Sodium activity of insect blood: physiological significance and relevance to the design of physiological saline

Abstract

The apparent activity coefficients for sodium (γ ′_Na) in the blood of six insect species have been calculated from measurements made with sodium-selective electrodes and a flame photometer. In every case γ’_Na was significantly lower than that for this cation in free solution (γ _Na). In Periplaneta americana γ ′_Na varied considerably, during a period of 90 days, so that a relatively constant sodium activity (a_Na) was maintained in the blood in the face of large variations in the total sodium content measured by flame photometry. Despite the relative constancy of a_Na (of around 0·088M) appreciable fluctuations were observed in the sodium and potassium content of nervous connectives over a period of 140 days. The values of a_Na and a_K were used to devise a satisfactory cockroach saline for use in experiments with isolated nerve cords.