Frequency-dependent effects of activation and inhibition of protein kinase C on neurohypophysial release of oxytocin and vasopressin.

Isolated rat neurohypophyses were superfused in vitro and the release of vasopressin and oxytocin into the medium was determined by specific radioimmunoassays. Hormone secretion was increased by electrical stimulation of the pituitary stalk at different frequencies. The effects of several phorbol esters, known to activate phorbol 12,13-dibutyrate, PDB) or not to affect (4 alpha-phorbol 12,13-dideconate and phorbol 12-monoacetate) protein kinase C, and of the direct protein kinase C inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7) were tested. Electrical stimulation with 450 pulses caused the release of about 45 microU vasopressin and 55 microU oxytocin, when a frequency of 3 Hz was applied, and of about 500 microU vasopressin and oxytocin, when a frequency of 15 Hz was used. PDB (1 mumol/l) increased the release of vasopressin evoked by 15 Hz stimulation maximally by about 40-50% and that evoked by 3 Hz stimulation by about 150%. The release of oxytocin evoked by 15 Hz stimulation was increased by about 150% and that evoked by 3 Hz stimulation by about 400-500% in the presence of PDB. Both inactive phorbol esters had no effects on the evoked release of vasopressin or oxytocin. The effect of PDB on the release of vasopressin and oxytocin was blocked by H7 (10-30 mumol/l). H7 (30 mumol/l) alone reduced the release of vasopressin evoked by stimulation at 15 Hz by 50%. The release of oxytocin was not significantly affected by H7. In the presence of naloxone (1 mumol/l) the release of oxytocin evoked by 3 and 15 Hz stimulation was increased by about 175 and 105%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)