BLOCKAGE OF K+ CHANNELS REVERSES THE INHIBITORY-ACTION OF ATRIOPEPTIN ON SECRETAGOGUE-STIMULATED ACTH RELEASE BY PERIFUSED ISOLATED RAT ANTERIOR-PITUITARY-CELLS

The aim of the present study was to investigate how atriopeptin inhibits secretagogue-stimulated ACTH secretion in vitro. Perifused isolated rat anterior pituitary cells were used throughout; the ACTH content of the perifusate was measured by radioimunnoassay. In the presence of a constant (0.05 nmol/l) concentration of 41-residue corticotrophin-releasing factor (CRF), arginine vasopressin (AVP; 0.05-50 nmol/l) stimulated ACTH secretion in a concentration-dependent manner, the combination of 0.05 nmol CRF/l and 0.5 nmol AVP/l (CRF/AVP) stimulated ACTH release to six- to eightfold above baseline. The effect of CRF/AVP was not modified by tetrodotoxin, but was abolished by CoCl2 and reduced to about 70% of the control stimulus by nifedipine. Application of 103-126 residue atriopeptin for 10 min before and 2.5 min during the CRF/AVP stimulus strongly suppressed the evoked release of ACTH, the maximal inhibition was 75-90% at 10 nmol atriopeptin/l. The calcium ionophore ionomycin (200 nmol/l) reversed the effect of atriopeptin while it had no secretagogue activity of its own, and did not enhance the response to CRF/AVP. A variety of blockers of K+ channels, 4-amino pyridine, tetra-blockers of K+ channels, 4-amino pyridine, tetraethylammonium, apamine, quinine, but not tolbutamide, effectively antagonized the inhibitory action of atriopeptin (10 nmol/l). None of these drugs altered ACTH release evoked by CRF/AVP. In concentration-response experiments, the half effective concentration of 4-aminopyridine and tetraethylammonium were around 1 mmol/l and 10 nmol/l for apamine. Finally, tetraethylammonium and apamine also antagonized the inhibition of CRF/AVP-evoked ACTH release by 8-Br-cGMP. These data suggest that (1) at least two types of K+ channels, a delayed rectifier and the apamine-sensitive Ca2+-activated channel, are functionally important in pituitary corticotroph cells; (2) atriopeptin inhibits CRF/AVP-stimulated ACTH secretion by hyperpolarizing the plasma membrane and thus reducing the uptake of Ca2+ into the cells; (3) cGMP is the intracellular mediator of the action of atriopeptin on corticotroph cells.