Discrepancies in the extracellular space of sympathetic ganglia measured using different isotopes of mannitol and sucrose.

The extracellular space of rat superior cervical ganglia in vitro was measured using mannitol and sucrose labelled with tritium and carbon-14. The volumes of distribution of the 3H-labelled derivatives, especially [3H]mannitol, exceeded those of the 14C-derivatives. The divergence increased with increasing lengths of incubation. Thus, after 30 min incubation, 'spaces' (ml . g-1) were: [14Cu]mannitol, 0.407; [3H]mannitol 0.447; [14C]mannitol, 0.458; [3H]mannitol, 0.645; [14C]sucrose, 0.430; [3H]sucrose, 0.497. Using thin layer chromatography, it was shown that an average of 22% of the label in ganglia incubated for 120 min with [3H]mannitol, but only 4% with [14C]mannitol, was not associated with the parent compound. Both [3H]- and [14C]sucrose appeared to be metabolized by 11%. It is concluded that mannitol and sucrose can be metabolized in isolated ganglia and that this may lead to substantial errors in estimating the extracellular space, particularly when [3H]markers are used.