Effects of MeCh, thapsigargin, and La3+ on plasmalemmal and intracellular Ca2+ transport in lacrimal acinar cells

C. Y. Kwan, H. Takemura, J. F. Obie, O. Thastrup and J. W. Putney Jr Calcium Regulation Section, National Institute of Environmental Health Sciences, National Institutes of Health, Research Triangle Park, North Carolina 27709. The Ca2(+)-mobilizing actions of the muscarinic receptor agonist, methacholine (MeCh), and the microsomal Ca2+ pump inhibitor, thapsigargin, were investigated in lacrimal acinar cells. As previously shown for parotid cells (J. Biol. Chem. 264: 12266-12271, 1989), thapsigargin activates both internal Ca2+ release and Ca2+ entry from the extracellular space without increasing cellular inositol phosphates. The inorganic Ca2+ antagonist La3+ inhibited MeCh- or thapsigargin-activated Ca2+ entry. However, when added before MeCh or thapsigargin, La3+ inhibited the extrusion of Ca2+ at the plasma membrane. This phenomenon was exploited in protocols designed to investigate the pathways for filling agonist-sensitive Ca2+ stores in lacrimal cells. The results show that, in contrast to previous suggestions that external Ca2+ is required to replenish agonist-regulated Ca2+ stores, the inhibition of Ca2+ extrusion permits recycling of Ca2+ released by MeCh back into an MeCh- and thapsigargin-sensitive pool. Thus, although extracellular Ca2+ is the major source for refilling the intracellular Ca2+ stores under physiological conditions, the pathway by which this Ca2+ enters the pool need not be a direct one. These results are consistent with the recently revised capacitative model for the refilling of intracellular Ca2+ stores through Ca2+ influx subsequent to Ca2+ depletion, according to which refilling of intracellular Ca2+ stores occurs via a cytoplasmic route rather than a direct channel between intracellular Ca2+ stores and the extracellular space.