Renal vasoconstrictor response to hypertonic saline in the dog: effects of prostaglandins, indomethacin and theophylline

Methylxanthines and the kidney

This chapter describes the effects of the natural methylxanthines caffeine and theophylline on kidney function. Theophylline in particular was used traditionally to increase urine out put until more potent diuretics became available in the middle of the last century. The mildly diuretic actions of both methylxanthines are mainly the result of inhibition of tubular fluid reabsorption along the renal proximal tubule. Based upon the use of specific adenosine receptor antagonists and the observation of a complete loss of diuresis in mice with targeted deletion of the A1AR gene, transport inhibition by methylxanthines is mediated mainly by antagonism of adenosine A1 receptors (A1AR) in the proximal tubule. Methylxanthines are weak renal vasodilators, and they act as competitive antagonists against adenosine-induced preglomerular vasoconstriction. Caffeine and theophylline stimulate the secretion of renin by inhibition of adenosine receptors and removal of the general inhibitory brake function of endogenous adenosine. Since enhanced intrarenal adenosine levels lead to reduced glomerular filtration rate in several pathological conditions theophylline has been tested for its therapeutic potential in the renal impairment following administration of nephrotoxic substances such as radiocontrast media, cisplatin, calcineurin inhibitors or following ischemia-reperfusion injury. In experimental animals functional improvements have been observed in all of these conditions, but available clinical data in humans are insufficient to affirm a definite therapeutic efficacy of methylxanthines in the prevention of nephrotoxic or postischemic renal injury.

Renal haemodynamics during hyperchloraemia in the anaesthetized dog: effects of captopril

1. An increase in plasma chloride concentration (PC1) decreases renal blood flow (RBF) and glomerular filtration rate (GFR) and inhibits the intrarenal release of renin and angiotensin II (AII). Captopril was administered to indomethacin-treated dogs to assess the role of AII independent of prostaglandins (PGs) in the haemodynamic response to hyperchloraemia. Studies were performed on kidneys that were denervated by autotransplantation. 2. Anaesthetized greyhounds received an intrarenal infusion of 0.616 M-sodium acetate, which was changed to 0.616 M-NaCl (hyperchloraemia). These infusions increased the plasma sodium and osmolality at the experimental kidney by 7-11% throughout but increased the PCl during the hypertonic NaCl infusions only (122 +/- 3 to 136 +/- 3 mM). 3. In vehicle-treated dogs (n = 8), hyperchloraemia reduced the GFR (1.4 +/- 0.1 to 1.0 +/- 0.1 ml min-1 kg-1; P less than 0.05) and the RBF (13.0 +/- 1.4 to 8.3 +/- 0.6 ml min-1 kg-1; P less than 0.01); these changes were reversible on return to the 0.616 M-sodium acetate infusion. Hyperchloraemia reduced the release of AII into renal lymph (2.5 +/- 0.9 to 1.2 +/- 0.4 pg min-1 kg-1; P less than 0.01). 4. Indomethacin (0.6 mg kg-1 and 0.2 mg kg-1 h-1 intrarenally; n = 4) blunted (P less than 0.05) the Cl--induced fall in RBF (10.4 +/- 1.1 to 8.2 +/- 0.6 ml min-1 kg-1) without changing significantly the fall in GFR or the release of AII into renal lymph.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of external compression and movement in lymph propulsion in the sheep hind limb

1. Pressure fluctuations and lymph flow were measured in metatarsal lymphatics in anaesthetized sheep. 2. Intermittent compression significantly increased lymph flow when this was applied over the hoof but did not increase flow significantly when applied over the metatarsal region. 3. In a second preparation a 15 cm length of metatarsal lymphatic was cannulated at both ends and measurements were made of the ability of the duct to pump saline from an inflow reservoir through an outflow at the same height. 4. In the absence of external forces fluid was propelled by the lymphatic's intrinsic contractions but when intermittent compression was applied over the metatarsal region flow increased almost fourfold. 5. When animals with the doubly cannulated duct were allowed to recover, the effect of normal limb movements on fluid propulsion was examined. Under these conditions flow only occurred in response to intrinsic lymphatic contractions and appeared to be unaffected by the animal moving round the cage. 6. These results suggest that the effects of external forces on lymph flow are more dependent on compression of tissues in the lymphatic drainage area than on compression of the main lymphatic ducts. External compression can increase fluid propulsion by these vessels but, since forces of adequate magnitude appear not to be encountered in normal hind-limb movements, lymph propulsion in this region must depend on intrinsic lymphatic pumping.