Plasma growth hormone and prolactin responses to TRH and arginine given before and during bromocriptine therapy to patients with acromegaly

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Abstract
: The chronic effects of bromocriptine (CB-154) administration on pituitary somatotrophs and lactotrophs were investigated in 9 patients with acromegaly. Following therapy with CB-154 for 3 to 20 weeks, 6 of 9 cases showed a normalization of plasma GH (below 5 ng/ml) and a clinical improvement. These cases were termed 'CB-154-responders'. Plasma GH responses to TRH were observed in all cases of'CB-154-responders' before the therapy and were significantly suppressed during CB-154 therapy, although the response patterns expressed in logarithms were retained. When TRH was administered in 3 cases of acromegaly after acute administration of CB-154 and before chronic CB-154 therapy, the peak plasma GH responses were between those responses seen before and during CB-154 therapy. After iv infusion of arginine, 5 cases of 'CB-154-responders' showed an increase in plasma GH levels. These responses were also significantly suppressed during CB-154 therapy, while the response patterns were also preserved. In these same patients, basal plasma prolactin was consistently suppressed to low levels (mean ± sem: 3.4 ± 0.9 ng/ml) in all 9 cases during CB-154 therapy and plasma prolactin responses to TRH almost disappeared after either acute or chronic administration of CB-154. These findings demonstrate: 1) chronic CB-154 therapy in acromegalic patients gradually reduces the activity of the somatotrophs to synthesize and to release GH while it causes a rapid and marked suppression of lactotrophs; 2) the sites of action to CB-154 on the somatotrophs may be different from those to TRH and arginine (possibly GH-RF),while CB-154 and TRH appear to have a close relationship in regard to sites of action on the lactotrophs

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