In vivo demonstration of load-induced fluid flow in the rat tibia and its potential implications for processes associated with functional adaptation

Load-induced extravascular fluid flow has been postulated to play a role in mechanotransduction of physiological loads at the cellular level. Furtherm ore, the displaced fluid serves as a carrier for metabolites, nutrients, mi neral precursors and osteotropic agents important for cellular activity. We hypothesise that load-induced fluid flow enhances the transport of these k ey substances, thus helping to regulate cellular activity associated with p rocesses of functional adaptation and remodelling. To test this hypothesis, molecular tracer methods developed previously by our group were applied in vivo to observe and quantify the effects of load-induced fluid flow under four-point-bending loads. Preterminal tracer transport studies were carried out on 24 skeletally mature Sprague Dawley rats. Mechanical loading enhanc ed the transport of both small- and larger-molecular-mass tracers within th e bony tissue of the tibial mid-diaphysis. Mechanical loading showed a high ly significant effect on the number of periosteocytic spaces exhibiting tra cer within the cross section of each bone. For all loading rates studied, t he concentration of Procion Red tracer was consistently higher in the tibia subjected to pure bending loads than in the unloaded, contralateral tibia, Furthermore, the enhancement of transport was highly site-specific. In bon es subjected to pure bending loads, a greater number of periosteocytic spac es exhibited the presence of tracer in the tension band of the cross sectio n than in the compression band; this may reflect the higher strains induced in the tension band compared with the compression band within the mid-diap hysis of the rat tibia. Regardless of loading mode, the mean difference bet ween the loaded side and the unloaded contralateral control side decreased with increasing loading frequency. Whether this reflects the length of expo sure to the tracer or specific frequency effects cannot be determined by th is set of experiments. These in vivo experimental results corroborate those of previous ex vivo and in vitro studies, Strain-related differences in tr acer distribution provide support for the hypothesis that load-induced flui d flow plays a regulatory role in processes associated with functional adap tation.