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-labelled 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)- biphosphate. 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.