Calcium localization in the sympathetic ganglion of the bullfrog and effects of caffeine.

The localization of Ca2+ in the bullfrog sympathetic ganglion was studied using electron microscopy with Oschman and Wall's technique. When the ganglion was incubated and processed in an extremely high Ca2+ solution (20 mM) for electron microscopy, electron-dense deposits (EDD) were found at or in the plasma membranes, subsurface cisterns and mitochondria of the postganglionic neurons. These EDD were proved to contain calcium by X-ray microprobe analysis. On the other hand, they were not significant in the preganglionic terminals except those in the synaptic vesicles. Addition of caffeine (10 mM) to the incubation media and fixatives caused a drastic decrease in number of EDD of the subsynaptic membranes and the subsurface cisterns. Caffeine also reduced, but less markedly, the size and number of EDD in mitochondria. Caffeine (10 mM) prolonged the afterhyperpolarization of an action potential, reduced the amplitude of the ACh (nicotinic) potential and induced slow rhythmic hyperpolarizations in a 20 mM Ca2+ solution. These effects of caffeine which were presumably the result of an increase in the intracellular free Ca2+ were discussed in relation to the morphological data.