Characteristics of the desensitization of growth hormone and cyclic AMP responses to growth hormone-releasing factor and prostaglandin E 2 in rat anterior pituitary cells in culture

Abstract

Upon first exposure, synthetic human growth hormone-releasing factor (GRF) and prostaglandin E 2 (PGE 2) cause a rapid and marked stimulation of cyclic AMP accumulation and GH release in rat adenohypophysial cells in primary culture. However, a marked attenuation of these responses occurs following previous incubation with the 2 compounds. A 50% desensitization of the cyclic AMP and GH responses is observed after 100 and 150 min of preincubation with 300 nM GRF, respectively. After a prior exposure to 3 μM PGE 2, a 50% maximal decrease of the cyclic AMP and GH responsiveness to a subsequent 3 h incubation with PGE 2 is obtained at 90 and 120 min, respectively. Following preincubation with GRF, a loss of responsiveness of the cyclic AMP and GH responses is also observed after heterologous stimulation with PGE 2. A similar heterologous desensitization to the action of GRF is observed following pretreatment with PGE 2. The desensitizing action of GRF on the cyclic AMP and GH responses is obtained at respective IC 50 values of 2 and 7 nM for both the homologous and heterologous responses. The sensitivity of the desensitizing effect of GRF (7 nM) is thus identical to that of its stimulatory action on GH release (6.2 nM). The desensitization to GRF, in analogy to that to PGE 2, is mainly due to a decrease in the maximal action of GRF. Although GH cell content is decreased by previous exposure to GRF and/or PGE 2, the ability of forskolin, cholera toxin, 8-bromo 3',5'-adenosine cyclic monophosphate and 3-isobutyl-1-methylxanthine to stimulate GH release remains unchanged in cells pretreated with these compounds, thus indicating that the loss of responsiveness to GRF and PGE 2 is not due to a depletion of the releasable pool of GH. On the other hand, nifedipine, a potent calcium channel antagonist, completely abolishes the stimulatory effect of GRF on GH release while not affecting basal and GRF- or PGE 2-induced cyclic AMP accumulation. Preincubation with nifedipine has no influence on the desensitizing effect of GRF or PGE 2 on either the cyclic AMP or GH responses to the same stimuli. In addition to showing the cross-desensitization by GRF and PGE 2, the present results strongly suggest that the desensitization does not result from a depletion of the GH releasable pool but most likely results from a down-regulation and/or an impairment of coupling of a component of the adenylate cyclase system independent from calcium uptake.