Total exchangeable sodium in the menstrual cycle

Abstract

T HE purpose of the study was to do serial measurements of total c~~hangeable body sodium in women throughout the menstrual cycle to determinc~ whether there is a sodium retention during normal menstrual cycles. It is common belief that there are changes in body sodium content in pregnancy, tosemia of pregnancy, the premenstrual tension syndrome,g and perhaps also in the normal menstrual cycle. The intent was to establish normal control values and ranges for body sodium content, and to determine the daily pattern of body sodium for possible changes concomitant with the endocrinologic changes of the normal menstrual cycle. Serial measurements of total exchangeable hod? sodium were made on a daily basis with the use of isotope dilution techniques. Sodium metabolism can be studied by metabolic balance methods, or, as has been done more recently, by isotope dilution techniques.4, 16-19 When a known amount of radioactive isotope is administered, it exchanges or is diluted by the naturally occurring element in the body, and the total amount of bod3 element with which the tracer isotope has exchanged or mixed can bc calculated by the dilution principle. That is, if the concentration of radioactive isotope is known or measured before and after dilution in the body, t,he volume. or weight of the diluting substance can be calculated. The basis of the method is the chemical dilution principle that the weight or mass of the diluted solute or tracer substance remains constant before and after dilution, expressed by the formula C,V, = &V,, where C and V represc>nt concentration and volume before and after dilut,ion. The method is suitable for volume or ‘ ‘space, ’ ’ and solid or weight studies using either dyes ot’ isotopes, and was first used by Keith, Rowntrec, and Geraghty” in 1915 for dye studies measuring plasma volume. The method has been popularized in the United States by F. D. &loore and his co-workrrs. Dilution studies with the radioactive isotopes are based on observations that the tracer isotopes behave chemically and physically in the body in a maw ner identical with that of the natural isotopes of an element, except that a proportion of the radioactive atoms will (lisintegl-ale with the cmissiwl of ~xdiatiolr which can be detected, measured, and rc~c’otde(l.“”