Stimulation of lipolysis in adipose tissue in vitro by inhibitors of lipid mobilization

5-Methylpyrazole-3-carboxylic acid (U-19425) and nicotinic acid, which apparently inhibit lipolysis in vivo as indicated by low plasma FFA and glycerol concentrations, stimulate lipolysis in vitro in adipose tissue removed from fasted rats 30-90 min after treatment. This stimulation is not the result of low initial levels of FFA in adipose tissue. An increased rate of lipolysis is not induced in vitro by preincubating tissue of untreated rats with U-19425. The increase can be blocked in incubated adipose tissue of U-19425-treated animals if U-19425 is added to the incubation medium. The beta-adrenergic blocking agent propranolol, at a concentration which inhibits epinephrine-stimulated lipolysis, does not affect the increase produced by administration of U-19425.