HORMONAL-REGULATION OF LIPOPROTEIN-LIPASE ACTIVITY FROM 5-DAY-OLD RATHEPATOCYTES

Lipoprotein lipase (LPL) activity is known to be synthesized, active a nd functional in the 1-day-old rat liver: it peaks just at birth trigg ered by parturition. During suckling LPL mRNA, LPL synthesis and LPL a ctivity are still high at 5 days and then fade reaching adult values a t weaning. How LPL expression is gradually extinguished is not known. Therefore we studied the effect of different doses of several hormones on LPL activity released by incubated hepatocytes from 5-day-old rats . In the presence of heparin the release of LPL activity in the medium was linear until 3 h and was always significantly increased vs. witho ut heparin. At 3 h in the presence of heparin the main hormonal effect s were: dose-independent increase (30-60%) with dexamethasone; dose-de pendent increase (20-60%) with glucagon; dose-independent decrease (50 -60%) with ethinylestradiol, testosterone, progesterone and prolactin; no effect with insulin; 20-40% increase with adrenaline <1 mM but 40- 50% decrease with noradrenaline < 10 mu M. Increase of LPL release by glucagon and adrenaline agrees with the increased LPL expression we pr eviously found in an undifferentiated hepatoma cell line when the aden ylate cyclase/protein kinase A pathway was activated, The effect of gl ucagon is concordant with our previous observations that fasting incre ases liver LPL activity in neonatal rats. The high estradiol levels kn own to be present in male and female 9-19-day-old rats might contribut e to liver LPL extinction during suckling.