Effects of sphingosine 1-phosphate on pacemaker activity in rabbit sino-atrial node cells

The effects of sphingosine 1-phosphate (S-1-P) on pacemaker activity and un derlying membrane rents were studied in isolated rabbit sino-atrial (SA) no de cells. S-1-P (0.1 mu M) reversibly increased the cycle length of spontan eous pacemaker activity from 560 to 1434 ms, and hyperpolarized the maximal diastolic potential (MDP) from -67 to -70 mV. In voltage-clamp experiments , S-1-P (1 mu M) activated a pertussis toxin-sensitive, inwardly-rectifying , time-independent K+ current (I-K,I-ACh) that had a reversal potential of -88 mV (K+ equilibrium potential -86 mV). S-1-P (1 mu M) had no measurable effect on the L-type Ca2+ current (I-Ca,I-L) or the hyperpolarization-activ ated inward current (I-f) under basal conditions. In the presence of the be ta-adrenergic agonist, isoproterenol (ISO, 0.1 mu M), S-1-P (1 mu M) revers ed the ISO-induced increase in pacing rate, hyperpolarized the MDP and decr eased the ISO-induced enhancement of both I-Ca,I-L (from 171 to 118% of con trol) and I-f (from 211 to 135% of control). These results demonstrate that under basal conditions S-1-P can significantly slow spontaneous pacing in rabbit SA node cells mainly due to activation of a background, inwardly-rec tifying K+ current. In the presence of ISO, S-1-P also slows the spontaneou s pacing rate due to activation of the same K+ current, as well as inhibiti on of I-Ca,I-L and I-f.