Calcium dependency of hormone stimulated lipolysis in the perfused rat heart

Drug and hormone stimulated triglyceride mobilization in the Langendorff perfused rat heart in vitro was estimated by an automated assay of perfusate glycerol. The disappearance of heart triglyceride-glycerol during perfusion of hearts with isoproterenol in Krebs-Henseleit buffer (5.5 m m glucose, 2.5 m m Ca 2+) agreed closely with the perfusate glycerol output. At perfusate [Ca 2+] of 1.2 m m or below, bolus injections of isoproterenol and glucagon failed to increase glycerol output over baseline. Raising the perfusate [Ca 2+] from 1.2 m m to 1.8 m m resulted in a restoration of the glycerol response following injection of isoproterenol or glucagon. Determination of protein kinase activity ratios after injection of lipolytic doses of glucagon and isoproterenol demonstrated that these agents were able to elevate the protein kinase activity ratio at all [Ca 2+] tested, including those at which an increase in glycerol release was not observed. These results suggest that an increase in cyclic AMP dependent protein kinase activity, following injection of glucagon or isoproterenol, does not lead to an increase in heart triglyceride mobilization unless a threshold perfusate [Ca 2+] is present.