An insulin-sensitive cation channel controls [Na+]i via [Ca2+]o-regulated Na+ and Ca2+ entry.
The insulin-stimulated cation channel previously identified in patch-clamped muscle preparations is here shown to be responsible for bulk Na+ entry into the cell. The mainly Na+ current of the channel was shown to be accompanied by an inhibitory Ca2+ component responsible for oscillations. Here, using quantitative fluorescence imaging of Fura-2- and SBFI-loaded soleus muscle, we measure changes in [Na+]i and [Ca2+]i related to channel function. Insulin increased [Na+]i and [Ca+]i in a transient spike of < 1-min duration. There was a momentary dip in [Na+]i related to inhibition of the channel by the Ca2+ spike, and changes in external Ca2+ were shown to alter [Na+]i via the cation channel, all effects being blocked by the specific channel inhibitor mu-conotoxin, but not by tetrodotoxin. The [Ca2+]i spike could also be induced by 8-bromo cyclic-guanosine 5'-monophosphate, an analogue of the channel-activator cyclic-guanosine 5'-monophosphate (cGMP). In addition it was noted that insulin reduced the [Ca2+]i rise upon subsequent muscle depolarization by a factor of 3.5. Insulin could be substituted with phorbol ester for the same effect and HA1004, a protein kinase inhibitor, blocked the reduction.
McGeoch
J E
JE
Department of Biochemistry and Molecular Biology, Harvard University, Cambridge, Massachusetts 02138.
Morielli
A D
AD
D000818:Animals; D000831:Animals, Newborn; D015220:Calcium Channels / Q000378:metabolism*; D020916:Conotoxins; D006152:Cyclic GMP / Q000031:analogs & derivatives / Q000494:pharmacology; D066298:In Vitro Techniques; D007328:Insulin / Q000494:pharmacology*; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D008564:Membrane Potentials; D009132:Muscles / Q000378:metabolism*; D010042:Ouabain / Q000494:pharmacology; D010456:Peptides, Cyclic / Q000494:pharmacology; D011493:Protein Kinase C / Q000502:physiology; D051381:Rats; D026941:Sodium Channel Blockers; D015222:Sodium Channels / Q000378:metabolism*; D013779:Tetrodotoxin / Q000494:pharmacology
D015220:Calcium Channels; D020916:Conotoxins; D007328:Insulin; D010456:Peptides, Cyclic; D026941:Sodium Channel Blockers; D015222:Sodium Channels; C046101:conotoxin GIII; C016276:8-bromocyclic GMP; D013779:Tetrodotoxin; D010042:Ouabain; D011493:Protein Kinase C; D006152:Cyclic GMP