THE ISOLATION OF ADULT-RAT CARDIOMYOCYTES ACTIVATES INOSITOL(1,4,5)TRISPHOSPHATE 3'KINASE ACTIVITY

Addition of norepinephrine to [3H]inositol-labeled rat hearts or isolated left atria caused the release of inositol(1,4,5)trisphosphate which was metabolized by dephosphorylation to inositol(4)monophosphate. Products of the inositol(1,4,5)trisphosphate 3'-kinase pathway were not present at detectable levels. When norepinephrine was added to [3H]inositol-labeled isolated adult cardiomyocytes the released inositol(1,4,5)trisphosphate was metabolized partially by phosphorylation as evidenced by the presence of the immediate phosphorylation product inositol(1,3,4,5)-tetrakis-phosphate and its dephosphorylation products inositol (1,3,4)trisphosphate and inositol(1,3)-bisphosphate. Direct measurement of inositol trisphosphate 3'-kinase activity in cytosols prepared from intact hearts and isolated myocytes demonstrated activation of the enzyme during cell isolation from 20 ± 0.5 p mol/min/mg protein to 124 ± 14 (mean ± S.E.M., n = 3, P < 0.001). These data show that the kinase pathway is suppressed in healthy heart tissue and show that it can be activated under conditions such as those required for cell isolation. © 1993 Academic Press Limited.