Hypernatremia in twin lambs is a time-limited phenomenon and correlates with rapid catch-up growth

Background: Because twins are of lower weight at birth than singletons, they have been used repeatedly as a model for investigating proposed prenatal origins of adult disease. A recent study on 3-week-old twin lambs suggested programmed hypernatremia as a potential mechanism for sodium-dependent hypertension. To test the hypothesis that such hypernatremia is associated with rapid catch-up growth, a longitudinal analysis on lambs from twin and triplet pregnancies was performed.

Methods: We studied 15 twin or triplet lambs from Merino ewes, which were provided with food and water ad libitum before and after delivery. All newborn lambs received colostrum and were allowed nursing with the ewe ad libitum. Three weeks after birth they were either assigned to a group showing accelerated postnatal weight gain (>6%/day) or to a group with significantly lower average weight gain (<4.5%/day). A control group consisted of 9 singleton lambs. Blood samples were collected and analysed at 10, 21, 35, 56 and 84 days after birth.

Results: Differences in plasma sodium between multiples and singletons were only seen transiently. During the first 8 weeks plasma sodium levels remained highest in multiples with accelerated weight gain, although the differences between singletons and the latter group diminished. Plasma sodium levels of multiples with slow weight gain were consistently lower until these animals also exhibited catch-up growth. No differences were observed at 84 days after birth. However, after weaning the arterial blood pressure was still elevated in multiples showing rapid catch-up growth in comparison with the other groups.

Conclusion: Our data reveal a clear correlation of plasma sodium levels with the speed of catch-up growth, arguing against the hypothesis of programmed hypernatremic hypertension in ovine twins. Further long-term studies on appropriate models are required to understand how intrauterine growth restriction may influence blood pressure regulation.