An analysis of the regulation of sodium excretion during induced changes in plasma sodium concentration in anaesthetized dogs.

1. Renal sodium excretion (UNaV) was studied during acute changes in plasma sodium concentration (PNa) induced by altering the concentration of a sodium chloride infusion in anaesthetized dogs. 2. The change in PNa induced other changes, notably in blood pH, the degree of blood dilution, and in glomerular filtration rate and renal plasma flow. No attempt was made to restrain these changes and their effects on UNaV were assessed using factor analysis. This defined new variables ('factors') that are linear functions of the primary variables. Four factors were defined, two of which were related to UNaV. 3. Factor 1 was strongly correlated with UNaV and PNa: its other correlations described the acidosis and plasma dilution of hypernatraemia. Further examination of these inter-relationships by regression analysis and data selection showed that the increase in UNaV with rising PNa depended upon a fall in both the blood pH and PCO2. 4. Factor 2 was negatively correlated with UNaV: its correlations with plasma protein concentration, haematocrit, blood pressure, glomerular filtration rate and renal plasma flow implied that it represented post-glomerular plasma protein concentration and its influence on sodium reabsorption. 5. Thus, two independent processes may regulate UNaV during acute salt loading: one is initiated by changes in PNa and plasma dilution but requires a concomitant acidosis and ventilatory adjustment for its full expression; the second relates UNaV to several variables whose influences could be understood by changes they produce in the peritubular capillary plasma protein concentration.