THAPSIGARGIN-INDUCED CA2+ MOBILIZATION IN ACUTELY ISOLATED MOUSE LACRIMAL ACINAR-CELLS IS DEPENDENT ON A BASAL LEVEL OF INS(1,4,5)P-3 AND IS INHIBITED BY HEPARIN

The tumour-promoting agent thapsigargin has been shown to inhibit the microsomal Ca2+-ATPase and cause Ca2+ mobilization in a variety of cell types including exocrine acinar cells [Bird, Obie and Putney (1992) J. Biol. Chem. 267, 18382-18386]. When applied to acutely isolated lacrimal acinar cells, thapsigargin caused a slow biphasic activation of both the Ca2+-dependent K+ and Cl- currents measured using the whole-cell patch-clamp technique. If the only action of thapsigargin is to inhibit sequestration into Ca2+ pools, then Ca2+ mobilization following exposure to thapsigargin indicates that there is a significant 'leak' of Ca2+ into the cytoplasm, which is normally countered by Ca2+-ATPase activity. In the present study, we introduced the Ins(1,4,5)P-3 receptor antagonist heparin (200 mu g/ml) into lacrimal acinar cells via the patch-clamp pipette. Following a 5 min preincubation in the presence of heparin, neither acetylcholine (1 mu M) nor thapsigargin (1 mu M) caused any significant increase in either Ca2+-dependent current. Caffeine has been shown to suppress basal Ins(1,4,5)P-3 levels in exocrine acinar cells [Toescu, O'Neill, Petersen and Eisner (1992) J. Biol. Chem. 267, 23467-23470]. Preincubation with caffeine (10 mM) also inhibited the response to subsequent exposure to thapsigargin. These data suggest that, in acutely isolated lacrimal cells, the source of the Ca2+ leak which gives rise to Ca2+ mobilization following inhibition of Ca2+ re-uptake by thapsigargin is Ca2+ release, from Ins(1,4,5)P-3-dependent Ca2+ pools, caused by resting Ins(1,4,5)P-3 levels.