Sites of Sulfate Incorporation into Mammotrophs and Somatotrophs of the Rat Pituitary as Determined by Quantitative Electron Microscopic Autoradiography*

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Abstract
Dispersed pituitary cells were labeled with [35S]sulfate (1 h), followed by a chase incubation (up to 2 h), in order to study sulfate incorporation and transport in anterior pituitary cells. The initial site of incorporation of sulfate, the kinetics of sulfate transport, and the intracellular localization of incorporated sulfate were studied by quantitative electron microscope autoradiography. Analysis of autoradiograms from estrogen-treated female rats revealed that all granulated cell types incorporate sulfate. The labeling index (relative [35S]sulfate incorporation per unit area of cytoplasm) of the various cell types was greatest for mammotrophs, slightly less for corticotrophs, gonadotrophs, and thyrotrophs (grouped together), and least for somatotrophs. Results of grain counts on mammotrophs indicated that initially (end of pulse) approximately 70% of the grains were localized in the Golgi region. After 1 and 2 h of chase, there was a decline in radioactivity in the Golgi region (to ∼30% of the total grains), but the percentage of the grains associated with mature granules progressively increased (from 13% to ∼40% of the total). Analysis of the relative grain density (percentage of total grains/percentage of total area) indicated that at the end of a 1-h pulse, [35S]sulfate is most concentrated in immature PRL granules and the Golgi cisternae; during 1–2 of chase it becomes progressively concentrated in mature PRL granules. Findings in somatotrophs (and other cell types) from estrogen-treated female rats or normal males were similar to those in mammotrophs. These results indicate the [35S]sulfate is initially incorporated into the Golgi complex of all anterior pituitary cell types. The majority of the sulfate-labeled macromolecules are then packaged into immature secretion granules in the Golgi region, which become mature granules. In addition, a considerable amount (∼30% in mammotrophs) of the radioactivity remains associated within the olgi region for up to 2 h post pulse. The incorporation of sulfate into the Golgi complex and its transfer to secretory granule membranes and/or contents thus appears to be a general property of anterior pituitary cells.