Thrombopoietin Enhances Rapid Current Responses Mediated by P2X ₁ Receptors on Megakaryocytic Cells in Culture

The effect of thrombopoietin (TPO), a magakaryocytopoietic cytokine, on the functional maturation of megakaryocytes was studied by using cell culture and patch-clamp techniques focusing on purinergic 2X₁ (P2X ₁)-receptors, which are expressed specifically on platelets and their progenitors. Meg-01 cells, one of the typical human megakaryocytic cell lines, were cultured and studied by using a whole-cell patch electrode. In control cells cultured in RPMI1640 medium, an application of adenosine nucleotide (ADP, 40 μM) evoked transient inward currents with amplitudes of 45±19 pA (at -43 mV). Based on kinetic, ionic, and pharmacological properties as well as on previously reported findings, these currents were thought to be mediated by P2X₁ receptors. When Meg-01 cells were cultured for 7-9 d in a medium to which the differentiation-inducing agent phorbol ester (PMA; 10 nM) or TPO (100 ng/ml) had been added, the responses of the cells to ADP increased to about 150% of the control with PMA and to about 200% of the control value with TPO. A combination of the two agents enhanced the response of the cells to ADP to about 570% of the control value. These results suggest that phorbol ester and TPO cause cellular differentiation of Meg-01 cells and enhance the level of expression of P2X₁-receptors on cell membranes in a synergetic manner. The effect of TPO on the induction of P2X₁-receptors on mouse megakaryocytes in culture was more obvious.