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An experimental model was developed to study the kinetics of electrolytes under different physiological and/or pathological conditions. The model was applied to investigate in vivo the effect of a pharmacological dose of melatonin 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 of 8 rats, the electrolytes were measured by a flow-through system with highly sensitive ion-selective electrodes. The results were compared to a control group (n=8) which was treated with diluent (saline). The electrolytes were monitored continuously via an extracorporeal circulation, on-going for at 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 concentration did not increase significantly. The extracorporeal circulation provoked an elevation of K+ by hemolysis. This K+ increase was slightly diminished by melatonin (p<0.06). No melatonin effects were seen on Na+, pH and magnesium in blood and plasma, respectively. Also, the urine concentrations of the electrolytes were not altered by melatonin. The mechanism by which melatonin influences the blood concentrations of ionized calcium and potassium is not yet understood.

A rat model is introduced which enables investigations in anticoagulated blood with continuous measurements by a flow-through electrode system. In the present study, a potentiometric ion-selective electrode (ISE)-system was used for measuring Ca2+, K+, Na+ and pH in rats. The setup was adjusted to an extracorporeal blood-volume of 0.750 ml. This permits indirect measurements of the analytes via a dialysis membrane, with electrical separation of the ISE's and the animal. The flow-rates of blood and dialysis-solution were adjusted in such a way that water diffusing from the aqueous dialysis solution into the blood, across the dialysis membrane, does not alter the haematocrit. Polyethyleneglycol-hirudin was used for anticoagulation, since it was superior to heparin. The assembly enables continuous measurements in the living anaesthetized rat over a time period of at least 3 hours.

Monitoring the ionized magnesium (Mg2+) concentration in critically ill patients can prevent development of serious and potentially fatal complications. The analyzers KONE Microlyte 6 (KONE Instruments, Espoo, Finland) and NOVA CRT (NOVA Biomedical, Waltham, MA, USA) provide the discontinuous measurement of Mg2+ and were evaluated in several studies. It was our objective to integrate the Mg2+-selective electrodes into a device for continuous on-line measurements. This device is suitable not only for research but also for a specific evaluation of electrode characteristics. It allowed us to compare the genuine electrodes and reference systems independently of their specific analyzers. Precision, accuracy, response time, limit of detection, drift and interferences were assessed by continuous flow-through measurements and discussed in comparison to the results of previous studies. The NOVA electrode proved to be superior regarding accuracy, sensitivity and selectivity, especially with respect to calcium. It was demonstrated that current commercial serum-like control materials were not appropriate for quality control of the assessed Mg2+-electrodes. However, despite the fact that the electrodes are commercially used for discontinuous measurements, both sensor types can be used for continuous on-line measurements in an extracorporeal circulation in a rat model. The NOVA electrode showed superior characteristics with this application as well. This study should also be understood as a contribution to the development of devices for online analyzers used in point-of-care-testing.