Chick myotubes in culture express high-affinity receptors for calcitonin gene-related peptide

Calcitonin gene-related peptide (CGRP) is a 37-amino acid neuropeptide that is expressed by many neurons of the vertebrate nervous system, including motoneurons of many species. It has been detected immunohistochemically in both cell bodies and motor terminals of motoneurons, suggesting that it may play a role at the neuromuscular junction. In support of this idea, CGRP has been shown to produce a variety of effects on cultured myotubes and muscle explants, including elevation of cAMP levels, increase in cell-surface acetylcholine receptor (AChR) numbers, increase in AChR alpha-subunit mRNA transcript levels, alterations in contractile responses, alterations in the physiological properties of AChRs, and inhibition of insulin-induced changes in glycogen metabolism. CGRP binding sites have been detected in many tissues, but have not yet been demonstrated directly on muscle cells. Here we report that chick myotubes in culture express high-affinity binding sites for CGRP (Kd approximately 2-4 x 10(-10) M). In view of the known biological effects of CGRP on myotubes, we believe that these binding sites represent CGRP receptors. They are uniformly distributed over the surface of myotubes, and we have found no evidence of clustering in culture, in contrast to AChRs. We have found no evidence for more than one class of receptors.