Quantitative electron microscopic autoradiography on the mouse thyroid gland after administration of 3 H-L-DOPA.

Abstract

The cellular and subcellular distribution of radioactivity in the mouse thyroid gland different times (20 min — 8 hours) after intravenous administration of 3H-L-DOPA was studied by means of quantitative electron microscopic autoradiography. High concentrations of autoradiographic silver grains occur over parafollicular cells and adrenergic nerves while the labelling of follicular cells and lumina is low or absent and similar to the labelling of connective tissue cells at all observation times. Over the parafollicular cells high levels of radioactivity can be recorded already 20 min after administration of the labelled amino acid. The grain counts are highest at 1 hour and decrease then at 2.5 and 8 hours. The intracellular distribution of label is similar at all observation times; thus, the concentration of silver grains over the typical cytoplasmic granules of the parafollicular cells is 4–5 times higher compared to the concentration over the remainder of the cytoplasm and the nucleus. Treatment with a decarboxylase inhibitor prior to the injection of 3H-L-DOPA results in a low and uniform labelling of all thyroid cells. This finding, taken together with the observation that also pretreatment with reserpine abolishes the autoradiographic reaction over the cytoplasmic granules, gives strong support to the idea that the great majority of silver grains observed over parafollicular cells represents dopamine formed by decarboxylation of the labelled precursor.