Sphingosine-1-phosphate (S1P) induces a transient bradycardia in mammalian hearts through activation of an inwardly rectifying K+current ( IKACh) in the atrium that shortens action potential duration (APD) in the atrium. We have investigated probable mechanisms and receptor-subtype specificity for S1P-induced negative inotropy in isolated adult mouse ventricular myocytes. Activation of S1P receptors by S1P (100 nM) reduced cell shortening by ∼25% (vs. untreated controls) in field-stimulated myocytes. S1P1was shown to be involved by using the S1P1-selective agonist SEW2871 on myocytes isolated from S1P3-null mice. However, in these myocytes, S1P3can modulate a somewhat similar negative inotropy, as judged by the effects of the S1P1antagonist VPC23019 . Since S1P1activates Giexclusively, whereas S1P3activates both Giand Gq, these results strongly implicate the involvement of mainly Gi. Additional experiments using the IKAChblocker tertiapin demonstrated that IKAChcan contribute to the negative inotropy following S1P activation of S1P1(perhaps through Giβγsubunits). Mathematical modeling of the effects of S1P on APD in the mouse ventricle suggests that shortening of APD (e.g., as induced by IKACh) can reduce L-type calcium current and thus can decrease the intracellular Ca2+concentration ([Ca2+]i) transient. Both effects can contribute to the observed negative inotropic effects of S1P. In summary, these findings suggest that the negative inotropy observed in S1P-treated adult mouse ventricular myocytes may consist of two distinctive components: 1) one pathway that acts via Gito reduce L-type calcium channel current, blunt calcium-induced calcium release, and decrease [Ca2+]i; and 2) a second pathway that acts via Gito activate IKAChand reduce APD. This decrease in APD is expected to decrease Ca2+influx and reduce [Ca2+]iand myocyte contractility.