The isolated perfused rat heart releases atriopeptin-28 [AP28 (ANF99-126)], whereas the storage form of AP in the heart is the intact prohormone AP126 (ANF1-126). Right atrial stretch or phenylephrine (5 x 10(-5) M) stimulated the release of AP28. The processing of the prohormone during stretch was inhibited by infusion of the protease inhibitor aprotinin, resulting in the appearance of intact AP126 in the cardiac effluent. Other protease inhibitors including p-aminobenzamidine and soybean trypsin inhibitor did not alter prohormone processing by the isolated heart subjected to stretch. In contrast, aprotinin did not block the prohormone processing induced by phenylephrine. Ca+(+)-free medium markedly inhibited prohormone processing during stretch without a significant effect on AP release, whereas phenylephrine-stimulated AP release was completely suppressed by Ca+(+)-free medium. Exogenous AP126 could be cleaved by isolated rat hearts perfused either with Krebs-Henseleit solution or with Ca+(+)-free medium. However, amino acid sequence analysis revealed that the prohormone cleavage in Ca+(+)-free medium occurred at sites other than between Arg98 and Ser99 and that the resultant low molecular weight APs were not AP28. These findings suggest: 1) the characteristics of the enzyme(s) involved in the processing of AP prohormone in isolated perfused rat hearts are different from the described properties of purified enzymes; 2) in isolated perfused rat hearts the specific AP processing enzyme is Ca++ dependent, whereas nonspecific cleavage does not necessarily require Ca++ and 3) two independent AP processing pathways differentially activated by mechanical (stretch) and pharmacologic (alpha 1-adrenergic agonist) stimuli exist.
