The osmotically sensitive potassium and sodium compartments of synaptosomes.

1. Synaptosomes are pinched-off nerve terminals whose components can be liberated by osmotic ;shock'. A synaptosome preparation run through a Sephadex column that was eluted with an iso-osmotic solution retained its small ions, whereas when the column was eluted hypo-osmotically the small ions were lost. In this way the osmotically sensitive Na(+) and K(+) of synaptosomes were measured. Measurements of the lactate dehydrogenase occluded within the synaptosome were also made. The release of osmotically sensitive Na(+) and K(+) and occluded lactate dehydrogenase had similar characteristics with respect to the degree of osmotic ;shock' necessary and the action of lytic agents. 2. The distribution of osmotically sensitive Na(+), K(+) and occluded lactate dehydrogenase in the subfractions of a crude mitochondrial preparation was examined. The synaptosome fraction was the richest source of these constituents. 3. On standing at 5 degrees in iso-osmotic solution Na(+) and K(+) were lost from synaptosomes, whereas the amount of occluded lactate dehydrogenase remained stable, suggesting that the synaptosome membrane retained its integrity but that Na(+) and K(+) diffused through it out of the osmotically sensitive compartment. 4. The uptake of Na(+) and K(+) into the osmotically sensitive compartment was examined. At 5 degrees the rates of uptake of Na(+) and K(+) were found to be equal to the rates of loss of these ions when correction to a uniform concentration gradient had been made. K(+) travelled across the membrane slightly faster than Na(+), the rate of K(+) movement being about 1.0mumuequiv.cm.(-2)sec.(-1) under a concentration gradient of 0.1m. Active transport is not thought to contribute to the ion movements under the conditions used. 5. The amount of K(+) taken up into the osmotically sensitive compartment as a function of the external concentration was examined. Since the uncharged molecule d-[(14)C]galactose distributes across the synaptosome membrane similarly to K(+) there is not thought to be a synaptosomal trans-membrane potential. The volume of the osmotically sensitive compartment was measured by this method and found to agree with estimates of the synaptosomal volume made from morphological studies. In media of low ionic strength synaptosomes exhibit a Donnan effect. 6. It is concluded from these studies that the osmotically sensitive compartment represents the inner volume of the synaptosome, which is completely separated from the outside environment by a diffusion barrier having many of the general properties of a biological membrane.