"On" and "off" responses of K+-induced synaptosomal proline release: involvement of the sodium pump

The K+-induced release of the putative neurotransmitter proline in rat cerebral cortex synaptosomes was studied by measuring [5-3H]proline released upon exposure to high-K+ media. In contrast to previous experiments, K+ was not substituted for Na+, which was held constant. Under the resulting conditions of increased osmolarity, two peaks of proline release were obtained, one upon exposure to increased [K+], another upon restoring normal [K+]. The dependence of the latter peak on ionic composition of the medium, and its sensitivity to ouabain are discussed.