Norepinephrine-induced calcium signaling and expression of adrenoceptors in avian tendon cells

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Abstract
Sympathetic efferent nerves are present in tendons, but their function within tendon is unknown. α1-Adrenoceptors are expressed by a variety of cell types. In the presence of norepinephrine (NE), adrenoceptors activate Gq/11signaling pathways that subsequently increase intracellular Ca2+concentration ([Ca2+]ic). It was hypothesized that avian tendon cells express functional adrenoceptors that respond to NE by increasing [Ca2+]ic. Avian tendon cells were analyzed for mRNA expression of α1-adrenoceptors by RT-PCR. Avian tendons expressed the α1A- and α1B-adrenoceptor subtypes. Furthermore, both tendon surface epitenon cells and internal fibroblasts infused with a Ca2+-sensitive dye, fura 2, and stimulated with NE responded by increasing [Ca2+]ic. KMD-3213, an α1A-adrenoceptor antagonist, significantly reduced the Ca2+response. Other adrenoceptor antagonists had no effect on the Ca2+response. The absence of extracellular Ca2+also significantly reduced the response to NE, indicating that Ca2+influx contributed to the rise in [Ca2+]ic. This study provides the first evidence that tendon cells express adrenoceptors and that the NE-induced Ca2+response is coupled to the α1A-adrenoceptor subtype.

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