the Yb type dimers present among liver GST isoenzymes indicating subtle differences between these subunits of brain and liver.

Acetylcholine synthesis by adult bovine adrenal chromaffin cell cultures

Adrenal chromaffin cells normally synthesize and release catecholamines. In the present study, [3H]acetylcholine synthesis and another characteristic of cholinergic neurons, [3H]choline uptake, were studied in cultures of adult bovine adrenal chromaffin cells. Chromaffin cell cultures took up [3H]choline from the medium and acetylated the [3H]choline to form [3H]acetylcholine. The rate of [3H]acetylcholine synthesis increased after 19 days in culture and continued to increase up to 28 days in culture. [3H]Acetylcholine synthesis could be increased by stimulating the cells with a depolarizing concentration of K+. The ability for K+ to stimulate synthesis of [3H]acetylcholine developed only after 28 days in culture. [3H]Choline was taken up by the cultures through a single mechanism with a high (to intermediate) affinity for choline. [3H]Choline uptake was enhanced by Na+ omission in day-14 cultures, but was at least partially Na+-dependent in day-29 cultures. Hemicholinium-3 (IC50 less than 10 muM) inhibited [3H]choline uptake into chromaffin cell cultures. It is concluded that bovine adrenal chromaffin cells, maintained in culture, are able to exhibit cholinergic properties and this capacity is retained even by the mature adult cell.

High-affinity [3H]choline accumulation in cultured human skin fibroblasts

[3H]Choline can be transported across cell membranes by high-affinity (KT less than 5 microM) and low-affinity (KT much greater than 5 microM) systems. High-affinity choline accumulation (HACA) has been demonstrated in synaptosomes made from cholinergic brain regions such as the hippocampus and caudate-putamen. In cell culture, HACA has been demonstrated in glia and avian telencephalon, dissociated spinal cord, and muscle fibroblasts. We examined [3H]choline accumulation in a single normal human fibroblast line cultured from skin biopsy. [3H]Choline accumulation was temperature-dependent and linear with incubation time up to 6 min at 0.125 microM-choline. The apparent KT for [3H]choline was 5 microM, which is similar to that observed in avian fibroblasts. Isoosmotic replacement of Na+ with either Li+ (144 mM) or sucrose (288 mM) severely reduced [3H]choline accumulation (by 70-90%). Pre-incubation with ouabain (100 microM), sodium orthovanadate (100 microM), or 2,4-dinitrophenol (100 microM), or replacement of Ca2+ by Mg2+ had little or no effect on subsequent [3H]choline accumulation. [3H]Choline accumulation was inhibited by hemicholinium-3 (HC-3); after pre-incubation in HC-3 at 37 degrees C for 10 min, the IC50 (at 0.125 microM-choline) was 5.6 microM. The HC-3 sensitivity, Na+ dependence, and low KT suggest that human skin fibroblasts have a high-affinity transport system for choline.

The K+ liquid ion exchange electrode system: responses to drugs and neurotransmitters

K+-sensitive liquid ion exchange electrode systems respond with a slow potential change to acetylcholine, choline, anticholinergic drugs, biogenic amines, and glutamic acid. The response threshold has been defined, and in most cases it is at extremely low concentrations (10(-7)-10(-5)M). The direction of the potential change varies, but in most instances it is additive to that produced by external K+. The K+ electrode system is further sensitive to decreasing pH, within a narrow and possibly physiological range of pH. These findings suggest that small measured changes in extracellular K+ are biased by the chemosensitivity of the liquid ion exchange electrode system to some compounds of physiological importance.