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
Authors: Edwards C. A. , Dieguez C. , Ham J. , Peters J. R. , Scanlon M. F. 
Published: December 2008

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 μmol/l), forskolin (10 μmol/l) and GRF plus SRIF (10 nmol/l and 1 μmol/l added together) on the subsequent responses of GH to GRF (1 pmol/l–10 nmol/l), cholera toxin (10 nmol/l), 3-isobutyl-1-methylxanthine (IBMX) (100 μmol/l) and forskolin (10 μmol/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 < 0·001) and forskolin (98% of basal; P < 0·001), but were increased after pretreatment with SRIF (246% of basal; P < 0·02). However, GH responses after pretreatment with GRF plus SRIF were not significantly different from those of the control. The dose giving 50% maximum stimulation (ED50) of GRF action was unaltered by SRIF pretreatment (control 48·5 ± 9·2 (s.e.m.) pmol/l vs 140 ± 38 pmol/l; P > 0·05) or by forskolin pretreatment (140 ± 70 pmol/l; P > 0·05), but was increased by GRF pretreatment (900 ± 230 pmol/l; P < 0·05) and GRF plus SRIF pretreatment (1010 ± 730 pmol/l; P < 0·01). Growth hormone responses to forskolin (32% of basal after GRF pretreatment vs 121% of basal in the control saline-treated cells; P < 0·01), cholera toxin (40% of basal after GRF pretreatment vs 106% of basal in the control cells; P < 0·025) and IBMX (195% of basal in the GRF-pretreated cells vs 133% of basal in the control cells; P < 0·025) were also decreased after GRF pretreatment in comparison with controls. Responses of cAMP to 10 μmol forskolin/l were not different from controls after pretreatment with either GRF, SRIF or GRF plus SRIF. However, cAMP responses to 10 nmol GRF/l were reduced after pretreatment with GRF, SRIF, GRF plus SRIF or forskolin.

Reduction of Bmax GH responses to GRF, cholera toxin, IBMX and forskolin by GRF pretreatment, and the reversal of this action by combined SRIF plus GRF pretreatment, suggests that depletion of the GRF-sensitive releasable pool of GH may contribute to the desensitization phenomenon. In addition, however, the increase in ED50 of GRF following GRF or GRF plus SRIF pretreatment, as compared with control or SRIF pretreatment alone, together with decreased cAMP responses to GRF after GRF plus SRIF pretreatment, supports the view that uncoupling of the regulatory protein of adenylate cyclase, Ns, also plays a role in this phenomenon.

J. Endocr. (1988) 116, 185–190