Sodium-phosphate cotransporter in human salivary glands: molecular evidence for the involvement of NPT2b in acinar phosphate secretion and ductal phosphate reabsorption.
OBJECTIVE
In order to elucidate the cellular and molecular mechanisms of phosphate secretion by human salivary glands, the expression and intracellular distribution of sodium-phosphate cotransporters was investigated.


METHODS
Total RNA was extracted from 33 parotid gland (PG) and 35 submandibular gland (SMG) samples and RT-PCR was performed using gene specific primers for all known sodium-phosphate cotransporters. An antibody was raised against an NPT2b epitope and the cellular and intracellular distribution was investigated by immunohistochemistry.


RESULTS
No mRNA for the type I cotransporter NPT1 was found. Out of the type II phosphate cotransporters only message for NPT2b but not for NPT2a or NPT2c could be detected in about the same number of samples (76% in PG versus 69% in SMG). Type III cotransporter mRNA was also found in both glands, PIT1 gave positive results for 93% of PG samples compared to 69% of SMG samples. For PIT2 also, a higher expression was found in PG than in SMG, although the difference was smaller (79% versus 51%). Immunostaining for NPT2b was found both in the acini and in the ducts, with a stronger reaction in the latter. In acinar cells, NPT2b was restricted to the basal-lateral plasma membrane, in duct cells, a broad band of reactivity was located in the apical part of the cell.


CONCLUSIONS
These findings suggest a secondary active secretion of phosphate into the primary saliva. Ductal cells appear to be able to reabsorb phosphate, thereby modifying the phosphate concentration in the final saliva.
Homann
Veronika
V
Abteilung Epithelphysiologie, Max-Planck-Institut für molekulare Physiologie, 44139 Dortmund, Germany.
Rosin-Steiner
Sigrid
S
Stratmann
Tina
T
Arnold
Wolfgang H
WH
Gaengler
Peter
P
Kinne
Rolf K-H
RK
D000328:Adult; D005455:Fluorescent Antibody Technique / Q000379:methods; D005982:Glutathione Transferase / Q000378:metabolism; D006801:Humans; D007150:Immunohistochemistry / Q000379:methods; D010306:Parotid Gland / Q000378:metabolism; D010710:Phosphates / Q000378:metabolism; D012333:RNA, Messenger / Q000032:analysis; D020133:Reverse Transcriptase Polymerase Chain Reaction / Q000379:methods; D018987:Salivary Ducts / Q000378:metabolism; D012469:Salivary Glands / Q000378:metabolism*; D050609:Sodium-Phosphate Cotransporter Proteins, Type I / Q000235:genetics / Q000378:metabolism*; D050611:Sodium-Phosphate Cotransporter Proteins, Type III / Q000235:genetics / Q000378:metabolism; D050614:Sodium-Phosphate Cotransporter Proteins, Type IIb / Q000235:genetics / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000032:analysis; D013363:Submandibular Gland / Q000378:metabolism
D010710:Phosphates; D012333:RNA, Messenger; D050609:Sodium-Phosphate Cotransporter Proteins, Type I; D050611:Sodium-Phosphate Cotransporter Proteins, Type III; D050614:Sodium-Phosphate Cotransporter Proteins, Type IIb; D005982:Glutathione Transferase; D000254:Sodium-Potassium-Exchanging ATPase