Responses of β-adrenoceptors in gastrocnemius of Japanese tree frogs to space night and in rat soleus and extensor digitorum longus (EDL) to hindlimb suspension were investigated. Furthermore, changes in β-adrenoceptors were investigated in these rat muscles when creatine was depleted chronically by feeding creatine analogue β-guanidinopropionic acid. The maximum binding capacity (Bmax) and the dissociation constant (Kd) were analyzed autoradiographicaIIy by using
125I-iodocyanopindolol. Most of the subtypes of β-adrenoceptors in frog gastrocnemius and rat soleus and EDL were β2. The Bmax was highest in rat soleus and lowest in frog gastrunemius. The Bmax was lower in muscles exposed to space flight for 8 days and/or hindlimb suspension for 10 days relative to the respective controls. On the contrary, the Bmax was increased by creatine depletion which stimulates mitochondrial biogenesis due to the lowered high-energy phosphates. It was suggested that the changes in Bmax is due to the alteration of the receptor number, not the affinity, since the Kd values did not change significantly. It was also suggested that the number of β-adrenoceptors in skeletal muscles increases when mitochondrial biogenesis or enzyme synthesis is stimulated and
vice versa.
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