Evidence that growth hormone depletion and uncoupling of the regulatory protein of adenylate cyclase (Ns) both contribute to the desensitization of growth hormone responses to growth hormone-releasing factor

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Recent data suggest that the response of GH to GH-releasing factor (GRF) is reduced following prior exposure to high concentrations of GRF. However, it is unknown whether this is due to alterations in GRF receptors, adenylate cyclase activity or the size of a GRF-releasable storage pool of GH. In order to clarify these questions we have compared the effects of pretreatment with GRF (10 nmol/l every 2 h for 12 h) with those of pretreatment with somatostatin (SRIF; 1 mumol/l), forskolin (10 mumol/l) and GRF plus SRIF (10 nmol/l and 1 mumol/l added together) on the subsequent responses of GH to GRF (1 pmol/l-10 nmol/l), cholera toxin (10 nmol/l), 3-isobutyl-l-methylxanthine (IBMX) (100 mumol/l) and forskolin (10 mumol/l). Experiments were performed on 4-day monolayer cultures of rat anterior pituitary cells. The cells were pretreated with test substances every 2 h for 12 h and incubated with GRF or forskolin for 3 h. Per cent maximal (Bmax) GH responses to GRF (10 nmol/l) were reduced after pretreatment with both GRF (control, 173% of basal; GRF, 25% of basal; P less than 0.001) and forskolin (98% of basal; P less than 0.001), but were increased after pretreatment with SRIF (246% of basal; P less than 0.02). However, GH responses after pretreatment with GRF plus SRIF were not significantly different from those of the control.(ABSTRACT TRUNCATED AT 250 WORDS)