Can the human blood platelet be used as a model for the cholinergic nerve ending?

Diurnal oscillation of choline acetyltransferase activity in human blood

The present study reports on the diurnal variation of choline acetyltransferase (ChAT; EC 2.3.1.6.) activity observed in human blood. The product of the enzymatic reaction, acetylcholine, was identified by thin-layer chromatography. The oscillation, first found in the blood of a male volunteer, was confirmed in the blood of a man and a woman volunteers during several days. The maximum enzyme activity appeared to be localized in the afternoon hours and the minimum in the morning hours. Similar results were found on several volunteers of both sexes who showed a statistically significant difference in ChAT activity between 11:30 and 18:30 h. No statistically significant sexual differences was observed. The results are related to the diurnal oscillation of the cholinergic system in rat brain and would indicate a cellular localization of the rhythm.

Human red blood cell choline uptake with age and Alzheimer's disease

Since previous studies suggested that blood choline homeostasis is altered in aging and in Alzheimer's disease, choline uptake was examined in human red blood cells (RBC) from young adults, intellectually-intact elderly controls and outpatients with Alzheimer's disease. Eadie-Hofstee analysis of uptake by RBC from young controls indicated two components; thus, group comparisons were done with 1 and 50 microM choline in the media. Temperature-dependent choline uptake at low and high choline concentrations increased in RBC from elderly controls (62-66%) or Alzheimer patients (52-54%) compared to young controls. These changes in transport were not directly related to altered RBC choline content, since RBC choline concentrations did not vary significantly between groups. However, plasma choline content was significantly elevated in elderly controls and Alzheimer patients compared to young control values. The RBC to plasma ratio of choline was reduced in elderly compared to young controls, whereas the ratio in Alzheimer patients was between the two other groups. Thus, abnormalities in RBC choline uptake and plasma choline content were not exacerbated in Alzheimer patients, and these results do not support suggestions that Alzheimer's disease is a form of generalized accelerated aging. The striking changes in RBC choline uptake and plasma choline content in elderly subjects do indicate age-related changes in systemic choline homeostasis and these abnormalities may contribute to the predisposition of the elderly to neurological diseases.

Uptake and incorporation of choline by Schistosoma mansoni adults

Choline uptake and incorporation into Schistosoma mansoni is used as a model for investigating transport across and formation of a double bilayer surface of a syncytial transporting epithelium. Choline uptake reached a maximal rate during the first 2 min (Vmax = 0.27 mumol mg-1 protein min-1; Km = 36 microM). Choline uptake during a 30 min incubation was similar to that of single bilayer transport systems described in the literature. Choline incorporation into phosphatidylcholine was saturated above 40 microM external choline concentration (Vmax = 3.7 pmol mg-1 protein min-1; Km = 7 microM). The low rate of choline efflux and the half life of the tissue choline pool (T 1/2 = 3 h), suggests that free choline pools available for efflux in S. mansoni are small. This model allows the determination of whether a proposed effector of membrane phosphatidylcholine synthesis and turnover alters surface bilayer formation through changes in transport of the precursor across the apical surface.

Cholinesterase activity and choline uptake in intact nerve cell cultures

Choline uptake and ecto-cholinesterase activities have been measured in intact astroblast and neuroblast cultures. The data show that choline uptake is dependent upon the ionic composition of the culture medium and is sensitive to metabolic inhibitors. However, the high concentrations of the inhibitors necessary for the inhibition of the uptake and some thermodynamic properties qould suggest a facilitated transport rather than an active uphill process. Preincubation of the cultures with various inhibitors of cholinesterases shows no direct parallelism between inhibition of choline high affinity uptake (apparent Km approximately equal to 10-6 M) and inhibition of ecto-acetylcholinesterase (EC 3.1.1.7).

Incorporation of choline, serine, ethanolamine and inositol into phospholipids of isolated rat mast cells

The incorporation of labelled phospholipid precursors, [Me-14C]-choline, L-[3(-14)C]-serine, [2(-14)C]-ethanolamine and [2(-3)H]-myoinositol into the phospholipids of isolated rat mast cells was studied. The label from the different precursors were found to be essentially associated with compounds with the t.1.c.-motility of the respective phospholipids. Whereas the incorporation of [Me(-14)c]-choline and L-[3(-14)C]-serine showed evidence of saturation the incorporation of [2(-14)C]-ethanolamine was linear with time (2 h) and it was not saturated by increasing the concentration from 0.07 mM to 2.07 mM. The incorporation of [2(-3)H]-myoinositol was stimulated by Ca2+ (1 mM) or Mg2+ (1 mM), while the incorporation of the other precursors was stimulated only in the presence of both Ca2+ and Mg2+ (1 mM). Antimycin A (1muM), an inhibitor of the respiratory chain, significantly ingibited the incorporation of [Me(-14)c]-choline, L-[3(-14)C]-serine and [2(-3)H]-myoinositol but not that of [2(-14)C]-ethanolamine. The experimental system used might be a useful model for studies on the turnover of membrane phospholipids during histamine release.