Methods

Preparation of animals and sample collection Microcatheterization studies were performed on male Sprague-Dawley rats (Cam Research, New Jersey, USA) weighing approximately 200 g. For micropuncture studies, smaller animals weighing approximately 100 g were used. Animals were allowed free access to food and tap water prior to the acute study. Animals for microcatheterization and micropuncture were each divided into three groups and prepared as follows:

Control group. Animals were fed regular rat chow (Ralston Purina, St. Louis, Missouri, USA), injected with a priming dose of 30 pCi of methoxy-3H-inulin and infused with 0.15 M NaCI, containing 30 pCi/mI of methoxy-3H-inulin, at 0.01 mllmin per 100 g body weight during the study. Acute metabolic acidosis group. Animals were fed regular rat chow (Ralston Purina) and infused with the same priming and sustaining infusions as the control group. At the initiation of the acute study NH4C1 was administered by gavage, in a dose of 8 mmoles/kg body wt. Pilot studies showed that this dose of NH4C1 was sufficient to induce severe acidosis with a fall in blood pH for approximately 3 hr. Acute metabolic acidosis with mannitol infusion group. Animals were prepared in a manner similar to the acute metabolic acidosis group, but during the acute study they were also infused with 5% mannitol in saline at 0.02 ml/min/l00 g body wt. This dose of mannitol was used after preliminary studies indicated that the medullary level of ammonia was reduced to near control values in acidotic animals at this rate of mannitol infusion.