Ca2+ signalling in exocrine acinar cells has been shown to be both polarized and pulsatile in all cell types tested, except acutely isolated mouse lacrimal acinar cells. Lacrimal cells are also unusual in that they display a very low sensitivity to Ins(1,4,5)P3 (InsP3) that may be enhanced by placing the cells in primary culture for 12—72h or by intracellular infusion of a low concentration of Ins(1,3,4,5)P4 (InsP4). We have proposed previously that this atypical behaviour stemmed from vesiculation of the endoplasmic reticulum (ER) incurred during isolation of the cells and, furthermore, that time in culture or InsP4 increased sensitivity to InsP3 by increasing ER integrity [Smith, Harmer, Letcher and Irvine (2000) Biochem. J. 347, 77—82]. We have measured the half time for fluorescence recovery after photobleaching (FRAP) of a fluorescent marker (Mag-fluo 4) loaded into the ER lumen in order to determine directly the functional integrity of the ER in lacrimal cells. The half-time for FRAP was increased (indicating a reduction in the functional integrity of the ER) following exposure to anti-microtubule agents (taxol and nocodazole) known to perturb ER structure and decreased (indicating an increase in the functional integrity of the ER) by time in culture and exposure to InsP4. The action of InsP4 was particularly pronounced because it occurred under patch-clamp whole-cell conditions that were themselves found to reduce ER functional integrity. These data show that ER remodelling could be a physiological regulator of Ca2+ signalling and indicate a role for InsP4 in control of this process.
Abbreviations used: AM, acetoxymethyl ester; [Ca2+]i, intracellular Ca2+ concentration; CICR, Ca2+-induced Ca2+ release; ER, endoplasmic reticulum; FRAP, fluorescence recovery after photobleaching; GAP, GTPase-activating protein; InsP3, Ins(1,4,5)P3; InsP4, Ins(1,3,4,5)P4; IP4BP, InsP4-binding protein.