Response of canine cardiocyte lysosomes to ATP

The present study was designed to evaluate the relationship of adenosine triphosphate (ATP) to maintenance of cardiac lysosome latency. Highly latent lysosomes were isolated from adult canine ventricular myocytes or cardiocytes and exhibited latency values (based on % free N-acetyl-beta-glucosaminidase, NAGA) of 29.3 +/- 4.7% (SE), minimal cross contamination by mitochondria (less than 2% cardiolipin by weight) and only 1.68% of the total cytochrome c oxidase activity; enzymatic enrichments ranged from 10-fold for NAGA to almost 50-fold for cathepsin B. Incubations of cardiocyte lysosomes at pH 7.0 and 25 degrees C for up to 1 h resulted in changes in the rate of loss in lysosomal latency when ATP levels were adjusted from 0.0 to 5.0 mM; under these conditions as ED50 of 0.62 mM ATP was determined. The protection afforded by ATP was reversed by addition of the H+ ionophore m-chlorocarbonylcyanide phenylhydrazone (CCCP) at 10 microM to the lysosomal suspensions. A significant increase in lysosomal membrane fluidity, measured by fluorescent polarization of diphenylhexatriene, was also seen in the absence of ATP. Adenosine diphosphate (ADP) afforded significant protection only at the very highest concentration (5.0 mM); inorganic pyrophosphate (PPi) did not protect against loss of latency at any concentration. Thus ATP exerts a significant stabilizing effect on cardio(myo)cyte lysosomes in vitro and may be one of the metabolites regulating lysosomal integrity in vivo.