Effect of antecedent hormone administration on lipolysis in the perifused isolated fat cell

Development of a perifusion system utilizing isolated fat cells (IFC) has converted the static incubation system into one capable of measuring dynamic changes in adipose tissue metabolism. Using this new system, we have studied the initiation by epinephrine and inhibition by insulin of lipolysis in IFC. Perifusion with "low" (10 muU per milliliter) or "high" (50 muU per milliliter) insulin was performed (1) prior to, (2) simultaneous with, and (3) after infusion of epinephrine. Exposure to low insulin prior to or simultaneously with epinephrine produced a greater amount of antilipolysis than the addition of insulin to cells already stimulated by epinephrine. Similar results were obtained with proinsulin. Insulin (50 muU per milliliter) when given prior to or simultaneously with 0.035 mug per milliliter of epinephrine bitartrate completely blocked lipolysis but the antilipolytic effect of insulin could be overcome by increasing the concentration of epinephrine bitartrate. Insulin (50 muU per milliliter) also decreased the sensitivity of the fat cells to subsequent doses of epinephrine bitartrate even after apparent removal of all immunoreactive insulin by continuing buffer infusion. After exposure to 50 muU per milliliter of insulin, followed by buffer alone, the cells failed to release glycerol in response to 0.035 mug per milliliter of epinephrine bitartrate but did respond to 3.5 mug per milliliter of epinephrine bitartrate. Thus, the sequence of hormonal administration as well as hormonal concentration greatly altered the responsiveness of the adipose tissue to both lipolytic and antilipolytic stimuli.