Renal concentration in the normal dog: effect of an acute reduction in salt excretion

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Abstract
In normal anesthetized hydropenic dogs, the TcH2O-Cosm relation generated by infusing hypertonic solutions was variable. In ten dogs TcH2O rose as Cosm increased, attaining a maximum of 3.7–6.3 ml/min ("high curve"). In seven dogs a maximum TcH2O of only 1.4–2.5 ml/min was observed at low Cosm and CH2O was often generated at moderate Cosm ("low curve"). The acute reproducibility of this TcH2O-Cosm relation was examined by allowing Cosm to return to low levels and again infusing the same hypertonic solution. The TCH2O-Cosm relation was duplicated only in dogs initially exhibiting a high curve. Therefore, the effects on TcH2O of acutely reduced sodium excretion were examined only in dogs with an initial high curve relation. After the first diuresis, a phlebotomy or prolonged infusion of 10% mannitol was utilized to induce salt retention. If sodium excretion was reduced to less than 40 µEq/min at low Cosm, TcH2O was decreased during the subsequent diuresis. This impairment in TcH2O generation is attributed to decreased availability of salt for transport into the medulla.