ENDOGENOUS VARIATIONS AND SODIUM INTAKE-DEPENDENT COMPONENTS OF DIURNAL SODIUM-EXCRETION PATTERNS IN DOGS

1. Automated, sequential, 20 min urine collections were made to provid e a record of diurnal variations of urinary sodium excretion (UNaV) in seven dogs, in which the same daily intake of sodium, potassium and w ater was administered, at first orally (between 08.30 and 08.50 h) on day 1 and then by I.V. infusion at a constant rate on days 2 and 3. Th is basic protocol was employed for two different levels of sodium inta ke: normal (NSI; 2 5 mmol (kg body wt)(-1) (24 h)(-1)) and high (HSI; 14.5 mmol (kg body wt)(-1) (24 h)(-1)). 2. The aims were: firstly, to establish the diurnal pattern of UNaV under these circumstances; secon dly, to find out whether the quantity of sodium administered influence s this diurnal pattern; and thirdly, to distinguish endogenous fluctua tions from intake-dependent components in the UNaV excretion patterns. 3. On day 1 (oral intake) all dogs exhibited a similar excretion patt ern, which peaked between 13.00 and 15.00 h on both diets and then dim inished again over the remainder of the 24 h period. 4. On days 2 and 3 (infusion) UNaV fluctuated within a considerable range. 5. On HSI, t he maximal UNaV rates on day 1 were about double those observed on inf usion days. On HSI, UNaV during infusion days seems to consist of a co nstant basal component of about 5-6 mu mol (kg body wt)(-1) min(-1) up on which a fluctuating component is superimposed. The basal component may be a reactive homeostatic response to the high sodium intake, wher eas the superimposed fluctuating component may reflect endogenous vari ations. 6. On NSI, mean UNaV, variance and peak UNaV were nearly the s ame during oral intake and on infusion days. Thus, on NSI, UNaV variat ions seem to be caused primarily by endogenous mechanisms.