The effects of Calcitonin Gene-Related Peptide (CGRP) on skeletal muscle with respect to lipid metabolism

Abstract

The neuro-peptide calcitonin gene-related peptide (CGRP) and the endocrine hormone amylin share approximately 50% amino acid sequence homology, and have many biological effects in common. Both oppose the effects of insulin in skeletal musele, via mechanisms that may include the induction of lipolysis. This lipolysis may be a cause of insulin resistance, and thus the pathology of diabetes mellitus type 2 (DMT2). CGRP acts as a neuronal messenger at the motor end plate in skeletal muscle, while amylin is co-secreted with insulin from pancreatic islet β-cells. Results presented in this thesis show that CGRP α evokes lipolysis and release of free fatty acids in isolated skeletal muscle, at concentrations of 1 pM to 100 pM, which are similar to those present in blood or muscle tissue. These concentrations are significantly lower than those that elicit insulin resistance and inhibition of glycogen synthesis in isolated muscle preparations (Leighton and Cooper 1988; Leighton, Foot et al. 1989). The identity of the receptor for this 'high sensitivity effect' is unknown but evidence presented in this thesis suggests that it is likely coupled to a cAMP-dependent process. A second process becomes apparent at 1 nM to 100 nM, which occurs at similar concentrations of CGRP α and β and amylin. This second mechanism is likely to be effected through an amylin/CGRP receptor such as that produced by molecular RAMP-C1ins- interactions; this receptor is known to mediate the effects of the hormones on carbohydrate metabolism. High fat feeding of animals results in a two-fold elevation of triglyceride and free fatty acid in soleus muscle. Subsequent incubation of this skeletal muscle with picomolar doses of CGRP α, results in normalization of triglyceride compared to normal muscle; the receptors that mediate these effects are likely to be the same in both normal and insulin resistant muscle. In both the high fat- and standard chow- fed animal models the ability of both CGRP α (at both 'high' and 'low' concentrations) and amylin (high concentration only) to raise intramuscular FFA can be blocked at the receptor and post receptor levels. The specific amylin and CGRP receplor antagonists reduced FFA increments evoked by both amylin and CGRP α. In addition, the lipoxygenase inhibitor, Masoprocol, significanlly Inhibited the ability of CGRP α to raise FFA and decrease triglyceride in the soleus muscle of high fat fed animals. Infusion studies in normal rats confirmed effects of CGRP α on lipid content in oxidative muscle as well as corresponding effects in plasma and liver. Infusion of the CGRP antagonist, CGRP a-(8-37), resulted in opposite effects from those of CGRP α on liver, muscle and plasma; this is consistent with syntopic antagonism. Co-infusion of the CGRP antagonist with CGRP α inhibited any effects on tissue lipid content.