Pituitary stores of rat GH (rGH) and PRL (rPRL) are divisible into immediately releasable and more stable compartments representing either compartmentalized hormone within individual cells of a homogeneous population or responses of specialized cell subsets in a functionally heterogeneous population. In addition, newly synthesized rGH and rPRL can be processed either into intracellular storage or toward direct release. Fractional assignment of new hormone to these two paths can be influenced in the somatotroph by GHRH and may also represent either intracellular processes or functional heterogeneity of cells. We investigated the source, newly synthesized or stored, of hormone refilling the somatotroph and lactotroph immediately releasable pools (IRP) after their discharge by 21 mM potassium ion, 1 mM (Bu)2cAMP, 3 nM human GHRH-44, or 3 Î¼M prostaglandin Ei. Experiments were performed using perifused pituitary fragments exposed sequentially to [14C]- and [3H]leucine in association with stimulation by two 30-min pulses of the same secretagogue. Therefore, only [14C]hormone was available for release by the first stimulus, whereas both [14C]- and [3H]hormone were available for release by the second stimulus. Analysis was by specific immunoprecipitation. The first episode of stored [14C]rGH release exceeded the second episode of stored [14C]rGH release in response to each secretagogue. However, release of newly synthesized [3H]rGH in response to the second episode of stimulation exceeded the simultaneous release of stored [14C]rGH while matching or exceeding the [14C] rGH release that had occurred in the same experiment in response to the first episode of stimulation. Refilling both GH and PRL IRP stores drew primarily upon newly synthesized hormone, but with different secretagogue-specific patterns. These data confirm differential handling of new and stored rGH and rPRL within the pituitary. They are consistent with either (1) the enhanced shunting of newly synthesized hormone to IRPs within cells that are capable of compartmentalized intracellular hormone storage, or (2) the relatively complete discharge of a subset of somatotrophs and lactotrophs that are specialized to deliver pulsed hormone release, after which they are refilled by newly synthesized hormone.
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