Continuous servo-controlled replacement of urinary sodium loss in conscious dogs

A method for continuous servo-controlled infusion of sodium chloride to dogs is reported. The servo system consists of a sodium-sensitive electrode, a modified commercial flowmeter, a thermistor, a control unit, and a pump. Based on analog inputs from electrode, flowmeter, and thermistor, the control unit generates appropriate numbers of voltage steps, which drive the pump infusing a concentrated solution of sodium chloride. Inputs from the thermistor are necessary to correct for the influence of fluctuations in urine temperature. The sodium servo system has been tested together with a weight servo mechanism in conscious water-diuretic dogs. Furosemide (1 microgram.kg-1.min-1) was infused to change the rate of sodium excretion by a factor of 17 from 3 +/- 1 to 50 +/- 15 mumols/min. Within this range, the apparatus replaced the sodium losses with great accuracy. From seven experiments the average system response in micromoles per minute (NaIn) measured in 10-min periods against mean renal excretion of sodium (NaEx) was NaIn = 0.98.NaEx - 1.37. The servo system provides a tool for accurate maintenance of body sodium irrespective of large changes in the rate of sodium excretion.