1,2-Diacylglycerol and ceramide levels in insulin-resistant tissues of the rat in vivo

Abstract

Phorbol esters have been reported to decrease sensitivity or responsiveness to insulin in cells in vitro. Since phorbol esters are analogues of endogenously produced 1,2-diacylglycerol, the present study investigated whether 1,2-diacylglycerol concentration is elevated in insulin-resistant tissues of the rat in vivo. Studies were done on 11-12-week-old genetically obese Zucker rats, which are insulin-resistant. Lean Zucker rats served as controls. Levels of 1,2-diacylglycerol in obese rats were increased 82% in liver, 136% in calf muscles, 72% in soleus muscle, a slow-twitch muscle, and 40% in plantaris muscle, a fast-twitch muscle. Ceramide levels in the same tissues were increased 26, 52, 69, and 13%, respectively. Studies were also done on normal, non-obese Sprague-Dawley rats 3 h, 1, 3, 8, and 15 days after interrupting the nerve supply to hindlimb muscles. We have previously shown that 3-17 days after denervation, soleus muscles are completely unresponsive to insulin and do not increase glucose uptake in response to insulin stimulation in vivo, whereas plantaris muscles show a normal glucose uptake when stimulated by insulin; however, the insulin-induced increment in glucose uptake is reduced 68% because it is superimposed on already elevated basal glucose uptake (Turinsky, J. (1987) Am. J. Physiol. 252, R531-R537). In the present study, the denervated soleus muscles exhibited a sustained increase of 23-56% in 1,2-diacylglycerol concentration between 3 h and 15 days after interruption of nerve supply. The denervated soleus muscles also showed 34 and 42% increases in ceramide concentration at 3 and 8 days after denervation, respectively. In contrast, no increases in 1,2-diacylglycerol concentration were observed in plantaris muscles at shorter intervals than 15 days after denervation. Ceramide concentrations in plantaris muscles were increased 43 and 75% at 8 and 15 days after denervation, respectively. These observations demonstrate that tissue insulin resistance is frequently associated with a long term increase in tissue 1,2-diacylglycerol concentration. This suggests the possibility that augmented 1,2-diacylglycerol levels contribute to the development of some types of tissue insulin resistance.