The plasma-membrane Ca2+-ATPase is a key enzyme in the regulation of the intracellular Ca2+ concentration. On the other hand, sphingolipids have been recognized recently as important second messengers, acting in many systems in combination with Ca2+. In view of the fact that the Ca2+-ATPase is stimulated by ethanol, and since sphingolipids possess free hydroxy groups, we decided to study the possible effect of ceramide and sphingosine on this calcium pump. Here we show that ceramide stimulates the Ca2+-ATPase in a dose-dependent manner and additively to the activation observed in the presence of calmodulin or ethanol, when compared with any of these effectors added alone. Ceramide affects both the affinity for Ca2+ and the Vmax of the enzyme. Furthermore, this second messenger also stimulates Ca2+ transport in inside—out plasma-membrane vesicles from erythro cytes. Conversely, sphingosine, which is reported to act in many systems antagonistically with ceramide, showed an inhibitory effect on Ca2+-ATPase activity. This inhibition was also observed on the calmodulin-stimulated enzyme. These results, taken together, suggest that ceramide and sphingosine act antagonistically on the plasma-membrane Ca2+-ATPase. This is in accordance with the frequently reported opposite effect of these sphingolipids on intracellular Ca2+ concentration.
Abbreviations used: CaM, calmodulin; IOV, inside—out plasma-membrane vesicle.