ACTIVATION OF MUSCARINIC K-PIG ATRIAL MYOCYTES BY SPHINGOSINE-1-PHOSPHATE( CURRENT IN GUINEA)

1. Activation of muscarinic K+ current (I-K(ACh)) by sphingosine-1-pho sphate (Sph-1-P) was studied in isolated cultured guinea-pig atrial my ocytes using whole-cell voltage clamp. 2. Sph-1-P caused activation of I-K(ACh) with an EC(50) of 1.2 nM. The maximal current that could be activated by Sph-1-P amounted to about 90% of the I-K(ACh) caused by a saturating concentration of acetylcholine (ACh, 10 mu M). Sphingosine (1 mu M), which can mimic the signalling effects of Sph-1-P in other cells, failed to cause measurable activation of IK(ACh). 3. I-K(ACh) a ctivation by Sph-1-P was completely suppressed in cells treated with p ertussis toxin. 4. Desensitization of muscarinic receptors by pre-incu bation of the cells with carbachol did not affect the response to Sph- 1-P; likewise, pre-incubation of the cells with Sph-1-P resulted in a reduced sensitivity to the phospholipid but not to ACh. In contrast, p re-incubation with either Sph-1-P or a serum phospholipid previously d escribed as activating atrial I-K(ACh) resulted in reduced sensitivity to both phospholipids. 5. It is concluded that activation of I-K(ACh) by Sph-1-P in atrial myocytes is induced by binding to a novel G prot ein-coupled phospholipid receptor.