Perturbation of the platelet plasma membrane is not sufficient for inhibition of thrombin-induced PKC-activity

This work was carried out to decide whether a non-specific perturbation of the platelet membrane with exogenous amphiphiles affects protein phosphorylation in platelets, especially phosphorylation mediated by PKC. Effects of amphiphiles per se on protein phosphorylation were also recorded. (i) Sublytic concentrations of the differently charged model surfactants cetyltrimethylammonium bromide (CTAB), Zwittergent 3-16, sodium tetradecyl sulphate, and octaethyleneglycol hexadecyl ether, as well as chlorpromazine, and Triton X-100, did not affect the thrombin-induced, PKC-mediated phosphorylation of pleckstrin, whereas sphingosine blocked this phosphorylation. (ii) The sphingosine-mediated phosphorylation blockade is not related to a non-specific perturbation of the membrane, but can instead be attributed to specific properties of sphingosine. (iii) The amphiphiles, per se, had differential effects on protein phosphorylation at sublytic concentrations: a treatment with CTAB, Zwittergent 3-16, and sodium tetradecyl sulphate for 1 min led to phosphorylation of a 49-kDa protein, while treatment with sphingosine for 1 min led to a transient phosphorylation of the myosin light chain as well as a weak phosphorylation of pleckstrin.

Sphingosine induced release of K+ and 5-HT in platelets should not be confused with membrane leakiness

The release of 43K+, lactate dehydrogenase (LDH) and [14C]-5-hydroxytryptamine ([14C]-5-HT) from platelets treated with sphingosine and four differently charged model amphiphiles was studied. Sphingosine was found to differ from the detergents because it induced a concentration-dependent release of both 43K+ and [14C]-5-HT without causing a release of LDH. The release of [14C]-5-HT preceded the release of 43K+ and it is concluded that these effects are associated with platelet activation. The detergents caused a release of 43K+ followed by a release of LDH without causing a release of [14C]-5-HT. These effects are attributed to a non-specific perturbation of the platelet plasma membrane.