Asymmetrical, agonist-induced fluctuations in local extracellular [Ca2+] in intact polarized epithelia

We recently proposed that extracellular Ca2+ ions participate in a novel form of intercellular communication involving the extracellular Ca2+-sensing receptor (CaR). Here, using Ca2+-selective microelectrodes, we directly measured the profile of agonist-induced [Ca2+](ext) changes in restricted domains near the basolateral or luminal membranes of polarized gastric acid-secreting cells. The Ca2+-mobilizing agonist carbachol elicited a transient, La3+-sensitive decrease in basolateral [Ca2+] (average approximate to 250 muM, but as large as 530 muM). Conversely, carbachol evoked an HgCl2-sensitive increase in [Ca2+] (average approximate to 400 muM, but as large as 520 muM) in the lumen of single gastric glands. Both responses were significantly reduced by pretreatment with sarco-endoplasmic reticulum Ca2+ ATPase (SERCA) pump inhibitors or with the intracellular Ca2+ chelator BAPTA-AM. Immunofluorescence experiments demonstrated an asymmetric localization of plasma membrane Ca2+ ATPase (PMCA), which appeared to be partially co-localized with Calk and the gastric H+/K+-ATPase in the apical membrane of the acid-secreting cells. Our data indicate that agonist stimulation results in local fluctuations in [Ca2+](ext) that would be sufficient to modulate the activity of the CaR on neighboring cells.