THE VALIDITY OF THE CALCULATION OF SECRETION RATES FROM THE SPECIFIC ACTIVITY OF A URINARY METABOLITE

The validity of the calculation of secretion rates from the specific activity of a urinary metabolite after the injection of the radioactive hormone (secretion rate equals radioactivity injected divided by the specific activity of a urinary metabolite ) has been critically examined. Although in previous applications of the method the expression has been assumed to be valid on an intuitive basis, it is concluded that this is not justified unless after continuous infusion the specific activity of the hormone is constant throughout the body. In the case of a single injection, where W has been termed the single injection factor, and must be equal to one for the method to be valid. If the transport and metabolism of the hormone can be described in terms of an outer and inner pool, it has been shown that the single injection factor is nearly equal to one unless the metabolite which is analyzed is formed mostly in one pool but the overall metabolism largely occurs in the other space.Other assumptions and sources of error in the method are discussed. It is concluded that a comparison of the specific activities of various metabolites is the most generally applicable experimental test of the validity of the method. On this basis, applications of the method to obtain the secretion rates of aldosterone and cortisol seem to be justified.