Role of vasopressin in regulation of sodium excretion

Urinary ET-1, AVP and sodium in premature infants treated with indomethacin and ibuprofen for patent ductus arteriosus

The relative potency and interrelationship between vasoactive and natriuretic mediators are thought to be important in the transition from fetal to neonatal life. The relationship between urinary vasoactive factors and sodium excretion has not been adequately addressed in premature infants receiving indomethacin and ibuprofen for therapy of patent ductus arteriosus. Excretion rates of AVP, ET-1 and sodium were measured in premature infants with RDS receiving indomethacin or ibuprofen. Forty-four RDS premature infants (<34-week gestation) with PDA received either ibuprofen (n=22) in an initial dose of 10 mg/kg followed by two doses of 5 mg/kg each after 24 and 48 h or 3 doses at 12-h intervals of indomethacin (n=24), 0.2 mg/kg, infused continuously over a period of 15 min. Urinary ET-1, AVP and sodium excretion were measured before and after treatment. Indomethacin treatment caused a significant decrease in urinary ET-1 and AVP excretion (UET-1/Ucr 0.14+/-0.01 vs. 0.10+/-0.05 fenton/mmol; P<0.05; 24.42+/-6.18 vs. 12.63+/-3.06 pg/mmol; P<0.05, respectively), along with a significant reduction in urinary sodium (92.1+/-36.1 vs. 64.8+/-35.6 mmol/l; P<0.01), fractional excretion of sodium (6.8+/-37.1 vs. 4.5+/-37.1%; P<0.01) and urinary osmolality (276.2+/-103.9 vs. 226.4+/-60.3 mOsmol/kg; P<0.05). Ibuprofen treatment caused a significant decrease in urinary AVP (UAVP/Ucr 24.5+/-3.4 vs. 16.3+/-2.04 pg/mmol; P<0.01), along with a significant decrease in urinary sodium (78.0+/-8.4 vs. 57.0+/-8.0 mmol/l; P<0.05) and in fractional excretion of sodium (7.5+/-1.3 vs. 3.9+/-3.0%; P<0.05), while it did not modify urinary ET-1 excretion. The association of renal ET-1 and AVP activity with sodium excretion in premature infants treated with indomethacin and ibuprofen supports the hypothesis that these factors may play a role in the physiologic changes in sodium excretion.

Role of nitric oxide in systemic vasopressin-induced hypothermia

It has been reported that arginine vasopressin (AVP) plays a thermoregulatory action, but very little is known about the mechanisms involved. In the present study, we tested the hypothesis that nitric oxide (NO) plays a role in systemic AVP-induced hypothermia. Rectal temperature was measured before and after AVP, AVP blocker, or NG-nitro-L-arginine methyl ester (L-NAME; NO synthase inhibitor) injection. Control animals received saline injections of the same volume. The basal body temperature (Tb) measured in control animals was 36.53 +/- 0.08 degreesC. We observed a significant (P < 0.05) reduction in Tb to 35.44 +/- 0.19 degreesC after intravenous injection of AVP (2 micrograms/kg) and to 35.74 +/- 0. 10 degreesC after intravenous injection of L-NAME (30 mg/kg). The systemic injection of the AVP blocker [beta-mercapto-beta, beta-cyclopentamethylenepropionyl1,O-Et-Tyr2,Val4,Arg8]vasopressin (10 micrograms/kg) caused a significant increase in Tb to 37.33 +/- 0.23 degreesC, indicating that AVP plays a tonic role by reducing Tb. When the treatments with AVP and L-NAME were combined, systemically injected L-NAME blunted AVP-induced hypothermia. To assess the role of central thermoregulatory mechanisms, a smaller dose of L-NAME (1 mg/kg) was injected into the third cerebral ventricle. Intracerebroventricular injection of L-NAME caused an increase in Tb, but when intracerebroventricular L-NAME was combined with systemic AVP injection (2 micrograms/kg), no change in Tb was observed. The data indicate that central NO plays a major role mediating systemic AVP-induced hypothermia.

