Uptake and Intracellular Sequestration of Divalent Cations in Resting and Methacholine-stimulated Mouse Lacrimal Acinar Cells DISSOCIATION BY Sr*’ AND Ba2+ OF AGONIST-STIMULATED DIVALENT CATION ENTRY FROM THE REFILLING OF THE AGONIST-SENSITIVE INTRACELLULAR POOL*

The abilities of various divalent cations to enter the cytoplasm of mouse lacrimal acinar cells was examined under resting and agonist-stimulated conditions, by monitoring their effects on the fluorescence of cytosolic fura. In vitro Ni’+, Co2+, and Mn2* quenched the furafluoresceAce, whereas Sr’+, Ba”, and La3+ produced an excitation spectrum and maximum brightness similar to Ca’+. Stimulation of mouse lacrimal acinar cells with methacholine (MeCh) caused a biphasic elevation of intracellular Ca2+ concentration ([Ca”+]i) resulting from a release of Ca2+ from intracellular pools followed by a sustained entry of extracellular Ca2’. Neither La3+ nor Ni2+ entered the cells under resting or stimulated conditions, but both blocked Ca2’ entry. Although both Co2+ and Mn2+ entered unstimulated cells, this process was not increased by MeCh. Both Sr2+ and Ba2+ were capable of supporting a sustained increase in furafluorescence in response to MeCh, indicating that these cations can enter the cells through the agonist-regulated channels. However, Sr2*, but not Ba2+, was capable of refilling the agonist-sensitive intracellular stores. These findings demonstrate dissociation of agonist-induced Ca2+ entry from intracellular Ca2+ pool refilling and thereby provide strong support for the recently modified version of the capacitative Ca2+ entry model according to which influx into the cytoplasm occurs directly across the plasma membrane and does not require a specialized cation channel directly linking the extracellular space and the intracellular Ca2+ stores.