Studies of the biological effect and degradation of glucagon in the rat perifused isolated adipose cell.

The time relationships between glucagon-stimulated lipolysis and hormonal degradation are incompletely understood. These studies were designed to examine the rat adipocyte response to hormonal stimulation in the perifusion system. The perifused isolated adipocyte responded to glucagon concentrations as low as 10(-11) M, with maximum lipolysis at 10(-7) M glucagon. The lipolytic response corresponded in timing with exposure to glucagon. At lower concentrations, there was an appreciable lag between exposure to hormone and initiation of lipolytic response. Termination of response occurred with removal of hormone from the perifusion medium. The percentage of undegraded glucagon in the perifusate fractions (as determined by precipitability with trichloroacetic acid) remained above 80% until the point where free hormone was cleared from the system and lipolysis had ceased. The adipocytes continued to release both intact and degraded glucagon into the medium, the latter increasing with time. Release showed first order kinetics for both undegraded and degraded glucagon, with rate constants of 0.0656 +/- 0.0053 and 0.0716 +/- 0.0073 min-1, respectively. These perifusion studies provide a mechanism for assessing the time relationship between glucagon stimulation of lipolysis and its removal and degradation. Furthermore, the increased sensitivity of the fat cell to glucagon demonstrated herein with perifusion suggests that the importance of glucagon as a physiological regulator of lipolysis may have been underestimated by earlier researchers.