Calcium-activated nonselective cation channels (NSCCa) 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 mM 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 mM AMP, but it was unaffected by 10 mM cAMP, CMP, GMP, IMP, TMP or UMP. Dose-response studies yielded IC50-values of 4 mM for AMP and 32 mM for cAMP. dAMP was as effective as AMP, but all 5?-phosphate group modifications on AMP dramatically lowered the inhibitory effect. 10 mM of the AMP precursor adenylosuccinate weakly inhibited the channel activity. An increase in AMP concentration from 1 to 10 mM shifted the EC50 for Ca2+ activation almost 1 order of magnitude; a Schild plot analysis yielded a KB value of 0.3 mM for AMP. We conclude that AMP is the most efficacious endogenous nucleotide inhibitor of the brown adipocyte nonselective cation channel (NSCCa/AMP) yet identified and that there is functional competition between Ca2+ and AMP. The brown adipocyte NSCCa/AMP thus appears to be functionally different from the NSCCa,PKA in the exocrine pancreas and the NSCCa,cAMP in the endocrine pancreas, but similar to the NSCCa/AMP in the endocrine pancreas.
The Journal of Membrane Biology – Springer Journals
Published: Mar 18, 2014
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