Effect of melatonin on osmoregulation of gulls (Larus glaucescens)

Melatonin (MT) is largely known for its role in regulation of biological rhythms in many species. However, several studies suggest that MT may also have an osmoregulatory role. For example, Ching et al. (1999) showed that salt glands of Pekin ducks have MT receptors (MR) and that MR increased in number when ducks drank saline. They also found that elevating plasma MT to its night time level inhibited salt gland secretion (SGS). The gut and kidneys of Pekin ducks also have MR. Since Pekin ducks are not marine birds, I followed and extended the protocols of Ching et al. to examine effects of MT on simultaneous salt gland and kidney function of Glaucous-winged gulls (Larus glaucescens) that regularly have a high NaCl intake. I infused gulls during the day with 500mM NaCl and made timed SGS, urine, and plasma collections. In the first experiment, each bird received two treatments (given a week apart): 1) 500mM NaCl only (control treatment) and 2) plasma MT concentration ([MT]pi) was initially elevated to and sustained at night time level during infusion of 500mM NaCl (MT treatment). Sample collections were made for an hour in each treatment. In the second experiment, after 60 minutes of control treatment [MT]pi was elevated to and sustained at night time level for the second hour. When [MT]P] was elevated prior to salt loading (experiment 1), the response of the salt glands was opposite to the inhibition observed in Pekin ducks and MT increased SGS rate of gulls. However, when [MT]pi was abruptly raised during ongoing SGS (experiment 2), its rate decreased during subsequent MT infusion, as it did in Pekin ducks. Kidney function was greatly affected by MT treatment in both experiments. GFR decreased when [MT]pi was abruptly increased during ongoing SGS. Urine flow rate increased especially when [MT]p l was elevated prior to initiation of SGS and renal Na+ excretion increased in both experiments. Simultaneous salt gland and renal excretion of Na+ exceeded excretion of water under both treatments and MT improved Na+ excretion. I concluded MT increased the efficiency of Na+ excretion in saline stressed gulls.Science, Faculty ofZoology, Department ofGraduat