Responses of β-adrenoceptor in rat soleus to phosphorus compound levels and/or unloading

Responses of β-adrenoceptor (β-AR) in rat soleus to gravitational unloading and/or changes in the levels of phosphorus compounds by feeding either creatine or its analogue β-guanidinopropionic acid (β-GPA) were studied. A decrease in the density of β-AR (about -35%) was induced by 10 days of hindlimb suspension, but the affinity of the receptor was unaffected. Suspension unloading tended to increase the levels of adenosine triphosphate and phosphocreatine and decrease inorganic phosphate. Even without unloading, the β-AR density decreased after an oral creatine supplementation (about - 20%), which also tended to elevate the high-energy phosphate levels in muscle. However, an elevation of β-AR density was induced (about +36%) after chronic depletion of high-energy phosphates by feeding β-GPA (about +125%). Data suggest that the density of β-AR in muscle is elevated if the high- energy phosphate contents are chronically decreased and vice versa. However, it may not be directly related to the degree of muscle contractile activity.