title: Effects of pH on apical calcium entry and active calcium transport in rabbit cortical collecting system.
abstract: Rabbit connecting tubules and cortical collecting ducts were isolated by immunodissection and cultured on permeable supports. The monolayers actively transported Ca2+ with a net transcellular rate of 92 +/- 3 nmol.h-1.cm-2. Methoxyverapamil, felodipine, diltiazem, omega-conotoxin GVIA, and omega-agatoxin IVA when added to the apical side had no effect on Ca2+ absorption. Neither hyperpolarization nor depolarization of the apical membrane affected Ca2+ transport rates significantly. Stepwise lowering of the apical pH (pHa) from 8.0 to 5.6 gradually inhibited Ca2+ transport from 88 +/- 5 to 7 +/- 2 nmol.h-1.cm-2. Measuring the intracellular pH (pHi) with 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein revealed that lowering the pHa from 8.0 to 5.6 decreased pHi from 7.8 to 6.7. To determine whether inhibition of Ca2+ absorption results from intracellular acidification, pHi was lowered using an NH4Cl pulse while extracellular pH was kept constant. Intracellular acidification from 7.4 +/- 0.2 to 6.9 +/- 0.1 reduced Ca2+ absorption by 26 +/- 6% only. In addition, lowering of the basolateral pH to 6.2 resulted in a pHi of 6.8 +/- 0.1, without affecting Ca2+ absorption rates. In conclusion, the basal Ca2+ influx mechanism in the apical membrane is most likely a voltage-independent Ca2+ transporter, insensitive to Ca2+ channel blockers, but strongly inhibited by apical acidification.
authors.affiliation: Department of Cell Physiology, University of Nijmegen, The Netherlands.
authors.forename: R J
authors.initials: RJ
authors.lastname: Bindels
authors.forename: A
authors.initials: A
authors.lastname: Hartog
authors.forename: S L
authors.initials: SL
authors.lastname: Abrahamse
authors.forename: C H
authors.initials: CH
authors.lastname: Van Os
mesh_terms: D000818:Animals; D017920:Antiporters / Q000378:metabolism; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002135:Calcium-Binding Proteins / Q000378:metabolism; D027682:Cation Transport Proteins; D002462:Cell Membrane / Q000378:metabolism / Q000502:physiology; D004594:Electrophysiology; D006863:Hydrogen-Ion Concentration; D007425:Intracellular Membranes / Q000378:metabolism; D007672:Kidney Cortex; D007685:Kidney Tubules, Collecting / Q000378:metabolism*; D011817:Rabbits
chemical_list: D017920:Antiporters; D002121:Calcium Channel Blockers; D002135:Calcium-Binding Proteins; D027682:Cation Transport Proteins; C027614:calcium-hydrogen antiporters; D002118:Calcium