Role of Microtubules in Basal and Stimulated Release of Growth Hormone and Prolactin in Rat Adenohypophysisin Vitro

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Abstract
A 20-65% inhibitory effect of 25 or 50% deuterium oxide on the basal and induced release of both growth hormone and prolactin from anterior pituitaries was already measured in the first 45 min of incubation at 37 C. Hormonal release was measured by densitometry of proteins of the incubation medium separated on polyacrylamide gels and by radioimmunoassay. Incubation up to 3 hr in 50% deuterium oxide did not cause any detectable morphological alteration of the somatotrophs (growth hormone-secreting cells) nor mammotrophs (prolactin-secreting cells), while microtubules in apparent normal number were seen scattered through the cytoplasm of both cell types. Although the inhibitory effect of 10-7 to 10-5M vincristine sulfate was usually slight in the first 45-min incubation period, this agent led, during the following 60 min of incubation, to a 50-70% inhibition of the release of growth hormone induced by 1 X 10-5M PGET, 25 HIM K+ or purified growth hormone-releasing hormone. Basal and cyclic AMP-induced release of prolactin were both markedly inhibited by vincristine. After prolonged incubation in the presence of vincristine (10-5M), both somatotrophs and mammotrophs contained numerous crystalline deposits scattered through the cytoplasm and presumably derived from the microtubules. The concentration of adenohypophyseal cyclic AMP was not altered by the presence of deuterium oxide or vincristine, indicating that microtubules act a step beyond formation of the cyclic nucleotide. These biochemical and morphological data give strong support to the hypothesis that the microtubule system is involved in both basal and induced secretion processes in growth hormone and prolactin- secreting cells in the adenohypophysis. (Endocrinology93: 903, 1973)

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