Osmoregulation in marine mammals. (vol 204, pg 1831, 2001)

Osmoregulation in marine mammals has been investigated for over a century; however, a review of recent advances in our understanding of water and elec trolyte balance and of renal function in marine mammals is warranted. The f ollowing topics are discussed: (i) kidney structure and urine concentrating ability, (il) sources of water, (iii) the effects of feeding, fasting and diving, (iv) the renal responses to infusions of varying salinity and (v) h ormonal regulation. The kidneys of pinnipeds and cetaceans are reniculate i n structure, unlike those of terrestrial mammals (except bears), but this d ifference does not confer any greater concentrating ability. Pinnipeds, cet aceans, manatees and sea otters can concentrate their urine above the conce ntration of sea water, but only pinnipeds and otters have been shown to pro duce urine concentrations of Na+ and Cl- that are similar to those in sea w ater. This could afford them the capacity to drink sea water and not lose f resh water. However, with few exceptions, drinking is not a common behavior in pinnipeds and cetaceans. Water balance is maintained in these animals v ia metabolic and dietary water, while incidental ingestion and dietary salt may help maintain electrolyte homeostasis. Unlike most other aquatic mamma ls, sea otters commonly drink sea water and manatees frequently drink fresh water. Among the various taxonomic groups of marine mammals, the sensitivi ty of the renin-angiotensin-aldosterone system appears to be influenced by the availability of Na+. The antidiuretic role of vasopressin remains incon clusive in marine mammals, while the natriuretic function of atrial natriur etic peptide has yet to be examined. Ideas on the direction of future studi es are presented.