Effects of cyclic neucleotide monophosphates (cAMP, cGMP, cIMP, dibutyryl-cAMP) on transtubular net fluxes in rat kidneys

Glomerular effects of cholera toxin in isolated perfused rat kidney: a potential role for platelet activating factor

Cholera toxin (MW 84 kDa) is now considered a pharmacological tool to study the adenylyl cyclase system and a stimulus to generate platelet activating factor in the intestinal tract. We used this toxin to evaluate the renal haemodynamics, glomerular filtration function, tubular sodium transport and toxicity in isolated perfused rat kidney. Kidneys from adult male Wistar rats were isolated for perfusion. The perfusion fluid was modified Krebs-Henseleit solution and the samples were analyzed for sodium, potassium, inulin and osmolality. Clearance techniques were used to calculate physiological parameters. Cholera toxin (1.0 microg/ml) caused a significant time-dependent reduction of glomerular filtration rate and urinary flow. This toxin also caused a small, but consistent reduction in fractional proximal sodium reabsortion (toxin = 67.43+/-2.42% versus control = 79.26+/-5.80%; P<0.025). WEB 2086, a platelet activating factor receptor antagonist at 100 microg/ml completely blocked the effects induced by cholera toxin on glomerular filtration rate, fractional proximal sodium reabsortion and urinary flow. In contrast to cholera toxin, dibutyryl-cyclic AMP (10(-5) M) significantly increased glomerular filtration rate (Db-cyclic AMP = 0.651+/-0.035 versus control = 0.514+/-0.043 ml x g(-1) x min(-1); P<0.025) in isolated perfused kidneys. Db-cyclic AMP caused a similar, but more severe reduction in fractional proximal sodium reabsortion (Db-cyclic AMP = 54.21+/-2.35% versus control = 70.10+/-3.24%; P<0.025). In addition Db-cyclic AMP increased significantly the urinary flow (Db-Cyclic AMP = 0.290+/-0.018 versus control = 0.179+/-0.026 ml x g(-1) x min.(-1); P<0.025). WEB 2086+ Db-cyclic AMP also caused a significant increase in the urinary flow with maximal effect at 90 min. (WEB+Db-cyclic AMP = 0.26+/-0.01 versus control = 0.15+/-0.01 ml x g(-1) x min.(-1); n = 8, P<0.025). Cholera toxin caused a decrease of urinary flow (toxin = 0.034+/-0.004 versus control = 0.145+/-0.02 ml x g(-1) x min.(-1); P<0.025), this effect was also completely abolished by WEB 2086 when it was injected previously to toxin. When only WEB 2086 was injected, the functional parameters remained stable throughout the perfusion time. Cholera toxin had no effect on renal vascular resistance, renal perfusate flow or tissue potassium, suggesting renal integrity in kidneys treated with this toxin. The results suggest that cholera toxin effects in the perfused rat kidney are primarily mediated by platelet activating factor.

The effects of Escherichia coli heat-stable enterotoxin in renal sodium tubular transport

To compare the function of sodium transport between intestine and renal tubule, we studied the effect of E. coli STa enterotoxin and 8-bromo cyclic GMP in perfused rat kidneys. Infusion of STa enterotoxin (0.017 and 0.1 micrograms/ml) caused a dose and time dependent decrease in total renal sodium tubular transport. The major site of STa effect was at the renal proximal tubule. Similar to Sta enterotoxin, 8-bromo cyclic GMP (10(-5) M) caused a significant decrease of fractional renal sodium proximal tubule transport. In contrast to STa enterotoxin, infusion of 8-bromo cyclic GMP resulted in a significant but short lasting (30 min.) increase in glomerular filtration rate. STa enterotoxin also decreased significantly the renal tissue potassium. This STa effect was related to a significant decrease in renal potassium tubular transport, resulting also in an increase of urinary potassium excretion. These studies demonstrate the specific functional effect of STa enterotoxin in promoting the decrease in renal proximal tubular sodium transport, similar to 8-bromo cyclic GMP. In perfused rat kidneys STa also decreased tissue potassium, mainly by a decrease in potassium transport and increase in urinary potassium excretion. These effects suggest the existence of an endogenous peptide resembling STa enterotoxin, that regulates the function of renal sodium tubular transport.

