Effects of forskolin and analogues on nicotinic receptor-mediated sodium flux, voltage-dependent calcium flux, and voltage-dependent rubidium efflux in pheochromocytoma PC12 cells.
1. Forskolin, a naturally occurring diterpene that activates adenylate cyclase, HL706, a water-soluble derivative of forskolin (6 beta-[(piperidino)acetoxy]-7-desacetylforskolin) that is less potent than forskolin in activating adenylate cyclase, and 1,9-dideoxyforskolin, an analogue that does not activate adenylate cyclase, were examined for effects on the nicotinic receptor-mediated 22Na+ flux, a high potassium-induced 45Ca2+ flux through L-type calcium channels, and a high potassium-induced 86Rb+ efflux through a calcium-dependent potassium channels in PC12 cells. 2. Forskolin and analogues at 30 microM completely blocked carbamylcholine-elicited flux of 22Na+ through the nicotinic receptor-gated channel. 1,9-Dideoxyforskolin had an IC50 value of 1.6 microM with forskolin and HL706 being two- to three fold less potent. 3. Forskolin and its analogues appear to be noncompetitive blockers of the neuronal nicotinic receptor-channel complex in PC12 cells, but unlike many noncompetitive blockers, did not markedly enhance desensitization. Instead, forskolin, but not HL706 or 1,9-dideoxyforskolin, slightly antagonized the desensitization evoked by high concentrations of carbamylcholine. N-Ethylcarboxamidoadenosine, an adenosine analogue that elevates cyclic AMP and 8-bromo-cyclic AMP had no effect on desensitization. 4. Forskolin, HL706, and 1,9-dideoxyforskolin in the presence of carbamylcholine inhibited the binding of a noncompetitive blocker, [3H]perhydrohistrionicotoxin, to the muscle-type nicotinic receptor-channel complex in Torpedo electroplax membranes with IC50 values of 20 microM. Forskolin had no effect on [3H]perhydrohistrionicotoxin binding in the absence of carbamylcholine, while HL706 and 1,9-dideoxyforskolin still inhibited binding in the absence of carbamylcholine. 5. Forskolin, but not HL706 or 1,9-dideoxyforskolin had a slight inhibitory effect on the binding of [125I]alpha-bungarotoxin to acetylcholine recognition sites in Torpedo membranes. 1,9-Dideoxyforskolin at 30 microM, but not forskolin or HL706, markedly inhibited depolarization-evoked 45Ca+ flux and 86Rb+ efflux in PC12 cells, suggesting that 1,9-dideoxyforskolin has nonspecific inhibitory effects on a variety of ion channels.
Nishizawa
Y
Y
Laboratory of Bioorganic Chemistry, National Institute of Diabetes, Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892.
Seamon
K B
KB
Daly
J W
JW
Aronstam
R S
RS
D000310:Adrenal Gland Neoplasms / Q000473:pathology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002217:Carbachol / Q000494:pharmacology; D005576:Colforsin / Q000031:analogs & derivatives* / Q000494:pharmacology*; D015640:Ion Channel Gating / Q000187:drug effects*; D007473:Ion Channels / Q000187:drug effects*; D009498:Neurotoxins / Q000494:pharmacology; D010673:Pheochromocytoma / Q000473:pathology; D011188:Potassium / Q000378:metabolism; D051381:Rats; D011978:Receptors, Nicotinic / Q000187:drug effects* / Q000502:physiology; D012413:Rubidium / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*; D014407:Tumor Cells, Cultured / Q000187:drug effects / Q000378:metabolism
D007473:Ion Channels; D009498:Neurotoxins; D011978:Receptors, Nicotinic; C066607:6-((piperidino)acetoxy)-7-desacetylforskolin; D005576:Colforsin; D002217:Carbachol; D012964:Sodium; D012413:Rubidium; C047983:1,9-dideoxyforskolin; D011188:Potassium; D002118:Calcium