Release of noradrenaline by splenic nerve stimulation and its dependence on calcium

Abstract

1. Cat spleens were perfused with Krebs bicarbonate solution using a constant flow pump. The amount of noradrenaline released during splenic nerve stimulation was measured at various frequencies. The dependence of noradrenaline release on the ionic composition of perfusion medium was also determined.2. The effect of frequency of stimulation on the output of noradrenaline was studied in both normal and phenoxybenzamine treated cats. In normal cats, the output was 0.33 ng/stimulus at 10/sec, whereas it was 1.21 ng/stimulus at 30/sec. In phenoxybenzamine-treated cats, the maximum output of noradrenaline of 4 ng/stimulus was obtained at 5 or 10/sec. Higher or lower frequencies of stimulation produced lower output.3. In both normal and phenoxybenzamine treated cats, removal of calcium from the perfusing medium nearly abolished the release of noradrenaline in response to nerve stimulation. Replacement of calcium restored the noradrenaline release. The noradrenaline output/stimulus was linearly related to the log of the external calcium concentration.4. Increasing the concentration of magnesium to (10-20 mM) reduced the noradrenaline output. This depressant effect of magnesium was partially antagonized by increasing the calcium concentration of the perfusion solution.5. Divalent alkali metal earths such as barium and strontium were able to substitute for calcium. Barium substitution nearly doubled the noradrenaline output/stimulus and increased the pressor activity of the samples taken just before nerve stimulation.6. Removal of potassium from the perfusion fluid or lowering the sodium concentration to 50 mM had little effect on the release of noradrenaline. Lowering the sodium concentration to 37.5 mM or less usually abolished the noradrenaline output; this effect is attributed to blockade of nerve conduction.7. It is suggested that depolarization of post-ganglionic sympathetic nerve terminals may increase the influx of calcium ions which in turn leads to the release of noradrenaline from the nerve terminals.