ABSTRACT
X-ray diffraction patterns of [N(CH3)4][CdCl3], tetramethylammonium trichlorocadmate(II), have been investigated in the temperature range 80-293 K, which includes two phase transitions at 118 and 104 K, respectively. The main interest in this compound is to establish the mechanism of the structural phase transitions common to other members of the isostructural family [(CH3)4N][MX3]. It is supposed to be related to the ordering of the organic part together with some small distortion of the inorganic chains. The origin of the order-disorder mechanism would be the orientationally disordered distribution of the tetramethylammonium tetrahedra at room temperature. Maximum Entropy Methods suggest that the most probable distribution of the organic groups can be described through the so-called two-well model, in which one threefold axis of the tetramethylammonium tetrahedron coincides with the crystallographic threefold axis of the structure. Below 118 K the reflections are split. However, the splitting cannot be fully explained by the ferroelastic domains expected to appear after the phase transitions. Recent NMR results [Mulla-Osman et al. (1998). J. Phys. Condensed Matter, 10, 2465-2476] corroborate the existence of more domains than expected from symmetry considerations. A model of ferroelastic domains which is in agreement with both X-ray diffraction diagram and NMR measurements is proposed.
ABSTRACT
Isolated ductal cells of rat submandibular gland phospholipid pools were labeled with [3H]arachidonic acid (AA). The tracer was incorporated preferentially to phosphatidylcholine (46% of the lipidic fraction). Extracellular ATP induced the release of [3H]AA to the extracellular medium in a time- and dose-dependent manner (EC50 = 220 microM). Among other agents tested, only 2', 3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate (Bz-ATP) was able to mimic the effect of ATP (EC50 = 15 microM), without activation of phospholipase C. The purinergic antagonists oxidized ATP, suramin, and Coomassie Blue partly inhibited the response to 1 mM ATP and 100 microM Bz-ATP; the response was also blocked by the addition of Mg2+ or Ni2+. Expression of P2X7 receptor mRNA in these cells was confirmed by reverse transcription-polymerase chain reaction. In the presence of extracellular calcium, the phospholipase A2 inhibitor 2-(p-amylcinnamoyl)amino-4-chlorobenzoic acid (a nonspecific inhibitor), arachidonyl trifluoromethylketone (AACOCF3, an inhibitor of the calcium-dependent cytosolic PLA2 (cPLA2)), and bromoenol lactone (an inhibitor of the calcium-independent PLA2 (iPLA2)) inhibited the release of [3H]AA induced by ATP and Bz-ATP. In the absence of extracellular calcium, the release of [3H]AA in response to the purinergic agonists was still observed; this response was not affected by AACOCF3 and completely blocked by bromoenol lactone. ATP and Bz-ATP stimulated a calcium-independent secretion of kallikrein, which could be blocked by BEL but which was enhanced by AACOCF3. It is concluded that the P2X7 receptor in ductal cells is coupled to kallikrein secretion through a calcium-dependent cPLA2 and a calcium-independent iPLA2.
