Release of noradrenaline from the cat spleen by sodium deprivation

Abstract

1 The endogenous noradrenaline content of cat spleen slices was markedly reduced when the slices were incubated at 37° C in a medium in which sodium was replaced by sucrose, lithium, choline or potassium. Depletion of tissue noradrenaline was accounted for by its release into the incubating medium. At an external sodium concentration of 20 mm, about 50% depletion was obtained in 2 hours. 2 The enhanced release induced by sodium deprivation occurred in the absence of calcium, with or without ethyleneglycol-bis (β-aminoethyl ether) N,N' tetraacetic acid. Manganese potentiated release, while magnesium was without effect. 3 Ouabain caused a dose-dependent release of noradrenaline which was partially calcium-dependent. Removal of potassium from the incubation medium caused some release, which was potentiated in 25 mm sodium Krebs solution or by ouabain. 4 At 4° C, the release did not occur in sodium-free medium. 5 Dinitrophenol did not affect the loss of noradrenaline caused by sodium withdrawal. Iodoacetic acid and N-ethylmaleimide caused a time-dependent depletion of noradrenaline. Tetracaine caused release and partly opposed the release caused by sodium deprivation. Tetrodotoxin had no effect. Guanethidine, but not phenoxybenzamine, released noradrenaline and potentiated the release induced by sodium withdrawal. 6 The rate of release of ³H-noradrenaline from reserpine-treated spleen slices was not altered by sodium withdrawal. 7 Uptake-retention of ³H-noradrenaline in slices depleted of their endogenous noradrenaline content by sodium deprivation was about 60% of the control slices. This was effectively blocked by cocaine. Release of ³H-noradrenaline evoked by high potassium from both control and treated slices was calcium-dependent. 8 It is suggested that sodium-potassium-activated ATPase maintains the integrity of the axonal membrane, and any procedure which depresses the activity of the enzyme or the sodium-potassium pump would cause transmitter release by causing temporary disturbance in the membrane. Evidence is presented to suggest that vesicles depleted of their endogenous noradrenaline content by sodium deprivation are re-used for the storage and release of transmitter.