Strontium-85 extraction during transcapillary passage in tibial bone

Area of Science:

• Physiology
• Biophysics
• Pharmacokinetics

Background:

• Understanding solute transport across bone capillaries is crucial for drug delivery and understanding bone physiology.
• Strontium-85 (85Sr) is a radiostrontium isotope used to study bone metabolism and transport.

Purpose of the Study:

• To determine the extraction fraction and capillary permeability of strontium-85 (85Sr) in the tibial diaphysis.
• To investigate the relationship between blood flow and 85Sr extraction in bone capillaries.

Main Methods:

• Indicator dilution technique using 85SrCl2 and T-1824-labeled albumin as tracers injected into the nutrient artery.
• Measurement of venous dilution curves and direct isotope counting of tibial 85Sr content.
• Estimation of tibial diaphyseal blood flow using iodoantipyrine washout curves.

Main Results:

• Maximal instantaneous extraction of 85Sr was 0.53 +/- 0.08, with a net retention of 0.41 +/- 0.06 after 10 minutes.
• Tibial 85Sr extraction was approximately 0.53 +/- 0.28 after intravenous injection.
• Permeability-surface area product (PS) for bone capillaries increased with blood flow, averaging 0.63 +/- 0.29.

Conclusions:

• The capillary membrane acts as the primary barrier to 85Sr and other small hydrophilic solute passage in the tibial diaphysis.
• Bone capillary extraction of 85Sr is significant and influenced by blood flow, suggesting capillary recruitment.
• These findings have implications for understanding the pharmacokinetics of bone-seeking radiopharmaceuticals and therapeutic agents.