Effects of human plasma proteins on endothelin binding and function.
We have previously shown that the endothelin (ET) isoforms ET-1 and ET-3 and ET receptor antagonists exhibit a high degree (> 98%) of binding to plasma proteins, especially serum albumin. This study examines the effects of human plasma proteins on ET-1 binding and ET-1 stimulated biologic responses. When ET-1 binding to rat pituitary MMQ cells was examined, human serum albumin (HSA) inhibited ET-1 binding in a concentration-dependent manner, with 92% inhibition observed at 5% (w/v) HSA. A similar observation was made when human plasma was tested. Surprisingly, addition of human plasma (0-100%) or HSA (0-5%) did not have a significant effect on ET-1-stimulated phosphatidylinositol (PI) hydrolysis in these cells. In human pericardial smooth-muscle cells, HSA again exhibited profound inhibitory effects on ET-1 binding; 5% HSA inhibited ET-1 binding by 84%. However, ET-1-stimulated arachidonic acid release in these cells was not significantly affected by HSA (0-5%). Addition of increasing concentrations of human plasma incrementally decreased the potency of PD-156707, and ETA-selective antagonist, on inhibiting ET-1-stimulated PI hydrolysis in the MMQ cells. These results suggest that, although ET-1 binding to the receptor is inhibited by human plasma proteins, especially HSA, ET-1-induced signal transduction is not significantly affected. Furthermore, plasma proteins decrease the potency of an ET receptor antagonist in inhibiting ET-1-stimulated biologic responses. This study may help to explain the discrepancy between in vitro and in vivo potencies of ET receptor antagonists.
Wu-Wong
J R
JR
Pharmaceutical Products Division, Abbott Laboratories, Abbott Park, IL 60064-3500, USA.
Dixon
D B
DB
Opgenorth
T J
TJ
D000818:Animals; D001095:Arachidonic Acids / Q000378:metabolism; D001798:Blood Proteins / Q000494:pharmacology*; D002478:Cells, Cultured; D004149:Dioxoles / Q000494:pharmacology; D065128:Endothelin Receptor Antagonists; D016232:Endothelins / Q000378:metabolism* / Q000502:physiology*; D006801:Humans; D008566:Membranes / Q000378:metabolism; D009130:Muscle, Smooth / Q000187:drug effects; D010716:Phosphatidylinositols / Q000378:metabolism; D010902:Pituitary Gland / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D011485:Protein Binding / Q000187:drug effects; D011869:Radioligand Assay; D051381:Rats; D044022:Receptor, Endothelin A; D012709:Serum Albumin / Q000494:pharmacology
D001095:Arachidonic Acids; D001798:Blood Proteins; D004149:Dioxoles; D065128:Endothelin Receptor Antagonists; D016232:Endothelins; C093321:PD 156707; D010716:Phosphatidylinositols; D044022:Receptor, Endothelin A; D012709:Serum Albumin