Respiratory and metabolic effects of decreased osmolality in conscious rats

We investigated the respiratory and metabolic effects of decreased osmolali ty, and the potential roles of angiotensin II (ANG II) and the subfornical organ (SFO) in mediating these effects, in conscious Sprague-Dawley (SD) ra ts. Gastric water loading was induced either by oral gavage or an externali zed indwelling stomach tube (20 mL.kg(-1) distilled water at body temperatu re). Repeated measurements after oral gavage were obtained with and without water loading and with and without ANG II receptor block (saralasin, 1.3 m ug.kg(-1).min(-1) iv). At 15 min after water loading by oral gavage, ventil ation ((V) over dot, 1.14 +/- 0.08 L.kg(-1).min(-1)) and tidal volume (10.7 +/- 0.6 mL.kg(-1)) were transiently higher (P < 0.05), at a time when plas ma osmolality was decreased (-8 +/- 1 mOsm), compared with gavage tube alon e (0.95 +/- 0.08 L.kg(-1).min(-1) and 9.1 +/- 0.7 mL.kg(-1), respectively). However, water loading via stomach tube did not stimulate (V) over dot; on ly during the 60-s period of water infusion did (V) over dot increase brief ly, but this was due to increased respiratory frequency. Dye indicators dem onstrated that oral gavage exposes upper airway and esophageal afferents to water, presumably accounting for respiratory stimulation. Lesions of the S FO did not affect respiration or metabolism. A decrease in osmolality, asso ciated with both water loading techniques, caused a sustained increase in o xygen consumption ((V) over dot(O2)) and a decrease in the (V) over dot/(V) over dot(O2) ratio. ANG II receptor block reduced the (V) over dot respons e and prevented the decrease in (V) over dot/(V) over dot(O2) following wat er loading by oral gavage, but did not affect the transient stimulation of (V) over dot. Unlike larger mammals, decreased osmolality does not stimulat e respiration in the SD rat.