Effect of luteal metoclopramide-induced hyperprolactinemia on pituitary and luteal responsiveness to gonadotropin-releasing hormone

The pituitary and corpus luteum responses to acute gonadotropin-releasing hormone (GnRH) administration at the mid-luteal phase (LP) were studied in 24 infertile women. Patients were randomly divided into two groups. In one group (n = 12) metoclopramide (MCP, 10 mg orally 3 times daily) was administered from day 0 or 1 of the LP for 7 days. On day 7 or 8 of LP blood samples were taken every 15 min for 180 min; then 25 micrograms GnRH were acutely administered intravenously and blood samples taken at 185, 195, 210, 225, 240, 255, 270, 285 and 300 min. In the other 12 patients the same experimental design was performed on day 7 or 8 of an untreated LP. Plasma prolactin (PRL), luteinizing hormone (LH), follicle-stimulating hormone (FSH), progesterone and estradiol (E2) were assayed. The responsiveness of the different hormones to GnRH was evaluated as the integrated secretory area for 120 min after injection (sISA = stimulated integrated secretory area) and as the percentage increase (delta A) with respect to the area under basal conditions before GnRH administration (bISA = basal integrated secretory area). MCP-treated women showed higher basal PRL levels (p less than 0.01) and lower basal plasma concentrations and bISA (p less than 0.01) values of LH than controls. After GnRH a more marked response of LH secretion was observed in the treated group (p less than 0.01), so that the absolute values of sISA were superimposable in both groups. Basal and stimulated FSH secretion did not differ significantly in the study groups. Basal plasma and bISA values of progesterone were also decreased in MCP-treated subjects. After GnRH injection the absolute values of progesterone sISA were greater in controls (p less than 0.01), but delta A values were similar in both groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Chemical modification of phosphorylase b by tetranitromethane. Identification of a functional tyrosyl residue

Tetranitromethane, C(NO2)4, a reagent for tyrosyl residues, was found to inactivate irreversibly rabbit skeletal muscle glycogen phosphorylase b. Under the chosen conditions seven tyrosyl residues, namely Tyr-75, 203, 262, 280, 403, 552 and 647, were found to be nitrated. Inactivation was prevented by the presence of the allosteric activator 5'-AMP during nitration. Under these latter conditions one of the reactive tyrosyl residues was not modified by C(NO2)4; thus, this residue appeared to be essential for either catalytic activity or allosteric activation. Tryptic digests of phosphorylase b, reacted with C(NO2)4 in the absence and presence of 5'AMP, were fractionated by gel filtration. The peptide mixtures were further purified by reverse-phase HPLC. One of the peptides contained the tyrosyl residue which was modified by C(NO2)4 only in the absence of 5'AMP. The sequence of this peptide was determined. The amino acid residue which is responsible for the loss of activity upon reaction with C(NO2)4 was identified in the amino acid sequence of phosphorylase b as tyrosine-75. Of the other residues modified in the presence and in the absence of C(NO2)4, tyrosine-403 contributes to the glycogen-storage site whereas Tyr-280 is close to the alpha-D-glucose-binding site. These residues, exposed to the solvent both in the presence and in the absence of 5'AMP, are not essential for catalytic activity.