Choline fluxes in primary nerve cell cultures. Correlation with the endocellular metabolism of choline

The fluxes of choline across the plasma membrane were measured in primary nerve cell cultures from chick embryo cerebral hemispheres containing neurons and supporting cells. The incubation of cells with exogenous concentrations of choline far below the concentrations present in the growth medium (?30-50 ?M) and in the range of the high affinity uptake mechanism (about 0.5 ?M) profoundly affected the steady state of the endocellular free choline levels. The kinetics of the uptake were dependent upon the endocellular status of the choline pool since after preincubation in the absence of choline two K(m)s are observed (K(m1): 0.8 ?M; V(max)(1): 44.8 pmol/mg protein/2 min; K(m)(2): 14.3 ?M, V(max)(2): 333.3 pmol/mg protein/2 min) while only one mechanism can be found when the endocellular pool of choline was kept in steady state conditions (K(m): 14.3 ?M, V(max): 545.5 pmol/mg protein/2 min). The presence of an homoexchange phenomenon was suspected since choline efflux could be increased by increasing the concentrations of choline in the incubation medium. The results suggest that the movement of choline into nerve cells in culture appears to be mediated by a single mechanism which is regulated by the endocellular status of the choline pool.