BICARBONATE DEPENDENCE OF ION CURRENT IN DAMAGED BONE

The aim of this work was to characterize the ion current that enters m ouse metatarsal bones following damage to the cortex. We assessed both the spatial distribution of this current and its dependence on the pr esence of bicarbonate in the medium. We used a voltage-sensitive probe system vibrating in two dimensions and recorded the signal as functio n of the position of the probe with respect to the site of damage and of ion substitutions in the medium. When the cortex was damaged (50 mu m cylindrical hole penetrating into the marrow cavity), we recorded a steady state net inward electrical current directed toward the site o f damage. in nonbicarbonate media, the density of the current was maxi mal near the center of the hole and ranged from 6 to 18 mu A/cm(2). As the probe was moved off the center of the hole, measured current dens ity decreased in a manner consistent with the hypothesis that the sour ce of the inward current is localized to the hole. After changing bica rbonate concentration in the medium from 0 to 42 mM, the current densi ty nearly doubled, then decayed back to its original level exponential ly over 35 minutes. When the diaphysis of living bone was left intact the current density was close to background level either in the presen ce or absence of bicarbonate in the medium. Damaged dead bone did not drive any current higher than background level. We conclude that the v ibrating probe technique is a powerful tool to characterize ion curren ts in injured bone, helping to understand the physiology of bone-plasm a interface and the bone healing processes. The current density transi ently doubled upon addition of bicarbonate, indicating that this ion m ay carry the electrical current in damaged bone, probably by pump-leak mechanisms operating at the bone-plasma interface.