Adenosine 5'-monophosphate is a selective inhibitor of the brown adipocyte nonselective cation channel.
Calcium-activated nonselective cation channels (NSC(Ca)) in brown adipocytes are inhibited by several nucleotides acting on the cytosolic side of the membrane. We used excised inside-out patches from rat brown adipocytes to identify important nucleotide structures for NSC-channel inhibition. We found that 100 microM 5'-AMP inhibited NSC-channel activity more than did ATP or ADP. Adenosine was a weak inhibitor, whereas adenine and ribose-5-phosphate had no effect. The channel activity was effectively blocked by 10 microM AMP, but it was unaffected by 10 microM cAMP, CMP, GMP, IMP, TMP or UMP. Dose-response studies yielded IC(50)-values of 4 microM for AMP and 32 microM for cAMP. dAMP was as effective as AMP, but all 5'-phosphate group modifications on AMP dramatically lowered the inhibitory effect. 10 microM of the AMP precursor adenylosuccinate weakly inhibited the channel activity. An increase in AMP concentration from 1 to 10 microM shifted the EC(50) for Ca(2+) activation almost 1 order of magnitude; a Schild plot analysis yielded a K(B) value of 0.3 microM for AMP. We conclude that AMP is the most efficacious endogenous nucleotide inhibitor of the brown adipocyte nonselective cation channel (NSC(Ca/AMP)) yet identified and that there is functional competition between Ca(2+) and AMP. The brown adipocyte NSC(Ca/AMP) thus appears to be functionally different from the NSC(Ca,PKA) in the exocrine pancreas and the NSC-(Ca,cAMP) in the endocrine pancreas, but similar to the NSC(Ca/AMP) in the endocrine pancreas.
Halonen
J
J
The Wenner-Gren Institute, The Arrhenius Laboratories F3, Stockholm University, SE-106 91 Stockholm, Sweden.
Nedergaard
J
J
D000249:Adenosine Monophosphate / Q000008:administration & dosage / Q000031:analogs & derivatives / Q000378:metabolism*; D002001:Adipose Tissue, Brown / Q000187:drug effects / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D027682:Cation Transport Proteins / Q000037:antagonists & inhibitors* / Q000378:metabolism*; D000242:Cyclic AMP / Q000008:administration & dosage / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004553:Electric Conductivity; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D018408:Patch-Clamp Techniques; D051381:Rats; D017207:Rats, Sprague-Dawley; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
D027682:Cation Transport Proteins; D000249:Adenosine Monophosphate; D000242:Cyclic AMP; D002118:Calcium