Effects of metabolic inhibitors on potassium- and acetylcholine-stimulated incorporation of phosphate into phospholipids of rat-brain cortex slices

Lipids of nervous tissue: composition and metabolism

As indicated in the Introduction, the many significant developments in the recent past in our knowledge of the lipids of the nervous system have been collated in this article. That there is a sustained interest in this field is evident from the rather long bibliography which is itself selective. Obviously, it is not possible to summarize a review in which the chemistry, distribution and metabolism of a great variety of lipids have been discussed. However, from the progress of research, some general conclusions may be drawn. The period of discovery of new lipids in the nervous system appears to be over. All the major lipid components have been discovered and a great deal is now known about their structure and metabolism. Analytical data on the lipid composition of the CNS are available for a number of species and such data on the major areas of the brain are also at hand but information on the various subregions is meagre. Such investigations may yet provide clues to the role of lipids in brain function. Compared to CNS, information on PNS is less adequate. Further research on PNS would be worthwhile as it is amenable for experimental manipulation and complex mechanisms such as myelination can be investigated in this tissue. There are reports correlating lipid constituents with the increased complexity in the organization of the nervous system during evolution. This line of investigation may prove useful. The basic aim of research on the lipids of the nervous tissue is to unravel their functional significance. Most of the hydrophobic moieties of the nervous tissue lipids are comprised of very long chain, highly unsaturated and in some cases hydroxylated residues, and recent studies have shown that each lipid class contains characteristic molecular species. Their contribution to the properties of neural membranes such as excitability remains to be elucidated. Similarly, a large proportion of the phospholipid molecules in the myelin membrane are ethanolamine plasmalogens and their importance in this membrane is not known. It is firmly established that phosphatidylinositol and possibly polyphosphoinositides are involved with events at the synapse during impulse propagation, but their precise role in molecular terms is not clear. Gangliosides, with their structural complexity and amphipathic nature, have been implicated in a number of biological events which include cellular recognition and acting as adjuncts at receptor sites. More recently, growth promoting and neuritogenic functions have been ascribed to gangliosides. These interesting properties of gangliosides wIll undoubtedly attract greater attention in the future.(ABSTRACT TRUNCATED AT 400 WORDS)

Turnover of inositol phospholipids and signal transduction

Various extracellular informational signals such as those from a group of hormones and some neurotransmitters appear to be passed from the cell surface into the cell interior by two routes, protein kinase C activation and Ca2+ mobilization. Both routes usually become available as the result of an interaction of a single ligand and a receptor and act synergistically to evoke subsequent cellular responses such as release reactions. The signal-dependent breakdown of inositol phospholipids, particularly phosphatidylinositol bisphosphate, now appears to be a key event for initiating these processes.

A phorbol ester and A23187 act synergistically to release acetylcholine from the guinea pig ileum

Electrical stimulation of guinea pig ileum preloaded with [3H]choline provokes the release of [3H]acetylcholine (ACh) in a Ca2+-dependent manner. This release was markedly increased by the tumor-promoting phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA). The combination of the ionophore A23187 and TPA produced the release of [3H]ACh up to a level equal to or exceeding a maximal response induced by electrical stimulation. A23187 alone gave only a minor response and TPA alone had no apparent effect on the [3H]ACh release. Thus, protein kinase C probably plays a role in cell surface signal transduction related to the release of transmitters from nerve endings.

Differences in Ca2+ mobilization induced by alpha-adrenergic agonist and phosphatidic acid in cultured hepatocytes

In an attempt to elucidate the relationship between phosphatidylinositol breakdown and alpha-adrenergic responses, effects of phosphatidic acid and phosphatidylinositol related metabolites on Ca2+ mobilization and glucose output in cultured hepatocytes were examined. Norepinephrine induced the net 45Ca2+ efflux from preloaded cells and stimulated glucose output via alpha-adrenergic receptor stimulation, whereas phosphatidic acid caused 45Ca2+ uptake to cells and did not stimulate glucose output. Myo-inositol-monophosphate, diglyceride and arachidonic acid, which are released by phosphatidylinositol breakdown, had no effect on 45Ca2+ efflux and glucose output in cells. These results suggest that phosphatidic acid and phosphatidylinositol related metabolites can not mimic the alpha-adrenergic actions in cultured hepatocytes.

