Vasopressin-related alterations of sodium reabsorption in the loop of Henle

Vasopressin-induced natriuresis in the conscious rat: role of blood pressure, renal prostaglandin synthesis and the peptide ANF

1. The response to arginine vasopressin (AVP) at doses of 5 and 10 pmol (100 g body weight)-1 h-1 was studied in conscious rats during the infusion of 1% (w/v) dextrose at 11.6 ml h-1 with and without pre-treatment with indomethacin. 2. In the absence of indomethacin AVP infusion induced dose-related increases in sodium output that were positively correlated with increases in mean arterial blood pressure (MAP) and plasma atrial natriuretic factor (ANF) immunoreactivity. Increases in renal prostaglandin E2 (PGE2) synthesis were also associated with AVP infusion. 3. Indomethacin pre-treatment abolished the AVP-induced increases in renal PGE2 synthesis and also the dose-related differences in ANF immunoreactivity. Increases in MAP and sodium output were unaffected at the 10 pmol (100 g body weight)-1 h-1 dose of AVP and only slightly attenuated for the 5 pmol (100 g body weight)-1 h-1 dose. 4. For both series AVP induced marked falls in glomerular filtration rate (GFR) but only small transient falls in effective renal plasma flow. The observed falls in GFR support the view that the natriuresis is due to changes in tubular handling and not in the filtered load of sodium. 5. It is concluded that the natriuresis elicited by AVP is closely related to the pressor action of the hormone but renal PGE2 synthesis and plasma ANF are not responsible for mediating this response.

Role of the adrenal cortex in the natriuresis associated with vasopressin infusion

We have compared the effects of a 48-h continuous infusion of arginine vasopressin (AVP) given to four subjects (one subject studied twice) with that given to four patients lacking adrenocortical function and maintained on fixed doses of mineralocorticoid and glucocorticoid replacement therapy. Both patients and normal subjects were subjected to dietary salt and water restriction during the period of the infusion, and all participants were in balance on a low sodium diet at the start of the infusion period. Infusion of vasopressin in normal subjects was associated with an increase in fractional excretion of sodium by the kidney, which continued during the day after the infusion was stopped. The infusion was also associated with a fall in plasma aldosterone as well as falls in whole blood packed cell volume, plasma renin activity and plasma noradrenaline. Infusion of vasopressin in patients lacking adrenocortical function was associated with a smaller increase in fractional excretion of sodium by the kidney in comparison with that seen in normal subjects, and the increase in sodium excretion was confined to the period of the infusion. Mineralocorticoid activity was maintained constant during the infusion, although a fall in whole blood packed cell volume, plasma renin activity and plasma noradrenaline were noted, in similarity to the study made in normal subjects. We conclude that changes in mineralocorticoid activity are the main factors contributing to the increase in renal sodium excretion seen during, and after, the continuous infusion of vasopressin in salt- and water-restricted man.

The effect of vasopressin on renal tubular 22Na efflux in the rat

1. Paired tracer microinjections of [3H]inulin and 22Na were performed at late proximal and early distal puncture sites in water diuresis and during vasopressin-induced antidiuresis in anaesthetized rats. Vasopressin caused a sixfold increase in Na and a fourfold increase in K excretion without significant changes in renal haemodynamics. 2. 22Na-recovery from late proximal injections increased nearly fivefold (from 0.98+/- 0.16 to 4.76 +/- 0.66%, P less than 0.001). A significant correlation could be detected between changes in sodium excretion and changes in late proximal 22Na recovery. Vasopressin had no significant effect on 22Na recovery from early distal injections. 3. These results suggest that vasopressin inhibits active salt reabsorption within the loop of Henle.

Effect of vasopressin on electrical potential difference and chloride transport in mouse medullary thick ascending limb of Henle's loop

Medullary thick ascending limbs of Henle's loop of the Swiss-Webster mouse were perfused in vitro with an isotonic perfusate and a Ringer's bathing medium. In five studies, addition of a supramaximal concentration of synthetic arginine vasopressin (AVP) to the bathing medium resulted in an increase in electrical potential difference (PD) from 5.0 +/- 1.5 mV, lumen positive, to 10.7 +/- 1.4 mV (P < 0.001). When AVP was removed, the PD returned to 2.6 +/- 0.9 mV (P < 0.001), then increased again to 6.9 +/- 1.7 mV (P < 0.01) when AVP was added a second time. A significant, but submaximal, increase in PD of 2.3 +/- 0.6 MV (P < 0.05) was observed in five medullary thick ascending limbs when AVP was added to the bathing medium at a concentration of 10 microunits/ml. This increase was approximately one-third of the response observed at a concentration of 100 microunits/ml in the same tubule. No further increment in PD was observed in five medullary thick ascending limbs when the AVP concentration was increased from 100 to 1,000 microunits/ml. In seven thick ascendcing limbs, the effect of AVP on PD was reproduced by the addition of 8-[p-chlorophenylthio]-cyclic 3',5'-adenosine monophosphate to the bathing medium at a final concentration of 0.1 mM. AVP increased unidirectional chloride flux from lumen to bath from 29.3 +/- 3.2 to 69.8 +/- 6.2 peq/cm per s (P < 0.001) in spite of an increase in the lumen positive PD from 1.6 +/- 0.5 mV to 7.0 +/- 0.6 mV (P < 0.001). Unidirectional chloride flux from bath to lumen was not affected by AVP. In another series of experiments, net chloride flux increased from 15.6 +/- 3.0 to 41.7 +/- 5.3 peq/cm per s (P < 0.05) after addition of AVP. The effect of AVP on hydraulic water permeability (Lp) was examined by adding raffinose to the bathing medium in both the presence and the absence of AVP. The calculated Lp of 16 +/- 2 nm/s per atm in the absence of AVP, although very low, was significantly different from zero (P < 0.01). However, the Lp did not increase significantly when AVP was added to the bathing medium. These results suggest that AVP has a second site of action in the kidney to increase chloride transport by the medullary thick ascending limb in addition to its well-known effect on the water permeability of the collecting tubule. The former effect would contribute to urinary concentrating ability by increasing the axial osmotic gradient in the renal medulla.

