Synthesis and use of radio cobaltic EDTA as an extracellular marker in rabbit heart

Abstract

A new gamma-labeled marker for extracellular space is the cobaltic form of 58Co-ethylenediaminetetraacetic acid (58Co-EDTA). The cobaltic ion has a much higher affinity for EDTA than the cobaltous ion; it is prepared as a potassium salt, K+(58Co3+-EDTA4-), and is apparently biologically inert. Testing by equilibration in intact rabbits and comparing the myocardial content with that of [14C]sucrose give values of the volume of distribution in the myocardium of 0.294 +/- 0.052 ml/g for 58Co-EDTA and 0.303 +/- 0.051 ml/g for [14C]sucrose (SD, n = 130, for two hearts), with the ratios of 58Co-EDTA/sucrose averaging 0.973 +/- 0.043 (n = 130). The average value of the extracellular fluid measured in isolated rabbit interventricular septum using Co-EDTA was 0.51 +/- 0.05 ml/g (SD, n = 16) and 0.46 +/- 0.04 ml/g using [14C]sucrose as an extracellular fluid space (ECF) marker. Flushing with a high concentration of nontracer Co-EDTA does not reveal any release from binding sites. The gamma-energy (811 KeV), long half-life (71.4 days), stability, and lack of binding to tissue components make 58Co-EDTA a useful marker for ECF.

FIG. 1

Disappearance of both ⁵⁸Co-EDTA (closed circles) and [¹⁴C]sucrose (open circles) from plasma in intact rabbit. C(t), plasma cpm at time t; C(t = 5 min), plasma cpm 5 min after injecting bolus of radioactivity. Extraction of these isotopes from plasma appears to cease after 20 min.

FIG. 2

Probability density functions of myocardial volumes of distribution for 2 extracellular markers, ⁵⁸Co-EDTA (solid line) and [¹⁴C]sucrose (broken line) in intact rabbit hearts. Area under each curve is unit. Mean ± SD was 0.294 ± 0.052 for ⁵⁸Co-EDTA and 0.303 ± 0.051 for [¹⁴C]sucrose.

FIG. 3

Scatter diagram of volumes of distribution of [¹⁴C]sucrose and ⁵⁸Co-EDTA in 2 intact rabbit hearts. Regression is V_suc = 1.001 ± 0.009 V_{Co EDTA} (SE) with correlation coefficient of 0.974 and SD around mean of 0.014. Line of identity is that expected of 2 markers having identical volumes of distribution.

FIG. 4

Uptake of ⁵⁸Co-EDTA from modified Tyrode solution containing Co-EDTA at concn of 1 mM (25°C). Ordinate normalized for maximum uptake.

FIG. 5

Uptake of ⁵⁸Co-EDTA from modified Tyrode solution at 25°C. Initially concn of nontracer Co-EDTA was less than 8 × 10⁻⁶ M. At 75 min concn of Co-EDTA was increased to 1 mM. Concn of ⁵⁸Co-EDTA remained constant throughout expt. Ordinate normalized to last point.

FIG. 6

Uptake of ⁵⁸Co-EDTA for 80 min at 37°C. Ordinate normalized for maximum uptake.