Renal glomerular filtration rate and hepatic blood flow during voluntary diving in Weddell seals

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Abstract
Renal and hepatic function were studied during voluntary dives in Weddell seals by measuring the clearance rate of inulin and indocyanine green (ICG). Inulin is cleared exclusively by the kidneys and measures renal glomerular filtration rate (GFR). ICG is cleared by the liver and is blood flow dependent at concentrations used. Studies were conducted from a portable hut with a trapdoor placed over an isolated hole in the sea ice near McMurdo Station, Antarctica. An intravertebral extradural catheter was inserted percutaneously under light anesthesia in subadult seals weighing 130-200 kg. When released into the ice hole, the seals made voluntary dives, but always had to return to breathe. Serial blood samples were taken after single injections of inulin and ICG and analyzed within 24 h. The mean half time (t 1/2) for inulin clearance while resting at the surface was 27.3 +/- 13.0 min (n = 43) and the mean t 1/2 for ICG clearance was 18.3 +/- 7.3 min (n = 23). The mean resting GFR was 3.6 ml X min-1 X kg-1 (range 3.2-3.9, n = 3). Inulin and ICG clearance rates did not change from resting levels during dives shorter than the seal's aerobic dive limit (ADL). Inulin clearance decreased over 90% during dives longer than the ADL, but there was no significant reduction in ICG clearance during dives lasting up to 23 min. It appears that normal renal GFR and hepatic blood flow continue during natural aerobic dives. During dives that exceed the ADL, GFR is reduced but hepatic blood flow may be maintained.