Effects of dopamine and somatostatin on pulsatile pituitary glycoprotein secretion.

The hypothalamic factors dopamine (DA) and somatostatin (SRIH) inhibit pituitary glycoprotein secretion, but little is known regarding the effects of these factors on glycoprotein pulses. To address this question, 12 healthy volunteers underwent frequent blood sampling over 12 h at baseline and during 12-h infusions of DA and/or SRIH. TSH, LH, FSH, and alpha-subunit (alpha) levels were measured in all samples, and hormone pulses were located by Cluster analysis. Both DA and SRIH suppressed TSH pulse amplitude by 70%, while SRIH decreased TSH pulse frequency as well. Both infusions decreased LH pulse amplitude by 30-35%, but had no effect on pulse frequency. In contrast, neither infusion significantly altered FSH pulse parameters, although mean FSH levels declined 15%. DA had no effect on pulsatile alpha secretion, while SRIH decreased alpha pulse frequency. Serum thyroid hormone levels declined during both infusions, but there were no major changes in serum sex steroid levels. Thus, the hypothalamic inhibitory factors DA and SRIH had divergent effects on glycoprotein hormone pulses. The major effects on pulse amplitude, rather than frequency, imply that these factors do not play major roles in the generation of glycoprotein pulses, although SRIH may directly affect the TSH and alpha pulse generators.