Sodium-Sulfate Symport by Aplysia californica Gut

Factors influencing the assimilation of arsenic in a deposit-feeding polychaete

We investigated mechanisms leading to the assimilation of particle-bound arsenic (As) ingested by the deposit-feeding polychaete Alitta succinea using a radiotracer approach. The release of As from different particle types into extracted gut fluid or bovine serum albumin (BSA), a gut fluid mimic, was measured. In addition, gut fluid proteins were analyzed by separating proteins via 2D gel electrophoresis, and protein peptide sequences were determined by mass spectrometry. Major ions in the gut fluid were measured by ion chromatography and metals by mass spectrometry. Percentages of particulate As release were related to As assimilation efficiencies (AEs) in polychaetes feeding on different particle types. AEs of As were highest from radiolabeled pure diatoms (72%) and radiolabeled diatoms added to sediment (51%), lower from radiolabeled sediment (10%), and lowest from a radiolabeled iron oxide mineral, goethite (2%). It appears that As release from particles is a necessary but not sufficient requirement of As assimilation. For example, 15% of As was released from goethite into the gut fluid but only 2% was assimilated by A. succinea. Our results suggest that the likelihood of As assimilation is higher when it is bound to an organic compound of nutritional value in the ingested particles.

Phosphate absorption in Aplysia californica gut: glucocorticoid inhibition

Apical membranes of Aplysia californica foregut epithelia contain a sodium-phosphate symporter. Dexamethasone inhibited the absorptive activity of the sodium-phosphate symporter, whereas amiloride had no effect on the sodium-phosphate symporter. It appears that glucocorticoids or their molluscan equivalent play a role in the overall regulation of phosphate homeostasis by the Aplysia californica gut.

Thyroid hormone-induced sulfate absorption in Aplysia californica gut is mediated by protein synthesis

Mucosal membranes of foregut epithelia of Aplysia californica contain a sodium-sulfate symporter that is stimulated by triiodothyronine. Actinomycin D, puromycin, or cycloheximide inhibited the triiodothyronine-stimulated sulfate absorption. It appears that thyroid hormone manifests its effects on sulfate absorption in the A. californica gut through protein synthesis.

Sulfate absorption in Aplysia californica gut: glucocorticoid inhibition

Mucosal membranes of foregut epithelia of Aplysia californica contain a sodium/sulfate symporter. Dexamethasone inhibited the absorptive activity of the sodium/sulfate symporter, whereas amiloride had no effect on it. It appears that glucocorticoids or their molluscan equivalent play a role in the overall regulation of sulfate homeostasis by the A. californica gut.

Energetics of potassium ion transport in Aplysia gut

Basolateral membranes of Aplysia californica foregut epithelia contain an ATP-dependent Na(+)/K(+) transporter (Na(+)/K(+) pump or Na(+)/K (+) -ATPase). This Na(+)/K(+) pump accounts for both the intracellular Na(+) electrochemical potential (micro) being less than the extracelluar Na(+) micro and the intracellular K(+) micro being more than the extracellular K(+ ) micro. Also, K(+) channel activity resides in both luminal and basolateral membranes of the Aplysia foregut epithelial cells. Increased activity of the Na(+)/K(+) pump, coupled to luminal and basolateral membrane depolarization altered the K(+) transport energetics across the basolateral membrane to a greater extent than the alteration in K(+) transport energetics across the luminal membrane. These results suggest that K(+) transport, either into or out of the Aplysia foregut epithelial cells, is rate-limiting at the basolateral membrane.

Sulfate absorption inAplysiacalifornicagut: thyroid hormone stimulation

Mucosal membranes of foregut epithelia of Aplysia californica contain a sodium/sulfate symporter. Mucosal or serosal application of triiodothyronine stimulated the absorptive activity of the sodium/sulfate symporter, whereas reverse triiodothyronine had no effect on it. It appears that thyroid hormone or its molluscan equivalent plays a role in the overall regulation of sulfate homeostasis by the A. californica gut.

Transport energetics of the Na+ pump in Aplysia californica gut

Basolateral membranes of Aplysia californica foregut epithelia contain an ATP-dependent Na+ transporter (Na+ pump). Increased activity of the Na+ pump, coupled to luminal Na+/AIB symporter activity and basolateral membrane depolarization, changed the Na+ transport energetics across the basolateral membrane to a greater extent than the change in Na+ transport energetics across the luminal membrane.Key words: Na+ pump, Na+/K+-ATPase, Na+ electrochemical potential.

Sulfate absorption in Aplysia californica gut: intracellular regulation by cyclic guanosine monophosphate

Luminal membranes of foregut epithelia of Aplysia californica contain a sodium–sulfate symporter. Cyclic guanosine monophosphate stimulated the absorptive activity of the sodium–sulfate symporter, whereas cyclic adenosine monophosphate had no effect on the sodium–sulfate symporter. It appears that guanylate cyclase plays a role in the overall regulation of sulfate homeostasis by the A. californica gut.