Lack of evidence for gastrointestinal control of sodium excretion in unanesthetized rabbits

Experiments were performed to determine whether unanesthetized rabbits exhibit a greater natriuresis after oral administration of a hypertonic sodium chloride solution (1.5 mmol NaCl/kg) than after the intravenous administration of the same solution. Male rabbits (New Zealand) were placed on a low sodium diet for 4 days, and on the fifth day a hypertonic NaCl solution (616 mM) was given either by stomach tube (GI) or intravenously (IV), while an equal volume of a hypotonic NaCl solution (31 mM) was given by the alternate route. The studies were repeated on each rabbit so that paired observations were obtained. No differences in plasma Na concentration, peripheral hematocrit, or urinary sodium excretion were observed between the GI and IV groups after administration of the hypertonic NaCl solution. In seven rabbits that were permitted access to food and water following NaCl administration by either route, urinary sodium excretion tended to be reduced (P = 0.08), but, again, no significant differences in plasma sodium concentration, peripheral hematocrit, or urinary sodium excretion were observed between the GI and IV routes of sodium administration. Accordingly, we could find no evidence to support the existence of a GI or portal Na receptor system that regulates urinary sodium excretion in the unanesthetized rabbit.