DIHYDROPYRIDINE LIGANDS INFLUENCE THE EVOKED RELEASE OF OXYTOCIN AND VASOPRESSIN DEPENDENT ON STIMULATION CONDITIONS

The effects of dihydropyridine ligands on the electrically evoked rele ase of neurohypophysial hormones from isolated, rat neurointermediate robes were investigated as a function of all combinations of two pulse widths (0.2 and 2 ms) and three stimulation frequencies (6.5, 13 and 30 Hz). The dihydropyridine agonist (S)-(+)-202-791 potentiated concen tration dependently the release of both oxytocin and vasopressin at a pulse width of 2 ms and a frequency of 6.5 Hz. This effect of (S)-(+)- 202-791 was abolished by the antagonist(-)-nitrendipine and stereospec ifically by (R)-(-)-202-791 (only vasopressin). The antagonist (R)-(-) -202-791 alone inhibited the release of oxytocin at 13 Hz and 2 ms. Th e results presented show that the effects of the dihydropyridine ligan ds are dependent on the stimulation conditions, and thus demonstrate t hat the entry of Ca2+ through the dihydropyridine sensitive L-type Ca2 + channel is associated with electrically evoked release of neurohypop hysial hormones under certain conditions.