Dibutyryl cyclic AMP inhibits transport dependent QO2 in cells isolated from the rabbit medullary ascending limb

Because the medullary thick ascending limb of the loop of Henle is the target of several polypeptide hormones that stimulate adenyl cyclase in this nephron segment, we examined the effects of cyclic AMP on thick ascending limb of the loop of Henle cells isolated by enzymatic digestion and density gradient centrifugation from the outer medulla of the rabbit kidney. The functional parameter that was measured was transport dependent oxygen consumption. Oxygen consumption was measured using a polarographic oxygen electrode in a constant temperature chamber. We found that dibutyryl cyclic AMP inhibited oxygen consumption in a dose dependent way. Maximal inhibition was observed at a concentration of 10(-5) M. The effect of dibutyryl cyclic AMP was not present in the absence of either sodium, chloride or both implying that its effect is restricted to the sodium and chloride dependent oxygen consumption. The effect of dibutyryl cyclic AMP was additive to that of furosemide 10(-4) M while that of furosemide was not additive to that of cyclic AMP suggesting that the site of action of cyclic AMP is distal to that of furosemide. The effect of dibutyryl cyclic AMP was not additive to that of ouabain and was absent in cells where oxygen consumption was stimulated with amphotericin B in the absence of chloride indicating that it has no effect on Na-K-ATPase.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in rat renal cortical and medullary guanosine 3'5'-monophosphate accumulation by oxygen- and calcium-dependent and -independent mechanisms: evidence for a calcium-independent action of oxygen in renal inner medulla

Factors influencing guanosine 3'5'-monophosphate (cGMP) metabolism were examined in slices of rat renal cortex, outer medulla, and inner medulla. In the presence of extracellular Ca2+ and O2, a gradation of steady-state cGMP levels was evident among the tissues (inner medulla greater than outer medulla greater than cortex). Carbamylcholine, bradykinin, histamine, and the divalent cation ionophore A23187 significantly increased cGMP in each tissue. The cGMP-stimulating action of these agents was reversibly abolished by exclusion of either Ca2+ or O2. The influence of Ca2+ and O2 on expression of effects of carbamylcholine and related cGMP agonists was interdependent in each region of the kidney, since both were required for expression of agonist action. By contrast, nitrite, nitroprusside, NH2OH, and nitrosoguanidine increased cGMP in the presence or absence of Ca2+ or O2. Thus, at least two distinct mechanisms for altering cGMP accumulation are operative or expressible in each region of the kidney: one that requires and one that does not require the presence of extracellular Ca2+ and O2. Results also suggested a role for transmembrane transport of Ca2+ in the maintenance of basal cGMP and in the expression of the responses to Ca2+-dependent agonists in renal cortex, outer and inner medulla. Thus, verapamil, which can block such transport, lowered basal cGMP and abolished these responses while ionophore A23187 enhanced cGMP in cortex and medulla only in the presence of Ca2+. The interrelationship of Ca2+ and O2 in control of basal cGMP levels clearly differed in cortex compared to inner medulla. In cortex, Ca2+ and/or O2 deprivation produced quantitatively similar reductions in cGMP. Moreover, expression of the action of O2 to increase cortical cGMP required Ca2+. Thus, O2 effects on cGMP in cortex were closely coupled with or mediated through Ca2+. By contrast, in inner medulla, O2 deprivation resulted in more pronounced reduction in basal cGMP than did Ca2+-deprivation, and O2 significantly increased inner medullary cGMP in the absence of extracellular Ca2+...

