A method for measuring the spatial distribution of tissue oxygen removal rates

Micromeasurement of total and regional CO2 productions in the one-day-old chick embryo

A conductometric micromethod combined with image analysis system has been developed allowing to determine the CO2 production within 'two-dimensional' tissues, i.e., flat and thin cell layers or epithelial sheets. The preparation was mounted into an airtight chamber separated in two compartments by a thin silicone membrane permeable to gases. The lower compartment contained the nutritive medium and the preparation. The upper compartment and a conductivity measuring capillary connected in series were perfused with a solution of Ba(OH)2. The CO2 produced by the tissue precipitated as BaCO3 and the resulting decrease of electrical conductivity was linearly related to the total CO2 production. In addition, the pattern of CO2 production was directly observable as the BaCO3 crystals formed upon the silicone membrane over the regions which produced CO2. The spatial distribution of the crystals was quantified by video image processing and the regional CO2 production evaluated with a spatial resolution of 100 microns. This new microtechnique was originally developed to study the CO2 production in the early chick blastoderm which is a disc 1-5 cells thick. At the stage of young neurula the CO2 production was found to be 235 +/- 37 nmol.h-1 (mean +/- SD; n = 10) per blastoderm and large variations of local CO2 production were detected from one region to another (from 0.6 to 6.5 nmol.h-1.mm-2). These results indicate a high metabolic and functional differentiation of cells within the blastoderm. The possible applications and improvements of such a microtechnique are discussed.

The diffusion coefficient of oxygen in respiring kidney and tumour tissue

The diffusion coefficient and permeability, together with the oxygen uptake rate, have been measured in slices of respiring tissue. Values of these quantities are given for mouse kidney cortex and for implanted tumours of Lewis lung carcinoma in the mouse, and the fibrosarcoma SSB1a in the rat. The diffusion coefficient at 22 degrees C is 1.3 . 10(-5) . cm2 . s(-1) in the kidney, and has the higher value of 1.6 . 10(-5) . cm2 . s(-1) in both tumours. The solubility in all three tissues is similar to that in saline.

The accuracy and spatial resolution of the measurement of tissue oxygen removal rate

An analysis is presented of factors determining the accuracy and spatial resolution of a method for the estimation of oxygen removal rate in tissues using a multi-cathode electrode placed on the tissue surface. The effect of oxygen consumption by the cathodes is shown to be small for 25-micrometer diameter cathodes, but covering the tissue surface with a thin oxygen oxygen-permeable plastic film can introduce a systematic error into the estimate of removal rate, the size of which depends upon the oxygen diffusion coefficient and solubility in the plastic compared with those in the tissue. Spatial resolution is shown to be adequate for distinguishing changes in removal rate over distances of a few hundred microns, and the averaging of removal rate in the direction normal to the tissue surface is weighted heavily towards the superficial tissue layer.

Permeability and diffusion coefficient of oxygen in membranes for oxygen electrodes

Oxygen diffusion coefficient and permeability in thin membranes of low-density polythene and laminated polytetrafluorethylene (PTFE) are measured over the temperature range of about 20 to 30 degrees C. The diffusion coefficient is estimated from the time response of a large area platinum electrode covered with the membrane material and exposed to a sudden change in PO2. outside the membrane. The electrode is operated at a very low polarising voltage so that consumption of oxygen at the cathode surface is made negligible. This method is considered to be more reliable than that based on operating the electrode at more normal polarising voltages at which the boundary condition for oxygen tension at the cathode surface is often ill-defined. Oxygen permeability is estimated from the flux of oxygen across an area of membrane separating a nitrogen gas stream from one containing oxygen, the oxygen being measured with a platinum/zirconium oxide reversible cell. Diffusion coefficient in the membranes is very temperature dependent, whereas oxygen solubility is only weakly so. The diffusion coefficient at 25 degrees C is 0.28 +/- 0.06 . 10(-6) and 1.0 +/- 0.2 . 10(-6) cm2 . sec-1 for PTFE and polythene respectively. The corresponding values for permeability are 0.80 +/- 0.08 . 10(-7) and 0.43 +/- 0.04 . 10(-7) ml O2 atm-1 . cm-1 . sec-1.

Comparison between the oxygen removal rate and the histological structure of normal and tumour tissues

A technique has been used to measure distributions of oxygen removal rate, QR, in samples of recently excised tissue where QR is defined as the ratio of the oxygen uptake rate to the oxygen solubility. QR was measured at 91 sampling points across a cross section of tissue of diameter 5 mm. By placing markers in the tissue and using special procedures to obtain sections of the tissue samples it was possible to compare the distributions of QR with the histological structure of the tissues. The overall resolution of the system is about 1/2 mm so that structures of the order of a millimetre or more could be examined. There are considerable inhomogeneities in QR within some normal tissues, for example 4-19 mm Hg/s in rat kidney and 0.4-1.5 mm Hg/s in human cervix. Inhomogeneities in tumour tissues are also associated with the histological structure and examples for the Lewis lung mouse carcinoma show a correlation between QR and regions of viable or necrotic tissue. For a biopsy sample of human carcinoma of the cervix QR correlates with the degree of local infiltration by tumour cells.

Independence of the radiobiological oxygen constant, K, and the respiration rate of mammalian cells

The radiosensitivity, and the radiobiological oxygen-constant K for mouse Ehrlich ascites cells depend on the pre-irradiation growth conditions. Measurements of the oxygen-consumption rate of cells grown by different methods have been made to ascertain whether the differences in radiobiological properties could be associated with different rates of respiration. Inhibition of respiration by sodium amytal had no significant effect on either the radiosensitivity or the estimate of K.