Discrete but simultaneous release of adenine nucleotides and serotonin from mouse megakaryocytes as detected with patch- and carbon-fiber electrodes

Using patch- and carbon-fiber electrodes, we studied release phenomena of adenine nucleotides and serotonin from megakaryocytes isolated from the bone marrow of the mouse. Megakaryocytes express ionotropic purinergic receptors on their surfaces. Under the condition of whole cell recording, the cells showed spikelike spontaneous inward currents. The spontaneous currents were carried by cations and had amplitudes of 30 - 800 pA at - 43 mV and durations of 0.1 - 0.3 s. Pyridoxalphosphate-6-azophenyl-2', 4'-disulfonic acid (PPADS; 100 muM) and suramin (100 muM), purinoceptor-blocking agents, depressed the currents reversibly. It is thought that the receptor involved was the P-2X1 subtype on the cell and that the currents were due to activation of the P2X1 receptor by adenine nucleotides released from the cell. The currents showed a skewed amplitude distribution, suggesting variation of vesicular contents and/or distinct localization or varied density of receptors on the cell. Frequency of the spontaneous inward currents was enhanced by external application of platelet-activating substances, thrombin (0.4 U/ml), phorbol ester (100 nM), and ADP (2 muM), at low concentrations. With a carbon-fiber electrode, which can detect oxidizable substances including serotonin, spikelike oxidation currents from the external surface of the megakaryocyte were detected. The frequency of the oxidation currents increased remarkably after the application of thrombin (10 U/ml). The majority of the oxidation currents coincided with the rising phase of the whole cell currents, suggesting corelease of serotonin and adenine nucleotide from the same vesicle. We concluded that megakaryocytes store adenine nucleotides and serotonin in the same vesicle and release them simultaneously in a discrete manner.