SUMMARY To assess the influence of late hyponatraemia on the renal responsiveness to endogenous arginine vasopressin (AVP), urinary excretion and plasma concentration of sodium, plasma and urine osmolality, free water clearance, and urinary AVP concentration and excretion were measured in 11 healthy premature infants with a mean birth weight of 1360 g and mean gestational age of 31 weeks. Studies were performed on days 1, 5, and 19.The development of late hyponatraemia was associated with a pronounced decline in urine osmolality, whereas urine flow rate and free water clearance increased significantly. Mean (SEM) urine AVP concentration and excretion also rose significantly from 2 15 (0-31) pg/ml and 0-36 (0.55) pg/min/m2 on the first day to 6-5 (0.96) pg/ml and 3-85 (0.63) pg/min/m2 on the 19th day, respectively.When renal response to AVP was compared at different ages the highest urine osmolality and steepest response curve was observed on the first day. With development of hyponatraemia the renal response became blunted.It is concluded that the limited tubular sodium transport and hyponatraemia hinders the establishment of intrarenal osmotic gradient, impairs renal response to AVP, and prevents excessive water retention and further fall of plasma sodium.In a recent study on the role of arginine vasopressin (AVP) in development of late hyponatraemia in premature infants we showed a steady rise in urinary AVP excretion with age in spite of the pronounced decline in plasma sodium concentration and osmolality.1 This finding was interpreted as indicating that the protracted volume contraction due to renal salt wasting stimulates AVP release, which, in turn, enhances free water reabsorption and contributes towards restoring the volume of body fluid compartments to normal. Interestingly, however, the rising AVP excretion was not associated with the expected rise in urine osmolality; what is more, the lowest mean value for urine osmolality was observed in the third week when hyponatraemia was already established.On the basis of these observations it could be assumed that contrary to the general view2 either no postnatal increase in renal responsiveness to endogenous AVP occurs over the age period studied or the renal response to AVP is blunted by late hyponatraemia.In the present study we attempted to determine whether the impaired renal sodium conservation and hyponatraemia might limit the ability of the kidneys of premature infants to concentrate urine independent of AVP. Patients and methodsEleven healthy male premature infants with a mean birth weight of 1360 g (range 1020-1620 g) and mean gestational age of 31 weeks (range 29-33 weeks) were studied. All infants were delivered vaginally after uncomplicated pregnancy and labour. There were no pathological events known to increase AVP secretion during the perinatal period-that is, perinatal asphyxia, cardiopulmonary distress, or perinatal infection. They were nursed in a thermally controlled environment and fed pooled human milk. On the first day 5% glucose infu...

The role of endogenous dopamine (DA) in regulating arginine vasopressin (AVP) release and renal water excretion was studied in 10 premature infants with a mean birth weight of 1,341 g (range 1,150–1,660 g) and a mean gestational age of 30.2 weeks (28–33 weeks), who were given metoclopramide (MTC), a specific DA antagonist. It was demonstrated that in response to MTC urine flow rate increased significantly from a basal value of 0.90 ± 0.07 to 1.27 ± 0.09 ml/min/1.73 m² (mean ± SE; p < 0.01), urinary sodium excretion from 6.10 ± 1.47 to 11.7 ± 2.24 μEq/min/1.73 m² (p < 0.025) and osmolar clearance from 0.38 ± 0.044 to 0.600 ± 0.075 ml/min/1.73 m² (p < 0.01). MTC administration did not cause any alterations in free water clearance, whereas urinary AVP excretion fell significantly from 49.38 ± 10.13 to 32.66 ± 6.53 ng/min/1.73 m² (p < 0.05) after MTC. It is concluded that, contrary to adults, in low birth weight premature infants endogenous DA is enhancing rather than inhibiting AVP release and the MTC-induced water diuresis is independent of the fall of AVP since free water clearance remained unaltered after MTC.

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