EFFECT OF FENOLDOPAM, A DOPAMINE D‐1 RECEPTOR AGONIST, ON PITUITARY, GONADAL AND THYROID HORMONE SECRETION

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Abstract
The influence of fenoldopam, a dopamine (DA) D‐1 receptor agonist, on basal and GnRH/TRH stimulated PRL, GH, LH, TSH, testosterone and thyroid hormone secretion was studied in nine normal men. All men received 4‐h infusions of either 0.9% saline or fenoldopam at an infusion rate of 0.5 μg/kg min, 12–16 ml/h, adjusted according to weight. After 3 h of infusion, 50 μg GnRH and 100 μg TRH was given i. v. Blood samples were collected every 15 min from 1 h before to 1 h after the infusion for a total of 6 h for measurements of PRL, LH, FSH, GH, TSH, testosterone, T4 and T3. The median PRL concentration increased significantly (P < 0.01) to 128%, range 87–287, of preinfusion levels, compared to the decline during control infusion (85%, 78–114). Basal TSH levels declined significantly to 71% (60–91) during fenoldopam compared with 82% (65–115) during control infusion (P < 0.05). Basal LH, FSH, GH and thyroid hormones were similar during fenoldopam and control infusions (P > 0.05). The LH response to GnRH/TRH was significantly (P < 0.02) increased by fenoldopam infusion. Basal and stimulated testosterone concentration was lower during fenoldopam (P > 0.01) infusion compared with control. Other hormones were similar after GnRH/TRH stimulation during fenoldopam and saline infusions. These results suggest that DA D‐1 receptors are involved in the modulation of pituitary hormone secretion. We suggest that the effect of fenoldopam on PRL and TSH is mainly at the hypothalamic level. Regarding the effect on LH concentrations, an additional direct effect of fenoldopam on testosterone regulation can not be excluded.