New Approach to Calculation of Renal Cation Clearance in Rats after V1a Receptor Stimulation

V1a receptor agonist [Phe², Ile³, Orn⁸]-vasopressin produces maximum diuretic and saluretic effects in a dose of 0.1 nmol/100 g body weight: renal excretion of Na⁺ increases by 50 times in comparison with the initial level over 90 min, excretion of K⁺, Mg²⁺, and Са²⁺ increases by 3, 2, and 16 times, respectively. New formulas were proposed for calculation of the clearance considering total cation content in the extracellular fluid and the time of total effect of the injected dose. After injection of V1a receptors agonist, clearance of Na⁺ was 7% of its total content in the extracellular fluid, the corresponding values for K⁺, Mg²⁺, and Ca²⁺ were 63, 35, and 6%, respectively. The proposed approach differs significantly from the standard cation excretion evaluation and characterizes selective shifts in ion balance induced by physiologically active substances and possible side effects due to unbalanced loss of certain ions.