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Notes: 1. In mammalian plasma, many different inhibitors of Na+/K+-ATPase are present, but it is not clear whether their net effect on Na+/K+-ATPase activity changes during the regulation of electrolyte and fluid balance. We studied Na+/K+-ATPase inhibition by plasma extracts in conscious rats during short-and long-term body fluid regulation.2. Male, adult, conscious, freely moving Wistar rats were subjected to one of the following protocols: (i) intracerebroventricular (i.c.v.) injections of angiotensin II (Angll; 1, 10 and 100 ng), the Angll receptor antagonist losartan (1μg), atrial natriuretic peptide (ANP-III; 1μg) or isotonic saline (IS); (ii) intra-arterial (i.a.) injections of IS (6 or 10 mL), hypertonic saline (HS; 1.2% NaCl, 5 mL) or hypertonic plasma expander (HPS; 3.5% hetastarch in HS, 5mL); or (iii) a low salt-high salt-low salt diet sequence (0.18/1.8/0.18% NaCl chow for S days each with controls receiving 0.18% NaCl on all days). Bodyweight, the intake of food and water, urine volume and Na+ concentration and weight of faeces were determined daily. Plasma samples were withdrawn repeatedly throughout the respective protocols, extracted on C18-reversed phase columns and assayed for their effect on the activity of different Na+/K+-ATPase preparations.3. The inhibition of rat brain Na+/K+-ATPase by plasma extracts was not significantly changed by i.c.v. injection of Angll, losartan, ANP-III and IS within the observation period (30 min from respective stimuli). Similarly, no significant changes occurred after acute volume expansion by i.a. injection of IS or HS within 120 min; upon HPS, however, Na+/K+-ATPase inhibition was decreased by approximately 20% (P〈0.05), probably due to passive dilution. During the high-salt diet, fluid retention was effectively counteracted by an adaptive increase of urinary sodium excretion. Throughout the protocol, inhibition of pig brain Na+/K+-ATPase by plasma extracts did not differ significantly between groups.4. It is concluded from these results that the short- or long-term control of body fluids in conscious rats is not associated with systematic changes in Na+/K+-ATPase inhibition by plasma factors.