Determination of the extracellular space with nonradioactive Co3+EDTA and simultaneous estimation of Na, K, Ca, and Mg contents in isolated guinea-pig heart preparations by atomic absorption spectrometry.

A nonradioactive assay for the estimation of the size of the extracellular space (ECS) was developed utilizing Co3+EDTA as ECS marker. Co was measured by atomic absorption spectrophotometry. In a Langendorff preparation the compound was pharmacologically inert at a concentration of 3 X 10(-5) mol/l. The size of the ECS was independent of the ECS marker concentration indicating the lack of tissue adsorption. The filling of the ECS is almost completed after 3 min of perfusion. An insignificant slope of the late phase of the uptake curves in guinea-pig Langendorff heart preparations and isolated atria indicated no or very slow uptake into deep compartments. A sufficiently good estimate of the size of the ECS can be found after 15 min of equilibration. Simultaneous measurements of the size of the ECS and of ions (Na, K, Ca, Mg) can be performed within the same sample. Calculations of cellular Na, K, Mg, and Ca concentrations are in reasonable agreement with literature data.