These tools will no longer be maintained as of December 31, 2024. Archived website can be found here. PubMed4Hh GitHub repository can be found here. Contact NLM Customer Service if you have questions.


Pubmed for Handhelds

PUBMED FOR HANDHELDS

Search MEDLINE/PubMed

  • Title: The inhibitory action of melatonin in the ovine pars tuberalis is not dependent on changes in plasma membrane potential.
    Author: McNulty S, Schurov IL, Morgan PJ, Hastings MH.
    Journal: J Endocrinol; 1995 Jun; 145(3):471-8. PubMed ID: 7636431.
    Abstract:
    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 microM) the stimulatory action of forskolin (1 microM) 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 microM) 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 micrograms/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.(ABSTRACT TRUNCATED AT 250 WORDS)
    [Abstract] [Full Text] [Related] [New Search]