Low urine flow reduces the capacity to excrete a sodium load in humans

Recent studies in rats suggest that vasopressin and the resulting urinary concentrating activity reduce the capacity of the kidney to excrete sodium. The present study investigates the influence of the level of hydration on the excretion of a sodium load in humans. Eight healthy male volunteers (18-35 yr) were studied twice, in random order, under either low (LowH) or high (HighH) hydration. They drank throughout the study either 0.25 (LowH) or 2.0 ml water/kg body wt (HighH) every 30 min. After 1 h equilibration, urine was collected for 2 h before (basal) and 10 h after the NaCl load (5 g NaCl in 250 ml, infused intravenously over 30 min). Differences in excretory patterns between LowH and HighH were mostly confined to the first 4 h after the load. The increase in Na excretion after the load was more intense under HighH than under LowH (+ 10.9 +/- 2.6 vs. + 5.8 +/- 2.7 mmol/h in the first 4 postload h; P < 0.001). Under HighH, urine flow rate (V) increased markedly (+ 41%), with little change in urinary Na concentration (UNa), whereas under LowH, V declined slightly and UNa rose significantly (+ 33%). The capacity to raise UNa seemed to reach a maximum at approximately 280 mM. In both conditions, the changes in UNa observed after the load were positively correlated with basal UNa. After the load, urea excretion increased under HighH and decreased under LowH, whereas K excretion was unaffected in either condition. These results show that sodium excretion is facilitated by an abundant water supply. The less efficient sodium excretion occurring at low V is probably due to the influence of vasopressin on water, urea, and sodium movements across the collecting ducts. These observations suggest that, in everyday life, a low water intake could limit the capacity to excrete sodium. Whether this could contribute to salt-sensitive hypertension remains to be evaluated.

Epidermal growth factor antagonizes vasopressin in vivo and in vitro

Since EGF causes diuresis through a renal action and may antagonize the hydroosmotic effect of AVP in vitro we investigated the antagonistic action of EGF with AVP in vivo and the mechanism of the antagonism in vitro. Conscious ewes received i.m. injections of a selective AVP V2-receptor agonist (1-desamino, D-Arg8 vasopressin acetate, DDAVP) every 12 hours for days 5 to 16. All ewes received an i.v. isotonic saline infusion (100 ml/day) for days 1 to 8 and days 13 to 16, and i.v. EGF in 100 ml saline/day at doses of 0 (N = 8) or 10 (N = 8) micrograms/hr for days 9 to 12. DDAVP reduced both urine volume and water intake, and increased urine osmolality. In contrast, simultaneous infusion of EGF reversed the DDAVP-induced responses, resulting in a transient negative fluid balance, kaliuresis and a transient natriuresis (all P < 0.05). When EGF treatment ceased, the effects of DDAVP treatment alone gradually became apparent. From the in vitro studies, the AVP-related peptides displaced specific AVP V1- and V2-receptor antagonist radioligands from rat renal inner medullary membranes, whereas EGF had no effect. However, EGF antagonized AVP V2-stimulated cAMP production in a dose-dependent way (IC50 = 2 x 10(-7) M). Therefore, the diuretic effect of EGF is not via direct antagonism of the antidiuretic AVP V2-receptor but seems mediated by inhibition of the antidiuretic AVP V2-receptor second messenger system.

Control of blood and extracellular volume

Blood and extracellular fluid volume are maintained within narrow limits despite considerable daily variations in the intake in salt and water. As summarized schematically in Figure 15, the urinary excretion of salt and water responds to changes in blood volume and arterial pressure. Volume-sensitive receptors located predominantly in the cardiac atria and arterial tree sense acute changes in the filling of the blood volume compartment, and urinary sodium excretion is adjusted in response to these detector mechanisms by virtue of alterations in both glomerular filtration rate and tubular sodium reabsorption. The reabsorption of sodium by the tubule responds to changes in extracellular fluid volume as well as to changes in filtered sodium load. Glomerular filtration rate and tubular reabsorption of sodium are influenced importantly by physical properties of the plasma in glomerular and peritubular capillaries and by the composition of the tubular fluid. The renal arterial perfusion pressure is a major factor regulating tubular reabsorption of sodium and water as signalled via changes in renal interstitial hydrostatic fluid pressure. Renal nerves and a variety of systemic and local hormones also influence tubular reabsorption of sodium and water directly by effects on transepithelial sodium transport and/or indirectly by altering renal medullary haemodynamics and the pressure-natriuresis-diuresis relationships. Thus, utilizing a variety of overlapping effector mechanisms that influence renal sodium and water excretion, mammalian organisms have achieved a high degree of stability of body fluid volumes. The fundamental relationship between arterial pressure and renal excretion appears to be the major mechanism which provides for the long-term control of body fluid volume. The sensitivity of the pressure-natriuresis-diuresis relationship is modified by the efferent pathways of the rapid-acting reflex and mechanoreceptor detectors of volume. Working together, these mechanisms provide a remarkable degree of rapid and long-term extracellular and blood volume stability.