Renal function in Palestine sunbirds: elimination of excess water does not constrain energy intake

Abstract

SUMMARY Although the renal responses of birds to dehydration have received significant attention, the consequences of ingesting and processing large quantities of water have been less studied. Nectar-feeding birds must often deal with exceptionally high water intake rates in order to meet their high mass-specific energy demands. Birds that ingest large volumes of water may either eliminate excess water in the kidney or regulate the volume of water absorbed in the gastrointestinal tract. Because water absorption in the gastrointestinal tract of Palestine sunbirds (Nectarinia osea)decreases with increasing water ingestion rate, we predicted that glomerular filtration rate (GFR) in these birds would not be unusually high in spite of large ingested water loads. When feeding on dilute sucrose solutions, sunbirds ingested between 4 and 6 times their body mass in nectar per day, yet they were able to compensate for varying nectar energy density and increased thermoregulatory energy demands with no apparent difficulty. GFR was lower than predicted (1976.22±91.95 μl h-1), and was not exceptionally sensitive to water loading. Plasma glucose concentrations were high, and varied 1.8-fold between fasted (16.08± 0.75 mmol l-1) and fed (28.18±0.68 mmol l-1) sunbirds, but because GFR was low, glucose filtered load also remained relatively low. Essentially the entire glucose filtered load (98%) was recovered by the kidney. Renal fractional water reabsorption (FWR) decreased from 0.98 to 0.64 with increasing water intake. The ability of Palestine sunbirds to reduce the absorption of ingested water in the gastrointestinal tract may resolve the potential conflict between filtering a large excess of absorbed water in the kidney and simultaneously retaining filtered metabolites.

A Comparison of Pharmacokinetic Methods for In Vivo Studies of Nonmediated Glucose Absorption

“…This has proven to be a distinct advantage of this protocol, and it has been widely applied recently (McWhorter and Martínez del Rio 1999;McWhorter et al 2003McWhorter et al , 2004McWhorter et al , 2006Hartman Bakken et al 2004;Hartman Bakken and Sabat 2006;Napier et al 2008b). Another advantage is that this method may be used to study the effects of diet energy density on the extent of paracellular absorption.…”

“…Elimination of l-glucose in the steady state feeding protocol (based on appearance of marker in excreta) is influenced by GFR, renal fractional water recovery (FWR), and rate of digesta passing through the intestine (McWhorter et al 2003). FWR is responsive to reduced water intake rate in red wattlebirds (Goldstein and Bradshaw 1998) and other nectarivorous birds (Hartman Bakken et al 2004;McWhorter et al 2004) while GFR has been shown to be responsive to more extensive water shortage (e.g., due to nocturnal fasting) in hummingbirds (Hartman Bakken et al 2004;Hartman Bakken and Sabat 2006); renal function in the nectarivorous birds studied to date may therefore be slowed or completely stop in response to reduced food (i.e., water) intake. When the birds are feeding ad lib.…”

“…Furthermore, the protocol allows for manipulation of the diet the animal is feeding on, allowing assessment of the effects of diet energy density or nutrient concentration on absorption. Since its adaptation by Karasov and Cork (1994), the steady state feeding method has been the protocol of choice for assessing paracellular nutrient absorption in small species (McWhorter et al 2006) and has also been adapted for studying intestinal water absorption and renal function (McWhorter and Martínez del Rio 1999;McWhorter et al 2003McWhorter et al , 2004Hartman Bakken et al 2004;Hartman Bakken and Sabat 2006).…”

“…Karasov and Cork (1994) showed that this was the case in nectarivorous rainbow lorikeets (Trichoglossus haematodus) by sampling both blood and excreta in the same birds after probe injection. Although the method has been widely used over the intervening years (e.g., McWhorter and Martínez del Rio 1999;McWhorter et al 2003McWhorter et al , 2004Hartman Bakken et al 2004;Hartman Bakken and Sabat 2006;Napier et al 2008b), this assumption has not been tested in any species other than the rainbow lorikeet. There has been no direct comparison of these two pharmacokinetic protocols in the same species.…”

Modeling Chick to Assess Diabetes Pathogenesis and Treatment

“…Hummingbirds and many other migratory birds maintain very high glucose and elevated body fat as an adaptive trait without developing diabetes [24][25][26]. These birds consume high sugar food (nectar) and accumulate over 40% body fat shortly before migration, but they do not develop diabetes, polyurea, polyphagia, or polydipsia [27][28][29]. They have a very high metabolic rate and lose most of the stored fat in 20 hours by flying up to 600 miles across the Gulf of Mexico.…”