Vol. 53, Issue 5, 862-869, May 1998

Guanine Nucleotide-Sensitive Inhibition of L-Type Ca²⁺ Current by Lysosphingolipids in RINm5F Insulinoma Cells

Herbert M. Himmel, Dagmar Meyer zu Heringdorf, Bernd Windorfer, Chris J. van Koppen, Ursula Ravens, and Karl H. Jakobs

Institut für Pharmakologie, Universitätsklinikum Essen, D-45122 Essen, Germany (H.M.H., D.M. zu H., B.W., C.J. van K., U.R., K.H.J.) and Institut für Pharmakologie und Toxikologie, Technische Universität Dresden, D-01109 Dresden, Germany (H.M.H., U.R.)

The lysosphingolipids sphingosine-1-phosphate (SPP) and sphingosylphosphorylcholine (SPPC) reportedly increase free cytosolic Ca²⁺ concentration ([Ca²⁺]_i) in a variety of cell types, apparently by activating G protein-coupled plasma membrane receptors. We investigated whether and how sphingolipids modulate Ca²⁺ homeostasis in the insulinoma cell line RINm5F. The addition of SPPC and glucopsychosine (GPS) did not affect basal [Ca²⁺]_i but inhibited the KCl (30 mM)-induced increase in [Ca²⁺]_i in a pertussis toxin-insensitive and concentration-dependent manner (EC₅₀ ~ 5 µM). Similar inhibitory effects were observed with dihydro-SPPC and psychosine, whereas SPP and various N-acylated sphingolipids (at 10 µM each) had little or no effect on the KCl-induced [Ca²⁺]_i increase. Because in RINm5F cells the primary pathway for depolarization-induced [Ca²⁺]_i increase are L-type Ca²⁺ channels, we studied whether sphingolipids reduce L-type Ca²⁺ current (I_Ca.L). When added to the bath, GPS and SPPC, but not SPP (10 µM each), rapidly reduced maximal I_Ca.L by ~35%, similar to the ₂-adrenoceptor agonist clonidine (30 µM). However, when applied internally, GPS had no effect on I_Ca.L. When the electrode solution contained the stable GDP analog guanosine-5'-O-(2-thio)diphosphate (1 and 10 mM), the inhibitory effect of GPS was abolished. In conclusion, a novel cellular action of lysosphingolipids is observed in RINm5F cells (i.e., a guanine nucleotide-sensitive inhibition of L-type Ca²⁺ currents). The pharmacological profile of this inhibition is unique and unlike any known lysosphingolipid receptor-mediated action.