EXPRESSION OF LIPOPROTEIN-LIPASE IN RAT MUSCLE - REGULATION BY FEEDING AND HYPOTHYROIDISM

Lipoprotein lipase (LPL) is a key enzyme in lipid metabolism and is fo und predominantly in adipose tissue and muscle. We examined the mechan ism of regulation of LPL in muscles composed of different fiber types (soleus, extensor digitorum longus, and heart) in fed, fasted, and hyp othyroid rats. In all muscles, the detergent-extractable (EXT) fractio n represented approximately 95% of total LPL activity and mass. LPL ac tivity was similar in the heparin-releasable (HR) fractions of heart a nd soleus (predominantly type I fibers), while in the EXT fraction LPL activity in soleus was 418 +/- 48 nEq/min per g, and in heart was 272 +/- 30 nEq/min per g (P < 0.05). However, LPL activity in extensor di gitorum longus (EDL, predominantly type II fibers) was considerably lo wer (7.9 +/- 0.8 nEq/min per g in EXT, P < 0.0001 versus heart and sol eus). LPL immunoreactive mass followed a pattern similar to LPL activi ty. LPL mRNA levels were quantitated by both Northern blotting and rev erse transcriptase-polymerase chain reaction (RT-PCR), and were approx imately equal in heart and soleus, and 5-fold lower in EDL. In respons e to feeding, LPL activity, mass, and mRNA levels in heart were 30% to 50% lower than in fasted rat heart, although feeding had no effect on soleus or EDL. In hypothyroid animals, muscle LPL activity was increa sed by 3- to 4-fold in the HR (but not EXT) fractions of heart and sol eus (P < 0.05), with no change in LPL mass or mRNA. Thus, muscles with oxidative, type I fibers expressed higher levels of LPL mRNA than mus cles containing glycolytic, type II fibers. Feeding affected heart LPL with no effect on soleus or EDL. Soleus and heart from hypothyroid ra ts demonstrated increased HR LPL, with no change in LPL mass or mRNA, suggesting posttranslational regulation by thyroid hormone.