1. The aim of this investigation was to study the role of prostaglandins in the impaired Na+ conservation of the ageing kidney.

2. We measured the urinary excretion of thromboxane B2, 6-keto-prostaglandin F and prostaglandin E2 in young (3–4 months) and old (20–21 months) rats after 12, 24 and 36 h of Na+ deprivation. In a separate protocol, we measured prostanoid synthesis by isolated glomeruli, cortical homogenates, medullary slices and papillary slices from young and old rats in basal conditions and after 15 days of dietary Na+ deprivation.

3. In the acute study, urinary excretion of 6-ketoprostaglandin F and prostaglandin E2 decreased in young but not in old rats. Urinary excretion of prostaglandin E2 was lower in old rats, but did not vary significantly with Na+ deprivation.

4. In old rats, thromboxane B2 synthesis was increased in all the portions of the kidney except the medulla. Production of 6-keto-prostaglandin F was elevated in glomeruli and tended to increase in the cortex. Prostaglandin E2 synthesis was also elevated in the cortex. Thromboxane B2 synthesis tended to increase in the medulla and was enhanced in the papilla. After Na+ deprivation, only glomerular prostaglandin E2 synthesis increased in young rats. In old rats, cortical and papillary synthesis of 6-ketoprostaglandin F increased, whereas prostaglandin E2 synthesis did not change.

5. The results suggest increased thromboxane synthesis in the ageing kidney. Increased prostacyclin and prostaglandin E2 synthesis may be an attempt to counteract enhanced thromboxane production. Blunting of this tendency in medulla and papilla could contribute to the conservation of medullary solute concentration, whereas inappropriate stimulation of prostacyclin synthesis during Na+ deprivation could impair Na+ conservation.

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