Investigation of the relationship between cell-surface calcium-ion gating and phosphatidylinositol turnover by comparison of the effects of elevated extracellular potassium ion concentration on ileium smooth muscle and pancreas

Incubation of fragments of guinea-pig ileum smooth muscle in the presence of an elevated extracellular K+ concentration, which causes an increase in cell-surface Ca2+ permeability and thus leads to contraction, caused a marked increase in phosphatidylinositol turnover, as assessed by incorporation of 32Pi. This response was not diminished by atropine or propylbenzilycholine mustard, two muscarinic cholinergic antagonists, and was therefore not caused by the release of endogenous acetylcholine within the tissue. In contrast, exposure of guinea-pig pancreas fragments to high extracellular [K+], which does not increase cell-surface Ca2+ permeability or evoke secretion, did not cause an increase in phosphatidylinositol turnover, even though such an increase was triggered by carbamoylcholine, which is a secretagogue. These observations are consistent with a suggested function for phosphatidylinositol breakdown in the mechanisms of cell-surface Ca2+ gates.

STUDIES OF THE CATIONIC, AND ACETYLCHOLINE, STIMULATION OF PHOSPHATE INCORPORATION INTO PHOSPHOLIPIDS IN RAT BRAIN CORTEX IN VITRO

The addition of 0.1 M KCl to, or the omission of CaCl2from, incubation media in which rat brain cortex slices are respiring, stimulates the incorporation of inorganic P32into phospholipids. It also stimulates the labelling of 7-minute hydrolyzable nucleotide phosphates, but decreases their levels. The stimulation of P32incorporation into total phospholipid takes place primarily into phosphatidic acid and phosphoinositide but not into phosphatidyl choline and phosphatidyl ethanolamine. The addition of succinate and γ-aminobutyrate to brain cortex slices, metabolizing glucose, markedly inhibits the labelling of phospholipids although the respiration of slices is not diminished. The potassium stimulation of phospholipid labelling only occurs if sodium ions are present in the incubation medium. The lesser stimulation due to absence of calcium ions seems, however, to be independent of the presence of sodium ions. Acetylcholine stimulation of P32incorporation into phospholipids is dependent on the presence of sodium ions. Such stimulations are suppressed by concentrations of malonate, iodoacetate, fluoride, and ethanol that have little effect on the unstimulated incorporation of P32. Atropine and hyoscine inhibit acetylcholine stimulation but not cationic stimulation of P32incorporation. It is suggested that the effect of acetylcholine on P32incorporation is mediated by cationic changes at the nerve cell membrane.

AMINO ACID TRANSPORT IN BRAIN CORTEX SLICES: III. THE UTILIZATION OF ENERGY FOR TRANSPORT

When rat brain cortex slices are incubated with amino acids in the presence of various sugars, the ratio of amino acid transported to the steady-state level of ATP (termed the transport quotient) is constant. The actual amount of ATP maintained is greatest with glucose and mannose but somewhat less with fructose and galactose. Certain α-keto acids (pyruvate, oxalacetate, α-ketoglutarate, or glutamate which can give rise to α-ketoglutarate) also support glycine transport but with a lesser efficiency. Fumarate, malate, and succinate have little ability to support amino acid transport.The transport supported by pyruvate is particularly resistant to the action of agents which markedly reduce the transport supported by glucose as substrate. These agents include dinitrophenol, digoxin, and uranyl ions. Inhibitory effects are obtained with high concentrations of potassium ions whether the substrate is glucose or pyruvate.These results are interpreted in terms of pyruvate dismutation and acetate thiokinase leading to ATP production and in terms of intracellular localization of ATP.