Phosphate deposition during and after hypokinesia in phosphate supplemented and unsupplemented rats

The objective of this study was to show that prolonged restriction of motor activity (hypokinesia) could reduce phosphate (P) deposition and contribute to P loss with tissue P depletion. To this end, measurements were made of tissue P content, P absorption, plasma P levels, urinary and fecal P excretion of rats during and after hypokinesia (HK) and daily phosphate supplementation. Studies were conducted on male Wistar rats during a pre-hypokinetic period, a hypokinetic period and a post-hypokinetic period. All rats were equally divided into four groups: unsupplemented vivarium control rats (UVCR), unsupplemented hypokinetic rats (UHKR), supplemented vivarium control rats (SVCR) and supplemented hypokinetic rats (SHKR). Bone and muscle P content, plasma intact parathyroid hormone (iPTH) levels, P absorption, plasma P levels and urinary and fecal P excretion did not change in SVCR and UVCR compared with their pre-HK values. During HK, plasma P levels, urinary and fecal P excretion increased significantly (p<0.05) while muscle and bone P content, P absorption and plasma iPTH levels decreased significantly (p<0.05) in SHKR and UHKR compared with their pre-HK values and the values in their respective vivarium controls (SVCR and UVCR). During the initial 9-days of post-HK, plasma, urinary and fecal P levels decreased significantly (p<0.05), and plasma iPTH levels, muscle and bone P levels remained significantly (p<0.05) depressed in hypokinetic rats compared with their pre-HK values and the values in their respective vivarium control rats. By the 15th day, these values approached the control values. During HK and post-HK, changes in P absorption, plasma iPTH levels, and P levels in muscle, bone, plasma, urine and feces were significantly (p<0.05) greater in SHKR than in UHKR. Decreased tissue P content with increased P loss in animals receiving and not receiving P supplementation demonstrates decreased P deposition during HK. Higher P excretion with lower tissue content in SHKR and UHKR demonstrates that P deposition is decreased more with P supplementation than without. Because SHKR with a lower tissue P content showed higher P excretion than UHKR it was concluded that the risk of decreased P deposition with greater tissue P depletion is inversely related to P intake, that is, the higher the P intake the greater the risk for decreased P deposition and the greater tissue P depletion. It was shown that P (regardless of the intensity of its tissue depletion) is lost during HK unless factors contributing to the decreased P deposition are partially or totally reversed. It was concluded that dissociation between (decreased) tissue P content and (increased) P uptake indicates decreased P (absorption and) deposition as the main mechanisms of tissue P depletion during prolonged HK.