CHANGES IN THE ENZYME ACTIVITY DURING INCUBATIONS OF ISOLATED CARDIAC- MUSCLE CELLS IN VITRO Paul CHOHAN and Anthony CRYER

1. Isolated cardiac-muscle cells from the hearts of adult rats were shown to retain a high amount of viability during 4h of incubation when viability was assessed by Trypan Blue stain exclusion and intracellular enzyme leakage. 2. The cells also retained their ability to take up °2 and utilize added substrates over the period of incubation at both 25 and 300C. 3. When cells from the hearts of fed rats were incubated in a bufferedsalts solution at pH 7.4 in the presence of amino acids and heparin, lipoprotein lipase activity in the medium increased progressively. 4. During these incubations the intracellular activity of the enzyme remained constant and the total activity of lipoprotein lipase in the system (cells+medium) increased by 80% over the 4h of incubation at 250C. 5. In the absence of heparin only low amounts of enzyme activity were detectable in the medium and the total lipoprotein lipase activity in the system remained constant. 6. The measurement of lipoprotein lipase activity in either fresh homogenates of the cells or in homogenates of acetone/diethyl ether-dried powders of the cells had no effect on the overall pattern of activity change during the incubations, although as reported previously the total activity detected with acetone/diethyl ether-dried preparations was approx. 3-fold higher than with fresh cell homogenates. 7. The observations were compared with published data on lipoprotein lipase activity changes in neonatal heart cell cultures maintained in vitro.