Effects of steady-state plasma vasopressin levels on the distribution of intrarenal blood flow on electrolyte excretion

1. In order to evaluate the effects of arginine vasopressin (AVP) on the distribution of intrarenal blood flow and on electrolyte excretion, steady-state plasma AVP levels (4-8, 19-1, 44-3, and 100-6 micro u./ml.) were produced in anaesthetized dogs, which were hydrated to minimize endogenous anti-diuretic hormone (ADH) release. 2. The urinary excretion of sodium and potassium increased without change in their filtered loads during AVP infusion. 3. Measurement by the 133xenon washout method revealed diphasic blood flow shifts, as a function of the plasma AVP level, between compartment 1 (outer cortex) and compartment 2 (inner cortex and outer medulla) without change in compartment 3 (inner medulla). 4. In a separate study, the radioactive microsphere (15 micronm) method was used with a plasma AVP levels of 19-8 micronu./ml. Blood flow (expressed as % flow/g tissue) decreased in the outer cortex and increased in the inner cortex. 5. Total renal blood flow did not change during infusion of AVP. However, the values measured by 133xenon were lower than those measured by the microsphere method. 6. There was agreement between these two independent methods that blood flow shifted from outer to inner cortex, with no change in total renal flow, at similar plasma AVP levels (19-1 and 19-8 micronu./ml.). The relationship of these intrarenal circulatory changes to the increased electrolyte excretion is discussed.

Vasopressin dependent adenylate cyclase in single segments of rabbit kidney tubule

AVP dependent adenylate cyclase activity was measured in single pieces of 8 different tubular segments isolated from collagenase treated rabbit kidneys. High responses were observed in all the tested portions of the collecting tubule, that is its cortical branched part (BCT), its cortical straight part (CCT) and its outer medullary part (MCT). Dose response curves indicated in CCT: 2 fold threshold stimulation at 10(-11) M AVP, 27 fold stimulation at 10(-6) M AVP, half maximal stimulation at about 10(-9) M AVP. Both the medullary (MAL) and, to a lesser extent, the cortical (CAL) portions of the thick ascending limb were also observed to contain AVP sensitive adenylate cyclase (for MAL: 2 fold threshold stimulation at 10(-9) M AVP, 9 fold stimulation at 10(-7) M AVP, half maximal stimulation at 5 X 10(-9) M AVP). In contrast, nearly no responsiveness to AVP was observed in the proximal convoluted tubule, in the thin descending limb of the loop and in the distal convoluted tubule (DCT). The limited response obtained in DCT (which is a structure generally considered as a target site for AVP) as well as the clearcut effect elicited by AVP in MAL (the functioning of which is not known to be controlled by ADH) were expected observations; their possible physiological implications will be discussed.

The effect of vasopressin on phosphate transport in the proximal tubule of the dog

Recollection micropuncture study was performed in 11 thyroparathyroidectomized dogs during antidiuresis to determine the effect of continuous vasopressin infusion at 50 mU/kg/h on proximal tubule phosphate and sodium transport. The animals were divided into two groups according to changes in mean arterial blood pressure. In the first group (five dogs) with increased blood pressure and glomerular filtration rate (GFR), mean proximal tubule fluid-to-plasma inulin ratio fell significantly from 1.69 to 1.53, whereas it remained unchanged at 1.60 in the second group (six dogs) with no change in blood pressure. In contrast, mean proximal tubule fluid-to-plasma ultrafilterable phosphate ratio increased consistently in both groups, regardless of blood pressure changes. Since natriuresis as well as phosphaturia were observed in all animals, the sodium effect of vasopressin in the distal nephron must be mainly responsible for the natriuresis. It was concluded that vasopressin, when given in the doses employed, inhibits phosphate transport in the proximal tubule and sodium reabsorption in the distal nephron. An additional effect on proximal tubule sodium reabsorption appears to be related to the rise in blood pressure and GFR secondary to vasopressin administration.