MEMBRANE-DELIMITED ACTIVATION OF MUSCARINIC-K CURRENT BY AN ALBUMIN-ASSOCIATED FACTOR IN GUINEA-PIG ATRIAL MYOCYTES

Atrial myocytes obtained by enzymatic perfusion of hearts from adult g uinea-pigs and cultured for 0-14 days were studied using different con figurations of the patch-clamp technique. Activation of muscarinic K c urrent [I-K(ACh)] in whole-cell voltage-clamp mode by strongly diluted sera from various sources could be mimicked by corresponding concentr ations of albumin, but not by delipidated (''fatty-acid-free'') sample s of albumin. In cell-attached membrane patches activity of I-K(ACh) c hannels was significantly higher than basal I-K(ACh) channel activity, if the pipette contained serum, whereas application of serum-containi ng solution to the cell outside the patch did not affect channel activ ity. In isolated inside-out membrane patches, strong I-K(ACh) activati on by internal guanosine triphosphate (GTP, 5 mu M) was observed if th e pipette contained serum. If no activator was presented to the outer face of the membrane, only weak opening activity was observed during b ath application of GTP. These results demonstrate that the serrum fact or which causes activation of I-K(ACh) is associated with albumin. Fur thermore activation of I-K(ACh) by that factor proceeds analogous to A Ch or adenosine, i. e. via a membrane-delimited receptor, G-protein, c hannel interaction.