Mechanisms of the negative inotropic effects of sphingosine-1-phosphate on adult mouse ventricular myocytes

1 Department of Bioengineering, University of California, San Diego, La Jolla, California; and 2 Department of Kinesiology, University of Calgary, Calgary, Alberta, Canada Submitted 13 March 2007 ; accepted in final form 12 November 2007 Sphingosine-1-phosphate (S1P) induces a transient bradycardia in mammalian hearts through activation of an inwardly rectifying K + current ( I K ACh ) 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. S1P 1 was shown to be involved by using the S1P 1 -selective agonist SEW2871 on myocytes isolated from S1P 3 -null mice. However, in these myocytes, S1P 3 can modulate a somewhat similar negative inotropy, as judged by the effects of the S1P 1 antagonist VPC23019 . Since S1P 1 activates G i exclusively, whereas S1P 3 activates both G i and G q , these results strongly implicate the involvement of mainly G i . Additional experiments using the I K ACh blocker tertiapin demonstrated that I K ACh can contribute to the negative inotropy following S1P activation of S1P 1 (perhaps through G iβ subunits). Mathematical modeling of the effects of S1P on APD in the mouse ventricle suggests that shortening of APD (e.g., as induced by I K ACh ) can reduce L-type calcium current and thus can decrease the intracellular Ca 2+ concentration ([Ca 2+ ] 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 G i to reduce L-type calcium channel current, blunt calcium-induced calcium release, and decrease [Ca 2+ ] i ; and 2 ) a second pathway that acts via G i to activate I K ACh and reduce APD. This decrease in APD is expected to decrease Ca 2+ influx and reduce [Ca 2+ ] i and myocyte contractility. calcium; contraction; cell shortening; inhibitory G protein; acetylcholine-sensitive potassium; myocyte Address for reprint requests and other correspondence: W. R. Giles, Dept. of Kinesiology, Univ. of Calgary, 2500 Univ. Dr. NW, Calgary, Alberta, Canada T2N 1N4 (e-mail: wgiles{at}ucalgary.ca )