Long-term metabolic labeling studies of endothelial cell multimerin. Multimerin inmmunoprecipitates of equivalent volumes of culture media (CM; from 18 hours [18h] and 3 days [3d] of labeling in media supplemented with 40% unlabeled media; 18-hour [C1] and 3-hour [C2] chases in unlabeled media; and ionophore A23187 [+] or buffer [−] treated cells) and endothelial cell lysates (Lys; ionophore A23187 and buffer-treated cells) were compared with¹²⁵ I labeled platelet multimerin (Plt), and immunoprecipitates prepared by using normal mouse IgG (mIgG). Reduced SDS-PAGE (A) and nonreduced multimer gels (B) analyses of single or double-immunoprecipitates (*) from two experiments are shown. The images represent 3-day (A, lanes 1-3), 20-day (A, lanes 4-10), and 32-day (A, lanes 11-16; B) exposures. The arrows indicate the location of the p155 and proM subunits. Small quantities of more fully proteolyzed multimerin were detected in the cell lysates after constitutive multimerin secretion was complete. Secretagogue stimulation did not release detectable amounts of multimerin into the culture media.