Previous results from our laboratory suggest that stored rat GH (rGH) in the pituitary is divisible into at least two functional compartments. An immediate release pool (IRP) responds quickly and can be exhausted. A larger and less labile pool responds continuously to long term stimulation. We previously demonstrated that the sum of IRP rGH discharged by (Bu)2cAMP and potassium ion (K+) in separate experiments exceeds by one third the amount released by the two agents administered simultaneously. This overlap suggested an IRP substructure. We used prelabeled rat pituitary fragments in an in vitro perifusion-immunoprecipitation system to define intracellular hormone storage and to track release of stored rGH and rat PRL (rPRL). We tested three secretagogues: 1) K+ to induce release without altering pituitary cAMP levels, 2) (Bu)2cAMP to introduce cAMP into cells without activating adenylate cyclase, and 3) prostaglandin E1 (PGE1) to produce a temporary, localized cAMP increase through adenylate cyclase activation. Prelabeled tissue in basal perifusion was first exposed to one secretagogue for 90 min. Then, while the first secretagogue was continued, a second secretagogue was added for a second 90-min period. Demonstrable alterations in tissue responses to secretagogues included: 1) K+ diminished PGEi-induced rGH release from the IRP by 69% but had a mixed effect on the response to (Bu)2cAMP; 2) (Bu)2cAMP enhanced K+-induced rGH release from the IRP by 71% but reduced PGE1-induced rGH release by 72%; 3) PGE1 diminished K+-induced rGH release by 13% and (Bu)2cAMP-induced rGH release by 23%; 4) combined K+ and (Bu)2cAMP reduced the rGH response to PGE1 stimulation by 81% whereas prior PGE1 enhanced the response to subsequent combined K+ and (Bu)2cAMP by 16%. We conclude that the somatotroph IRP consists of a K+-sensitive portion which overlaps with, but is not identical to, a (Bu)2cAMP-sensitive portion. The PGE1-sensitive portion of the IRP appears to be roughly equivalent to the shared fraction of the K+- and (Bu)2cAMP-sensitive portions of the IRP. These agents define a similar rPRL compartmentalization. However, the K+-sensitive portions of the somatotroph and lactotroph IRP differ in that the former is larger and expandable, whereas the latter is smaller and appears to be of limited capacity.
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