Intracellular signalling in the ovine pars tuberalis: an investigation using aluminium fluoride and melatonin.
The effect of aluminium fluoride (AlF4-) has been studied on inositol phosphate accumulation, calcium mobilization, cyclic AMP production and [2-125I]iodomelatonin binding in ovine pars tuberalis cells. These cells have high-affinity receptors for, and respond to, melatonin through inhibition of forskolin-stimulated adenylate cyclase. In the presence of 10 mM LiCl, AlF4- stimulated the net accumulation of inositol monophosphate and inositol bisphosphate. Consistent with these findings, AlF4- increased intracellular calcium; although this response was attenuated in calcium-depleted medium, indicating that the calcium response comprises both intracellular and extracellular components. Melatonin was ineffective on either basal or AlF4(-)-stimulated turnover of inositol phosphates. In concordance with the inositol phosphate response, melatonin had no effect on either the AlF4(-)-stimulated or the basal calcium levels. AlF4- blocked the increase in cyclic AMP stimulation by 1 microM forskolin, being as effective as melatonin, achieving approximately 90% inhibition. AlF4- also attenuated the binding of [2-125I]iodomelatonin to ovine pars tuberalis membranes by 15%. At the concentration used, these results are consistent with the interpretation that AlF4- activates many G protein-mediated responses, and thus imply that the inhibitory pathway for cyclic AMP predominates over the stimulatory arm, whereas there can only be a stimulatory pathway linked to phosphoinositide metabolism in ovine pars tuberalis cells.
Morgan
P J
PJ
Rowett Research Institute, Bucksburn, Aberdeen.
Hastings
M H
MH
Thompson
M
M
Barrett
P
P
Lawson
W
W
Davidson
G
G
D000535:Aluminum / Q000494:pharmacology*; D017607:Aluminum Compounds; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism; D005459:Fluorides / Q000494:pharmacology*; D007295:Inositol Phosphates / Q000378:metabolism; D008550:Melatonin / Q000031:analogs & derivatives / Q000378:metabolism / Q000494:pharmacology*; D010903:Pituitary Gland, Anterior / Q000378:metabolism*; D012756:Sheep; D015398:Signal Transduction / Q000187:drug effects*
D017607:Aluminum Compounds; D007295:Inositol Phosphates; D005576:Colforsin; C044556:2-iodomelatonin; D000535:Aluminum; D000242:Cyclic AMP; D008550:Melatonin; D005459:Fluorides; D002118:Calcium; C032311:aluminum fluoride