Adrenal medulla chromaffin cells are stimulated by acetylcholine released from sympathetic preganglionic neurons and secret catecholamines into the blood to accommodate short-term stress. Acetylcholine activates nicotinic and muscarinic receptors to elevate the intracellular Ca2+ concentration and causes the secretion of catecholamines. Lysophospholipids in the
serum, including sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA), are released primarily from immune cells during blood clotting and infection to modulate various physiological activities. To characterize the effects of lysophospholipids on Ca2+ responses in bovine chromaffin cells, cells were loaded with the Ca2+ indicator fura-2, and fluorescence intensity was used to monitor changes in intracellular Ca2+ concentration. Cells treated with 1 μM S1P or LPA for 0, 10, 20, 30, 40, 50, or 60 min were stimulated with acetylcholine (100 μM), muscarine (100 μM), DMPP (10 μM), or high K+ buffer (50 mM KCl) for 5 sec.
The results showed that the Ca2+ responses evoked by acetylcholine were significantly inhibited after incubation in LPA or S1P for 20 min. The Ca2+ response evoked by high K+ buffer was not inhibited by S1P pretreatment and was significantly facilitated by LPA pretreatment. Muscarine-evoked Ca2+ responses were slightly attenuated by LPA and S1P pretreatments. Nicotine-evoked responses were inhibited in both LPA- and S1P-pretreated cells. Our findings demonstrate that nicotinic receptors may be the main targets of lysophospholipids in inhibiting acetylcholine-evoked Ca2+ responses.