Involvement of carboxyl groups in the divalent cation permeability of rat parotid gland basolateral plasma membrane

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Abstract
Divalent cation permeability of rat parotid gland basolateral plasma membranes was examined in dispersed parotid acini (by Ca2+ or Mn2+ entry) and in isolated basolateral plasma membrane vesicles (BLMV, by45Ca2+ influx). Mn2+ entry (fura2 quenching) was about 1.6 fold higher in internal Ca2+ pool-depleted acini (Ca2+-depl acini) than in unstimulated cells. Mn2+ entry into Ca2+-depl acini was increased at external pH>7.4 and decreased at pH2+-depl acini with the relatively hydrophobic carboxylic group reagent, N,N′-dicyclohexylcarbodiimide (DCCD, 50 μM for 30 min) resulted in the inhibition of Mn2+ entry into Ca2+-depl acini to unstimulated levels. Another hydrophobic carboxyl group reagent, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) and the relatively hydrophilic carboxyl group reagents, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and 1-cyclohexyl-3-(2-morpholinoethyl)carbodiimide (CMCD) did not affect Mn2+ entry. Similar to the effects in intact acini, Ca2+ influx into BLMV was decreased when the external pH was lowered below 7.4. Also DCCD (5 mM, 30 min), but not EEDQ, decreased (40%) Ca2+ influx in BLMV. However, unlike in acini, the hydrophilic reagents, EDC, EAC, and CMCD decreased Ca2+ permeability in BLMV and the effects were nonadditive with the decrease induced by DCCD. The aggregate effects of carboxyl group reagents on the Ca2+ and Mn2+ permeability in BLMV and intact acini, respectively, suggest that a critical carboxyl group (most likely accessible from the cytoplasmic side of the plasma membrane) is involved in divalent cation flux in rat parotid acinar cells.