Uptake of adenosine and release of adenine derivatives in mammalian non‐myelinated nerve fibres at rest and during activity

Abstract

SUMMARY1. Influx of adenosine into rabbit non-myelinated nerve fibres was measured using [2-3H]adenosine. The uptake of radioactivity increased linearly with duration of incubation for up to 60 min and adenosine concentration up to 200 UM. The uptake at different adenosine concentrations showed a saturable component with a halfmaximal activation at 17-1 /M and a linear part.2. The radioactivity taken up was rapidly incorporated into AMP, ADP and ATP. Isotopic equilibrium between the nucleotides was achieved within 15 min.3. The uptake of 3H from 0-2 gM-adenosine was almost completely inhibited by addition of 200 gM-adenosine and to a similar extent by 200 /LM-tubercidin and AMP; a 70 % inhibition was found with ATP and ADP; a, , methylene-ADP had no effect.4. ATP, ADP and AMP added to the extracellular medium of a desheathed vagus were slowly hydrolysed.5. In preparations loaded with [2-3H]adenosine and then washed with adenosine and label-free solution there was a steady efflux of radioactivity amounting to 0 18 x 10-3/min. Addition of adenosine or tubercidin transiently increased the efflux. 6. Electrical stimulation caused an extra release of radioactivity. The extra fractional loss was 21-8 x 10-6/impulse in preparations that had rested for several hours; it decreased to 2-3 x 10-6/impulse when stimulation was applied after a 30 min rest.7. The radioactivity of the resting efflux and of the extra efflux after stimulation was found mostly in inosine and hypoxanthine; adenosine and adenine accounted for only 3 %, and the nucleotides for less than 1 % of the efflux.8. Adenosine added to the external medium of a desheathed nerve was slowly deaminated. 9. It is concluded that inosine and hypoxanthine found in the effluent from desheathed vagus nerve trunk result from release of these compounds from nerve fibres and not from extracellular breakdown of released ATP or adenosine.10. Electrical activity in non-myelinated nerve fibres of the nerve trunk thus causes the release of metabolites (inosine and hypoxanthine) together with small amounts of adenosine and adenine, while release of ATP and other nucleotides is almost completely absent.

Adenosine metabolism in neurons and astrocytes in primary cultures

“…Since synaptosomal preparations are contaminated by glial cells (see "Introduction"), a considerable part of the metabolism might, again, occur in astrocytic constituents. Desheathed nerve fibers (Maire et al, 1982) and sympathetic ganglion neurons (Wolinsky and Patterson, 1985;McCaman and McAfee, 1986) have, however, also been found to convert adenosine mainly to nucleotides.…”

Enhancement of tetrodotoxin‐induced axonal blockade by adenosine, adenosine analogues, dibutyryl cyclic AMP and methylxanthines in the frog sciatic nerve

“…Since electrical activity in nerve fibres causes the release of adenine, inosine and hypoxanthine (Maire et al, 1982), the effects of these substances on the axonal blockade induced by TTX (30-40 nM) were investigated in three experiments. Neither inosine (5 mM) nor hypoxanthine (5 mM) changed the inhibition induced by ITX on nerve conduction.…”

“…Adenosine is released from nerve fibres upon electrical stimulation (Maire et al, 1982). When applied exogenously, this substance does not affect either axonal (Okamoto et al, 1964;Ribeiro & Dominguez, 1978) or nerve terminal (Silinsky, 1984) action potentials.…”

Inhibitory effects of forskolin and papaverine on nerve conduction partially blocked by tetrodotoxin in the frog sciatic nerve

“…The enhancement of TTX-induced axonal block caused by papaverine may be attributed to its ability to inhibit phosphodiesterases (Kukovetz & Poch, 1970) and thus to increase cyclic AMP levels. However, papaverine is also an inhibitor of the uptake of adenosine (Huang & Daly, 1974), which is released from nerve fibres on electrical stimulation (Maire et al, 1982) and enhances TTX-induced axonal block (Ribeiro & Sebastiao, 1984). Thus one cannot preclude the possibility that a papaverine-induced inhibition of adenosine uptake by axonal membranes contributes to the inhibitory action of papaverine on nerve conduction observed in these experiments.…”