Ryanodine suppresses the frequency-dependent component of the spike after-hyperpolarization in the rat superior cervical ganglion.

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The spike after-hyperpolarization (AH), which is linked to the Ca-activated K conductance system, in the rat superior cervical ganglion was examined by means of intracellular recording. The AH was shortened during repetitive stimulation of the cell at 1 Hz and reached a steady state within 10 sec. These changes in the AH disappeared after a part of the AH was depressed by 5μM ryanodine. The steady state of AHs was dependent on the frequency of stimulation ranging from 0.005 Hz and 2 Hz; half depression was observed at 0.53 Hz. Half recovery time from the depression induced by conditioning stimuli at 2 Hz was approximately 14 sec. Caffeine (1 mM) or TEA (3 mM) enlarged the AH at low stimulation frequencies. Caffeine slightly shifted the frequency depressing the AH toward high frequencies, and TEA did not cause significant changes. These results suggest that the repetitive firing of cells decreases the intracellular Ca release by contributing to the generation of the AH, rather than by accelerating the loading of Ca to its storage sites.
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