The design of experiments using isotopes for the determination of the rates of disposal of blood-borne substrates in vivo with special reference to glucose, ketone bodies, free fatty acids and proteins

Abstract

1. The two well-known methods of estimating rates of irreversible disposal (R) of blood-borne substrates in vivo by isotope experiments involve estimating the specific radioactivity (S) of the substrate in blood either after single intravenous injection of labelled substrate or during its infusion at a constant rate. The value of R is calculated from the S-time curve, usually by assuming: (i) a metabolic steady state with respect to substrate, (ii) the passage of all substrate through the blood, and (iii) the absence of certain types of recycling via blood. 2. In a theoretical investigation we show how experiments can be performed and R calculated from analyses of blood when one or more of the above assumptions is unjustified, by using glucose, ketone bodies, plasma free fatty acids and proteins as examples. In general the methods require single injection procedures, with estimation of the total quantity of label in the substrate in blood and the substrate concentration instead of only S. Such values give estimates of R with standard errors even when only one blood specimen is taken from each of a group of animals, as is convenient when working with small animals or substrates in low concentration, and when the animals are in a non-steady state in which constant infusion procedures are invalid. 3. Similar methods give the fraction of label injected as one compound which passes through another (the isotopic yield). 4. The methods are not always applicable, and cannot be applied to plasma proteins in some pathological conditions. A questionnaire for assessing their applicability is given.

The estimation of rates of utilization of glucose and ketone bodies in the brain of the suckling rat using compartmental analysis of isotopic data

“…(1) amino acids and total non-amino acids. In each amino acid fraction the quantities of label were more constant at each time and fell on smoother curves than the specific radioactivities, indicating, as shown by Heath & Barton (1973), that the rate of utilization in the citrate cycle was more nearly proportional to ketone-body concentration in blood than it was Table 1, column 2. independent of it. Consistent with this conclusion the simplifying assumption of strict proportionality was made for all calculations.…”

“…As its effects on the estimates of rates proved 1974 acetate] was 3.10±0.18 (s.E.M., n = 7), and a value of 3.0 has been used throughout. The quantity of label in ketone bodies in blood (qk) was very much less variable than the specific radioactivity (F test on variance of replicates, P<0.01) as was to be expected if the rate of uptake of label from the peritoneal cavity was independent of the concentration of ketone bodies in the blood and if the rate of disposal of blood ketone bodies in the whole animal was roughly proportional to concentration (see Heath & Barton, 1973, for a discussion of this point), as it is in adults . Therefore only theqk-t curve is shown (Fig.…”

Effects of metformin on insulin resistance after injury in the rat

“…The clearance rate coefficient, k [ml whole blood, min -1. (100 g)-l], was assumed to remain constant during the experiment and calculated using equation (4.7) of Heath and Barton [25] with M = 1. The rate of glucose utilization was calculated as the product of k and the mean blood glucose concentration in each rat and is therefore a mean figure over the experimental period.…”

The calculation of errors and the choice of sampling times in the determination of irreversible disposal rates in vivo by isotope experiments

“…These are solely dependent on the variances of S or q. Heath & Barton (1973) proposed an approximate method for calculating the random errors from these variances. Their method can, however, sometimes be seriously misleading, and the purpose ofthe present paper is first to present a more rigorous, Gaussian method, the principles of which are well expounded by, among others, Deming (1964).…”

“…The rates are simple functions of the areas to infinity under the specific radioactivity-time (S-t) curves or quantity of plasma label-time (q-t) curves (Gurpide et al, 1963;Mann & Gurpide, 1966;Shipley & Clark, 1972). The method is often applicable even when the metabolic state is not steady, as is usually the case in conscious animals (Heath & Barton, 1973).…”