We examined whether the
cholinergic mechanism is involved in the paradoxical GH responses to
vasoactive intestinal peptide (VIP) and
peptide histidine methionine (PHM) in
acromegaly. 28 patients with active
acromegaly underwent i.v. bolus
injections of
thyrotropin-releasing hormone (TRH, 500 micrograms), GH-releasing
hormone (GHRH, 100 micrograms), VIP (100 micrograms), and PHM (100 micrograms) with or without a prior
atropine treatment (1 mg, i.m., 30 min before). Blood samples were collected before and at intervals up to 120 min after the injection, and plasma GH levels were measured. In response to TRH, GHRH, VIP and PHM, 23 (82%), 24 (86%), 13 (46%) and 7 (25%) patients, respectively, responded with a significant GH increase (> 50% and 6 micrograms/l above the basal level). The effect of
atropine pretreatment was examined in only these responders to the respective
peptides. When the GH responses were estimated by the area under the response curve, the
atropine pretreatment was able to significantly suppress the GH response to GHRH, but not to TRH, VIP, or PHM. Although the lack of
cholinergic involvement in the TRH-induced GH release in
acromegaly is confirmatory to previous reports, the same results with the VIP- and PHM-induced GH release are novel. The present study may suggest that in
acromegaly the physiological GH response is mediated by the
cholinergic mechanism, but the paradoxical ones are not.