Capacitative Ca2+ entry (CCE) induced by luminal and basolateral ATP in polarised MDCK-C7 cells is restricted to the basolateral membrane

In previous studies we have characterised various properties of capacitative Ca2+ entry (CCE) in different epithelia. After Ca2+ store depletion with PLC/InsP(3)-coupled agonists or by inhibition of store Ca2+ uptake, with for example thapsigargin, Ca2+ influx is activated. This leads to a sustained cellular response (e.g. NaCl secretion). In the present study, we have investigated CCE in polarised MDCK-C7 cells grown on permeable supports in a chamber allowing for separate luminal and basolateral perfusion. The transepithelial resistance (R-te) and voltage (V-te) were measured simultaneously to verify the tightness of the epithelial monolayers. MDCK-C7 cells grew to very tight monolayers (R-te > 3000 Omega.cm(2)). Apical ATP (100 mu mol/l) led to a biphasic [Ca2+](i) increase. Removal of apical Ca2+ in the continuous presence of ATP did not reduce the stimulated plateau. However, removal of Ca2+ from the basolateral side rapidly and completely interrupted the [Ca2+](i) plateau to below basal values ([Ca2+](i) decrease during plateau phase after removal of basolateral Ca2+ = 213 +/- 15 nmol/l, n = 9). Furthermore, MDCK-C7 responded to basolateral ATP (100 mu mol/l) with a biphasic [Ca2+](i) transient. Again the plateau phase of the ATP-induced [Ca2+](i) effect was fully dependent on the presence of basolateral but not apical Ca2+([Ca2+](i) decrease during plateau phase after removal of basolateral Ca2+ = 196 +/- 5 nmol/l, n = 10). Receptor-independent depletion of cytosolic Ca2+ stores with thapsigargin from both sides led to a rise in [Ca2+](i), which was also exclusively dependent on the presence of basolateral Ca2+ (n = 8). These data indicate that MDCK-C7 cells express luminal and basolateral P2-receptors coupled to PLC/InsP(3)/Ca2+. ATP applied from both sides induced a sustained [Ca2+](i) plateau which was due to transmembrane Ca2+ influx. The ATP- and thapsigargin-induced Ca2+ influx pathway was exclusively located in the basolateral membrane.