Abstract

Interaction between 4-aminophrydine (4-AP) and nicotine on sympathetic nerve terminals was studied in the isolated cat spleen slices, labeled with [3H]norepinephrine ([3H]NE). Incubation of slices for 5 min at 37 degrees C in low (50 microM) and high (2 mM) concentrations of nicotine released 0.8 +/- 0.08 and 2.73 +/- 0.39% of tissue [3H]NE. Tetrodotoxin (TTX) blocked the response to low nicotine but not to high nicotine. Low nicotine did not release [3H]NE in the absence of calcium. Response to high nicotine which persisted in calcium-free solution was blocked by ethylene glycol bis(beta-aminoethyl ether)N,N'-tetra-acetic acid. 4-AP (1 mM) not only enhanced the response to low nicotine but it effectively antagonized the suppressant effects of TTX and calcium-free solution on release induced by low nitotine. Restoration of release by 4-AP from TTX-blocked preparations occurred in the absence of calcium in the perfusion medium, but lanthanum (1 mM) blocked it. Restoration of release from spleen slices incubated in calcium-free Krebs' solution by 4-AP was blocked by lanthanum and prolonged incubation in calcium-free ethylene glycol bis(beta-aminoethyl ether)N,N'-tetraaectic acid solution. It is concluded that at lower doses nicotine, by acting on nicotinic receptors, depolarizes the sympathetic nerve terminals to set off propagated action potentials which are responsible for NE release, and that 4-AP restores nicotine response in the presence of TTX or in the absence of calcium by mobilizing calcium both from extracellular and intracellular sources.