Effects of sequence and timing of hormonal additions on adipose tissue: Activation of low-Km cyclic adenosine monophosphate phosphodiesterase

Abstract

Epinephrine (EPI) is lipolytic and insulin (INS) antilipolytic in the isolated fat cell (IFC). We have previously demonstrated that in a perifusion system the antilipolytic action of INS is more powerful when IFC are exposed to INS before EPI. In contrast to their opposite effects on lipolysis, both INS and EPI stimulate low-Km cyclic adenosine monophosphate (cAMP) phosphodiesterase (PDE) in adipose tissue. In view of these observations, we decided to determine the effects of sequential addition of EPI and INS on stimulation of PDE from rat adipose tissue. Using previously published methods, the effects of INS and EPI on PDE were assessed alone, together with INS followed by EPI, and then with EPI followed by INS. The resulting data demonstrate that EPI and INS individually both stimulate PDE ( P < .001); EPI plus INS together stimulate PDE minimally compared with EPI or INS alone ( P < .001); when adipose tissue is included with INS first, then followed by EPI, activation of PDE is much less than INS or EPI alone ( P < .001); and when adipose tissue is stimulated by EPI when INS, there is no activation of PDE, different from EPI or INS alone ( P < .001). In conclusion, in perifused IFC, INS and EPI always oppose each other. In studies using activation of PDE, EPI and INS each stimulate PDE, but INS opposes EPI when incubated simultaneously. When adipose tissue is incubated first with INS followed by EPI, PDE is activated. In contrast, when the reverse order is applied, no activation of PDE is observed. Hence, these studies demonstrate that the sequence and timing of hormonal agents, not just concentration, is clearly an important regulatory mechanism in adipose tissue.