Interstitial Fluid Calcium, Magnesium and Phosphorus Concentrations in Bone, Muscle and Subcutaneous Tissue Sampled with Ultrafiltration Probes

probes has provided a tool for investigating the chemistry of the interstitial space. It has provided data which have expanded the understanding of both physiological and pathological conditions. Under a contract with NASA, we developed ultrafiltration probes which could be implanted in bone, muscle and subcutaneous tissue to sample the interstitial fluid. In this project, sheep were used as the animal model. This permitted the investigation of calcium, magnesium and phosphorus concentrations in locations not previously accessible to direct in vivo measurement. The goal of this project was to develop tools to study mineral metabolism. These tools will facilitate the development of a better understanding of bone physiology and pathology. Calcium, magnesium and phosphorus are the minerals present in the largest quantities in bone and are important in giving strength and shape to the bone. However, they have important physiological functions in other parts of the body as well. Calcium is an essential element in intercellular regulation and metabolism. Extracellular calcium is an essential component of cofactors required for bone formation, blood clotting, adhesions molecules, and as a first messenger in signaling functions (1). Many of its actions within cells are dependent on a constant extracellular pool of available calcium. The parathyroid hormoneVitamin D-calcitonin system and the parathyroid gland “set point” tightly regulate extracellular calcium concentrations. Magnesium is involved in neuromuscular transmission and is a cofactor in various enzyme reactions. It is important in ribosomal protein synthesis and ATP energy transfer. Phosphorus is ubiquitous throughout the body. Phosphates are a major component of the blood buffering system. Phosphorus is found in DNA, RNA, proteins and phospholipids and is important in energy metabolism. It therefore affects all aspects of physiology (2). One of the benefits of membrane probe sampling is that different tissues can be sampled simultaneously. This provides a tool to study the differences in distribution of these minerals. In this study, calcium, magnesium and phosphorus concentrations were compared in the different tissues and in blood. Calcium exists in blood in three forms: protein bound, complexed and ionized. Magnesium is also found in the blood in the protein bound or ionized forms. Complexed and ionized calcium can pass though the probe membrane but protein bound calcium cannot. The recovered calcium is called the ultrafilterable calcium. Ultrafi l trate membrane probes can also be used in vitro to separate protein bound calcium and magnesium from other forms of these minerals in blood. Interstitial Fluid Calcium, Magnesium and Phosphorus Concentrations in Bone, Muscle and Subcutaneous Tissue Sampled with Ultrafiltration Probes