Strontium-85 extraction during transcapillary passage in tibial bone

Abstract

Indicator dilution experiments were done to determine the extraction of 85Sr during a single passage through capillaries of the tibial diaphysis. Extraction was estimated by injection of 85SrCl2 and a nonpermeant, reference tracer, T-1824-labeled albumin, into the nutrient artery and recording of the effluent venous dilution curves (femoral vein). The mean (+/- SD) maximal instantaneous extraction was 0.53 +/- 0.08 (N = 12). Net retention after 10 min, estimated from venous curves, was 0.41 +/- 0.06 (N = 12), which appeared not substantially different from the retention estimated by direct isotope counting of the tibias for 85Sr, 0.35 +/- 0.06 (N = 12). In a second set of experiments in intact animals, tibial 85Sr extraction after intravenous injection was apparently higher, 0.53 +/- 0.28 (N = 15). Values of tibial diaphyseal blood flow, estimated from washout curves for iodoantipyrine after tibial nutrient artery injection, were 1.47 +/- 0.63 ml/min per 100 g (N = 27). The extraction was not much diminished by higher flows. The estimates of permeability-surface area product (PS) for bone capillaries did increase with flow, suggesting recruitment of more capillaries at higher flows. PS values averaged 0.63 +/- 0.29 (N = 12); we conclude that the capillary membrane is a primary barrier to the passage of 85Sr and presumably other small hydrophilic solutes.

FIG. 1

Normalized femoral vein concentration-vs.-time curves for T-1824 [h_N(t)] and Sr [h_Sr(t)] obtained after injection into the dog tibial nutrient artery. Instantaneous extraction, E(t), and net extraction, E_net(t), of ⁸⁵Sr were calculated by Eq. 5 and 6.

FIG. 2

Relationship of ⁸⁵Sr extraction, E_max, to flow, F_s. 1 – E_max is plotted on log scale versus the reciprocal of Sr-containing plasma or solvent flow. Encompassing and midrange theoretical lines of constant PS are shown. PS values tend strongly to increase as F_s increases.

FIG. 3

Theoretical clearance-vs.-blood flow curves at constant PS (solid lines) with experimentally measured (IAp) solvent flow data and ⁸⁵Sr clearance. The latter was determined both by measurement in the arteriovenous concentration difference group (open circles) and by calculation using F_s and E_max from the instantaneous extraction group (solid circles). In both sets of experimental data, Cl-F relationships tend to be more linear than are the theoretical curves, indicating an increase in PS with flow increase. Clearances are less than flow because extraction is incomplete.

FIG. 4

Net ⁸⁵Sr extraction [E_net(10 min)] versus estimated blood flow (F_IAp). Flow has a minimal effect on extraction. The regression equation is E_net(10 min) = 0.44 − 0.22 F_IAp (r = 0.20).