Reduction of Muscarinic K<SUP>&#8290;</SUP> Channel Activity by Transferrin in Ischemic Rat Atrial Myocytes

Reduction of Muscarinic K<SUP>&#8290;</SUP> Channel Activity by Transferrin in Ischemic Rat Atrial Myocytes

It has been demonstrated that an unidentified cytosolic factor(s) reduces K<SUB>ACh</SUB> channel function. Therefore, this study attempted to elucidate the cytosolic factor. Fresh cytosol isolated from normal heart (FC) depressed the K<SUB>ACh</SUB> channel activity, but cytosol isolated from the ischemic hearts (IC) did not modulate the channel function. Electrophorectic analysis revealed that a protein of <FONT FACE= 바탕 >∼80 kDa was markedly reduced or even lost in IC. By using peptide sequencing analysis and Western blot, this 80 kDa protein was identified as transferrin (receptor-mediated Fe<SUP>3</SUP><SUP>&#8290;</SUP> transporter, 76 kDa). Direct application of transferrin (100 nM) to the cytoplasmic side of inside-out patches decreased the open probability (P<SUB>o</SUB>, 12.7&#8273;6.4%, n=4) without change in mean open time (<FONT FACE= 바탕 >τ<SUB>o</SUB>, 98.5&#8273;1.3%, n=4). However, the equimolar apotransferrin, which is free of Fe<SUP>3</SUP><SUP>&#8290;</SUP>, had no effect on the channel activity (N*P<SUB>o</SUB>, 129.1&#8273;13.5%, n=3). Directly applied Fe<SUP>3</SUP><SUP>&#8290;</SUP> (100 nM) showed results similar to those of transferrin (N*P<SUB>o</SUB>: 21.1&#8273;3.9%, n=5). However Fe<SUP>2</SUP><SUP>&#8290;</SUP> failed to reduce the channel function (N*P<SUB>o</SUB>, 106.3&#8273;26.8%, n=5). Interestingly, trivalent cation La<SUP>3</SUP><SUP>&#8290;</SUP> inhibited N*P<SUB>o</SUB> of the channel (6.1&#8273;3.0%, n=3). Taken together, these results suggest that Fe<SUP>3</SUP><SUP>&#8290;</SUP> bound to transferrin can modulate the K<SUB>ACh</SUB> channel function by its electrical property as a polyvalent cation.