Calcium and Calcium-Antagonistic Effects on Prolactin and Growth Hormone Responses to Thyrotropin-Releasing Hormone and L-Dopa in Man

The significance of calcium for the responsiveness of human lactotrophs and somatotrophs to iv TRH and oral L-dopa was investigated in 11 young healthy women. Both TRH and L-dopa were administered on three different background infusions: 1) saline, 2) verapamil, and 3) calcium. Twenty-five micrograms of TRH raised the PRL level from 15.1 +/- 2.3 to 76.9 +/- 8.8 ng/ml in 15 min (P less than 0.001). Calcium infusion blunted this PRL response by 33 +/- 8% (P less than 0.02), whereas verapamil, known for its calcium-antagonistic properties, left in unaffected. Five hundred milligrams of L-dopa increased the GH level from 2.2 +/- 0.7 to 16.7 +/- 2.2 ng/ml in 60 min (P less than 0.002) and reduced the PRL level from 11.6 +/- 2.9 to 3.1 +/- 0.4 ng/ml in 150 min (P less than 0.05). Neither calcium nor verapamil influenced these GH and PRL responses significantly. These findings indicate that human somatotrophs may be less dependent than human lactotrophs on normocalcemia for adequate hormone secretion.