On-line analysis of electrolytes in extracorporeally circulating blood: Application of a rat model to examine the effect of a single pharmacological dose of melatonin on electrolyte levels in blood

An experimental model was developed to study the kinetics of electrolytes u nder different physiological and/or pathological conditions. The model was applied to investigate in vivo the effect of a pharmacological dose of mela tonin on the concentrations of Ca2+, K+, Na+, and pH in the anticoagulated blood of anaesthetized male Wistar rats (250-350 g). After the application of 0.25 mg melatonin/kg body weight, injected intraperitoneally into each o f 8 rats, the electrolytes were measured by a flow-through system with high ly sensitive ion-selective electrodes. The results were compared to a contr ol group (n=8) which was treated with diluent (saline). The electrolytes we re monitored continuously via an extracorporeal circulation, on-going for a t least 60 min. Melatonin induced a significant increase of blood Ca2+ (p<0 .02) by an average of 9.9% after 60 min. However, total calcium concentrati on did not increase significantly. The extracorporeal circulation provoked an elevation of K+ by hemolysis. This K+ increase was slightly diminished b y melatonin (p<0.06). No melatonin effects were seen on Na+, pH and magnesi um in blood and plasma, respectively. Also, the urine concentrations of the electrolytes were not altered by melatonin. The mechanism by which melaton in influences the blood concentrations of ionized calcium and potassium is not yet understood.