Effect of K+ and Na+ on calcium-dependent electron-dense particles in the monoaminergic synaptic vesicles of rat pineal nerves fixed in Ca2+-containing solutions.

The effect of K+ and Na+ on the Ca2+ binding site in the dense core of monoaminergic vesicles of pineal nerves was investigated in the rat. Rat pineal glands, bisected immediately after decapitation, were incubated at room temperature in solutions containing high K+ or high Na+ in the presence or absence of Ca2+. Fixation was performed in glutaraldehyde-osmium tetroxide in collidine buffer, with and without CaCl2. It was confirmed that, after fixation in Ca2+-containing solutions, an electron-dense particle, located in the vesicle core, which can be considered a calcium deposit, appears within the synaptic vesicles. It was observed that this Ca2+ deposit may be modified by incubation in a high K+ or high Na+ milieu before fixation in Ca2+ containing solutions. When the incubation was carried out with high K+ and high Ca2+ simultaneously, Ca2+ deposits were considerably increased. With K+ alone, no Ca2+ deposits were apparent, as when electrical stimulation is applied before fixation. This effect was not observed when the incubation was done in high Na+. Consecutive incubations in high K+ and high Na+, respectively, restored the capability of the vesicle cores to bind Ca2+. Prolonged incubation in high Na+ before fixation increased Ca2+ deposits within the vesicles. These findings are in line with data on the effect of these ions upon the storage and release of biogenic amines and suggest that these ions modify the capability of synaptic vesicles to bind Ca2+.