The effects of histidyl-proline-diketopiperazine, a metabolite of TRH, on prolactin release by the rat pituitary gland in vitro.

The effect of four different preparations of histidyl-proline-diketopiperazine (DKP) on prolactin (PRL) release by rat hemipituitary glands and cultured dispersed normal pituitary cells was studies. Three DKP preparations )0.1-1 micrometer) were ineffective on PRL release in vitro. By combining the results from 12 incubation vials in 3 experiments 1 micrometer of the fourth DKP preparation was shown to suppress PRL release slightly but significantly by 18%, (P less than 0.05). DKP did not enhance the dopamine-mediated inhibition of PRL release and it also did not reverse the TRH-mediated stimulation of PRL release by the pituitary gland in vitro. It was concluded that DKP (in micromolar concentrations) has a very weak PRL release-inhibiting action and is probably not a physiologically acting prolatin-inhibiting factor.