DISTRIBUTION AND KINETICS OF COEDTA IN SMOOTH MUSCLE AND ITS USE AS AN EXTRACELLULAR MARKER

1. [60Co]EDTA has been evaluated as an extracellular marker in guinea‐pig taenia coli and rabbit myometrium. 2. The complex was found to be stable, non‐toxic, convenient to use and had the advantage of being readily measured without interfering with the analysis of water and electrolyte contents. 3. In both smooth muscles [60Co]EDTA distributed rapidly into a volume equivalent to that available to [14C]sorbitol and slightly greater than that available to inulin. 4. The optimum concentrations of carrier required to effectively saturate any adsorption sites were 0·1 m M‐CoEDTA for taenia coli and 1·0 m M for myometrium. 5. [60Co]EDTA was distributed into a volume equivalent to the total water content of rabbit tendon, indicating that it was not excluded from connective tissue water. 6. About 5% of the [60Co]EDTA taken up by smooth muscle in 15 minutes exhibited slow efflux kinetics. This property was also exhibited by [14C]sorbitol and has been reported for other extracellular markers. 7. The efflux of 95% of [60Co]EDTA followed bulk diffusion kinetics for both taenia coli and myometrium. 8. The simultaneous efflux of 24Na indicated that 1‐2% exchanged slowly while 98‐99% of the 24Na exchange could be described by bulk diffusion kinetics, similar to [60Co]EDTA. About 15% of this 24Na was contained outside of the [60Co]EDTA space. 9. More detailed studies involving the analysis of simultaneous efflux of [60Co]EDTA and 24Na may help to locate and characterize the properties of the excess rapidly exchanging sodium (12‐15 m‐equiv/kg. wet wt.) in smooth muscle. © 1969 The Physiological Society