Effect of calcium on the relationship between frequency of stimulation and release of noradrenaline from the perfused spleen of the cat.

The sympathetic nerves of the cat spleen were stimulated electrically (20 v, 1 msec, 7 sec) with increasing frequencies (1-30 Hz) during perfusion of the spleen with normal or low-calcium Krebs solution. Noradrenaline (NA) output per stimulus was measured. In spleens perfused with normal Krebs solution, the NA output per stimulus at 5Hz was barely detectable, while the output at 30 Hz was about 5-fold greater than at 5 Hz. Reduction of the calcium concentration in the Krebs solution to 0.2-0.5 mM, while reducing outputs at both frequencies, resulted in an output at 30 Hz that was nearly 8 to 10 times that at 5 Hz. In spleens pretreated with phenoxybenzamine (PBZ) the NA output was increased at all frequencies, and the maximal output occurred at 5 Hz, whereas the output at 30 Hz was only one-third of the maximal output. Reduction of the calcium concentration of the perfusion solution to 0.5 mM, while reducing outputs from PBZ-treated spleens, increased the output at 30 Hz by about 5-fold over the output at 5 Hz. The muscarinic inhibition of stimulation-evoked release of NA by acetylcholine was much more pronounced at low than at high frequency of stimulation. The increase in the release of NA per stimulus with increase in the frequency of sympathetic nerve stimulation is explained on the assumption that the specific entry of calcium ions required for transmitter release is also frequency-dependent.