Depression of excitability by sphingosine 1-phosphate in rat ventricular myocytes

phate in rat ventricular myocytes. Am. J. Physiol. 275 (Heart Circ. Physiol. 44): H2291–H2299, 1998.—Sphingosine 1-phos-phate (S-1-P) is a bioactive sphingolipid that is released from activated platelets. Extracellular S-1-P augments an in-wardly rectifying potassium conductance in cultured atrial preparations, but the electrophysiological effects of this com-pound in the ventricle are unknown. The electrophysiological effects of S-1-P were examined in single myocytes from rat ventricular muscle. Action potential waveforms and underly-ing ionic currents in the presence and absence of 3 µM S-1-P (1–6 min) were recorded. S-1-P increased the minimum stimulus current needed to elicit an action potential by,100 pA. Pertussis toxin or preexposure to S-1-P did not alter this effect. The action potential waveform was unchanged by S-1-P. The inward sodium current (INa) was examined in