Ingestion, intestinal absorption, and elimination of seawater and salts in the southern flounder, Paralichthys lethostigma

Abstract

Simultaneous measurements of flow rates and major electrolyte composition of urine and rectal fluid were made on five southern flounders, Paralichthys lethostigma, averaging 1,19 kg body weight. Rectal fluid is a residue of seawater swallowed by the fish, isosmotic to the blood plasma (336 mosmoles/l), and composed principally of magnesium (average 180 mmoles/l), sulfate (average 105 mmoles/l), and chloride (average 126 mmoles/l). Only 15.5% of the magnesium and 11.3% of the sulfate swallowed was absorbed from the intestine; all absorbed was probably excreted entirely by the kidney. The rate of seawater ingestion calculated from the total (renal + rectal) excretion of magnesium and sulfate yielded estimates averaging 4.6 ml/h × kg body weight. Of this volume, 24% (average 1.1 ml/h × kg) was eliminated as rectal fluid, 4% (average 0,18 ml/h × kg) was excreted as urine and 72% (average 3.3 ml/h × kg) was lost osmotically across the body surface. Nearly 99% of the sodium, 98% of the potassium, and 93% of the chloride swallowed was removed by an extrarenal route, presumably the gills. Of the calcium swallowed, 32% remained in the rectal fluid residue, 8% was excreted in the urine, and the rest (60%) was removed by unknown extrarenal pathways.

Environmental factors responsible for switching on the SO42−excretory system in the kidney of seawater eels

“…Despite the importance of SO 4 2Ϫ regulation, the research on ion regulation has focused mainly on monovalent ions (Na ϩ and Cl Ϫ for osmoregulation and H ϩ and HCO 3 Ϫ for pH regulation) and divalent cations (Ca 2ϩ and Mg 2ϩ for regulation of muscular contraction and others). (6,21), but obligatory influx of SO 4 2Ϫ was nullified by excretion via the kidney in marine teleosts (3,7,8,28). We recently showed that the eel is unique in SO 4 2Ϫ regulation compared with other euryhaline teleosts because it has much higher plasma SO 4 2Ϫ in FW (ϳ6 mM) than in SW (ϳ1 mM) (35) as reported previously (23).…”

“…Mg 2ϩ is the second most abundant cation in SW (ϳ50 mM) that is actively excreted by the kidney of marine teleosts (7,8). Because of the huge difference in Mg 2ϩ concentration between plasma and SW, Mg 2ϩ regulation at the kidney is also thought to be altered by the environmental salinity in fishes (2,4).…”

“…In summary, it appears that Cl Ϫ ions in SW are taken up into the circulation via the NCC together with Na ϩ , and the resultant increase in plasma Cl Ϫ concentration enhances SO 4 2Ϫ excretion by the kidney through downregulation of absorptive Slc13a1 and upregulation of excretory Slc26a6a, resulting in low plasma SO 4 2Ϫ concentration in SW. 2Ϫ regulation when they move between FW and SW. It has been suggested that the gills and intestine are almost impermeable to SO 4 2Ϫ (6, 21), but obligatory influx of SO 4 2Ϫ was nullified by excretion via the kidney in marine teleosts (3,7,8,28). We recently showed that the eel is unique in SO 4 2Ϫ regulation compared with other euryhaline teleosts because it has much higher plasma SO 4 2Ϫ in FW (ϳ6 mM) than in SW (ϳ1 mM) (35) as reported previously (23).…”

Calcium balance in sea bream (Sparus aurata): the effect of oestradiol-17beta

“…Previous studies have shown that the proportion of the calcium taken up from water in the intestine of seawater fish is very variable. For example, the cod (Gadus morhua) absorbs about 30% of the calcium from drinking water (Sundell et al 1993), while the flounder (Paralichthys lethostigma) absorbs about 70% (Hickman 1968). In tilapia (Oreochromis mossambicus), it has been shown that the net calcium transport in the intestine of seawater-adapted fish is significantly lower than in their freshwater counterparts, in which the uptake of calcium by the intestine represents a high proportion of whole body calcium uptake (Schoenmakers et al 1993).…”

Role of tubular secretion and carbonic anhydrase in vertebrate renal sulfate excretion

“…Water absorption by the intestine generates a high [SO 4 2Ϫ ] in the intestinal lumen (50-100 mM), and the steep concentration gradient promotes passive SO 4 2Ϫ absorption across the intestinal mucosa (34,60,62). Although passive SO 4 2Ϫ absorption dominates, an active secretory flux driven by an electroneutral brush-border SO 4 2Ϫ /Cl Ϫ exchanger is also present (62).…”