THE INHIBITORY-ACTION OF MELATONIN IN THE OVINE PARS TUBERALIS IS NOT DEPENDENT ON CHANGES IN PLASMA-MEMBRANE POTENTIAL

Treatment of ovine pars tuberalis (oPT) cultures with forskolin activates adenylyl cyclase, leading to increased levels of cyclic AMP, activation of protein kinase A, phosphorylation of the calcium/cyclic AMP response-element binding protein and the increased synthesis and secretion of several proteins. Simultaneous treatment with melatonin inhibits or reverses these effects of forskolin. In the neonatal rat pituitary, the inhibitory effects of melatonin are mediated by changes in membrane potential. This study therefore investigated whether the inhibitory action of melatonin in oPT cultures is also dependent on the modulation of plasma membrane potential. Treatment of cultures with the ionophore valinomycin selectively permeabilised the cell plasma membrane to potassium, thereby causing membrane hyperpolarisation. In cultures of oPT, valinomycin inhibited in a concentration-dependent manner (maximal effect 2 mu M) the stimulatory action of forskolin (1 mu M) on intracellular levels of cyclic AMP, indicating that the activity of adenylyl cyclase in this tissue is sensitive to hyperpolarisation of the plasma membrane. However, increasing the extracellular concentration of potassium from 5 mM to 100 mM, which would depolarise the plasma membrane, had no effect on the inhibitory action of melatonin (1 mu M) in forskolin-stimulated cultures. This indicated that melatonin could be effective in cells with sustained depolarisation. To test directly whether integrity of the plasma membrane is essential for melatonin to inhibit adenylyl cyclase, cultures were treated with the cholesterol-chelating agent saponin (50 mu g/ml). Saponin increased cellular permeability to trypan blue and enhanced the release of the cytoplasmic enzyme lactate dehydrogenase to the extracellular medium, demonstrating that cell plasma membranes had been permeabilised, thereby abolishing membrane polarity. In cultures pretreated with saponin there was a tendency for levels of cyclic AMP to be reduced. However, permeabilisation did not block the forskolin-stimulated increases in cyclic AMP levels nor did it alter the ability of melatonin to inhibit the production of cyclic AMP in forskolin-stimulated cultures. This study demonstrated that, while it is possible to inhibit the stimulatory actions of forskolin in the oPT by increasing the permeability of cells to potassium and thereby hyperpolarising them, melatonin is able to inhibit cyclic AMP in permeabilised cells and so can act independently of changes in membrane potential.