Experiments were designed to study the inhibitory action of somatostatin (SRIF) on stimulated GH release. The studies were performed in a standard incubation- or perifusion- immunoprecipitation system in which rat anterior pituitaries were prelabeled with [3H]leucine to define a pool of stored rat GH ([3H]GH) whose temporal release could be followed separate from any contribution by new hormone synthesis. The responses to 1CT3 M dibutyryl cAMP (dbcAMP), to the depolarizing effect of 21 mM potassium ion (K+), to a maximally inhibitory concentration of SRIF (25 nM), and to the three in various combinations were defined. Stimulation of initial [3H]GH release above basal (0.065%/min) was immediate and rapid after either K+ (0.45%/min) or dbcAMP (0.37%/min); used together, the releasing effect of these two agents was almost additive (0.69%/min). However, while the K+ response was complete in 30—40 min, an increased rate of release in response to dbcAMP (0.24%/min) was sustained for the duration of that stimulus. Maximal SRIF prevented dbcAMP or K+ stimulation of [3H]GH release above pre-SRIF basal release rates (0.068%/miri), but despite maximal SRIF, the combination of dbcAMP and K+ stimulated [3H]GH release in a biphasic pattern (initial, 0.35%/min; sustained, 0.25%/min) similar to that produced by dbcAMP alone in the absence of SRIF. The interrelation of SRIF, K+, and dbcAMP effects upon [3H]GH release was further studied in pituitary explants which had been pretreated by exposure to SRIF plus dbcAMP, SRIF plus K+, or SRIF plus both K+ and dbcAMP. In each pretreated group, SRIF was withdrawn and [3H]GH release examined during dbcAMP, K+, or dbcAMP plus K+ stimulation and in the absence of stimulation. SRIF withdrawal after pretreatment by SRIF plus K+ resulted in no rebound [3H]rGH release, an absence of the immediate release response to dbcAMP stimulation, but persistence of K+-induced immediate release (0.33%/min) which could be enhanced by the addition of dbcAMP (0.60%/min overall). After SRIF plus dbcAMP pretreatment, immediate rebound release of [3H]GH followed SRIF withdrawal (0.25%/min), and stimulation by dbcAMP (0.59%/min), K+ (0.88%/min), or both together (1.05%/min) enhanced that immediate release. SRIF withdrawal after pretreatment with SRIF plus both K+ and dbcAMP resulted in a similar enhancement of rebound [3H]GH release (0.37%/min), combined K+ and dbcAMP stimulating more rapid [3H]GH release (1.29%/min) than did any other protocol. In conclusion: 1) in resting and stimulated explants, SRIF may inhibit stored [3H]GH release by more than one mechanism, 2) the immediate/brief phase of dbcAMP-stimulated [3H]GH release can be eliminated despite persistence of the late/sustained phase of dbcAMP-stimulated release, 3) depolarizing concentrations of K+ interfere with the inhibitory effect of SRIF upon dbcAMP-stimulated [3H]GH release, and 4) the immediately releasable pool of [3H]GH has both dbcAMP- and K+- sensitive components which overlap and whose content can be increased by the accumulation of stored hormone behing a SRIF block.
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