Effect of parathyroid hormone and cyclic adenosine 3',5'-monophosphate on isotonic fluid reabsorption: polarity of proximal tubular cells

Isotomic fluid reabsorption (JV) of rat renal proximal tubules was examined by the shrinking droplet method in combination with simulatneous perfusion of blood capillaries. Sensitivity of JV measurement was improved by using each punctured tubule for control measurements: 1) Parathyroid hormoen (PTH) on the contraluminal cell side reduced JV in a dose-response behavior. The maximal inhibition was achieved at a PTH concentration of 10(-5) M, the half maximal inhibition at a concentration of 3 X 10(-9) M. PTH on the luminal cell side had a small inhibitory effect. 2) Cyclic AMP inhibited JV preferentially when applied to the luminal cell side. On the luminal cell side, both cyclic AMP and dibutyryl cyclic AMP inhibited JV in a similar dose-dependent behavior. Concentrations of both nucleotides as low as 10(-10) M had a definite inhibitory effect. Tested at a high concentration, N6-butyryl cyclic AMP was almost as effective as cyclic AMP. Deoxy cyclic AMP, 5' AMP, cyclic guanosine monophosphate (cyclic GMP), dibutyryl cyclic GMP had no effect. ATP inhibited JV to a very small extent. 3) The reduction of JV after administration of PTH and dibutyryl cyclic AMP was not additive. The similar inhibitory effect of PTH at the contraluminal cell face and of cyclic AMP at the luminal cell face suggests the following sequence of events in the mediation of the action of PTH: 1) activation of adenylate cyclase by PTH in the contraluminal cell membrane, and 2) action of the generated cyclic AMP on the luminal cell membrane. The interaction of cyclic AMP and the luminal cell membrane is initiated at the luminal cell surface.

Indomethacin blockade of renal PGE-synthesis: effect on total renal and tubular function and plasma renin concentration in hydropenic rats and on their response to isotonic saline

The effects of indomethacin (I), a blocker of prostaglandin (PG)-synthetase, was studied in rats in an attempt to elucidate the physiological role of renal PGE. Plasma-I-concentrations of 13-14 mug/ml reduced renal venous plasma PGE-concentration significantly from 216 to 85 pg/ml within 45 min. Urine flow and solute excretion decreased by 42% and 20%, respectively, while urine osmolality increased 450 mOsm. Inulin clearance (CIN) and proximal reabsorption rate was unaffected, while renal plasma flow (RPF) decreased by 18%. Plasma renin concentration decreased slightly but significantly. An i.v. saline load equal to 1% b.wt. given to I-treated rats failed to elevate significantly either urine flow, solute excretion, CIN, RPF or proximal reabsorption rate, but plasma renin decreased further. The normal inverse relationship between plasma renin and proximal reabsorption rate was absent. The data are consistent with the concept that intrarenal PGE plays in adjustment of renal vascular resistance, and support the concept of a physiological role of intrarenal PGE in regulating salt and water excretion. The data do not support any major physiological role of PGE in regulating proximal tubular function.

A micropuncture study of the effect of isoprenaline on renal tubular fluid and electrolyte transport in the rat

To explain the mechanism of the isoprenaline induced antidiuresis the effect of this substance on fluid and electrolyte reabsorption in various nephron segments of the rat kidney has been studied using micropuncture techniques. Isoprenaline (1.5x10-9mol/kg-min i.v.) decreased GFR of superficial nephrons and increased fractional proximal fluid, sodium and potassium reabsorption. However, the fractions of filtered fluid and electrolytes, which had been reabsorbed up to early distal tubule were unchanged after isoprenaline. This indicated that the increased fractional proximal reabsorption has been completely compensated by decreased reabsorption from Henle's loops. In the distal convoluted tubules an increase in the fractional fluid and sodium reabsorption could be established using a recollection technique. Given directly into the distal tubular lumen isoprenaline stimulated the isotonic fluid reabsorption, which was measured by the Gertz split oil droplet method. From these results it is concluded that the decrease in superficial GFR and the increased distal tubular fluid and sodium reabsorption lead to the decrease in urine volume and in urinary electrolyte excretion after isoprenaline.