Activating Effect of NO on Ca2+-Dependent K+ Channels in Smooth Muscle Cells of the Main Pulmonary Artery of the Rabbit

Abstract

Using a patch-clamp technique in the whole-cell configuration, we studied the effect of a nitric oxide (NO) donor, nitroglycerin (NG), on outward transmembrane ion current in isolated smooth muscle cells (SMC) of the main pulmonary artery of the rabbit. We also studied the characteristics of unitary high-conductance Ca2+-dependent K+ channels (KCa channels) in the SMC membrane in the cell-attached and outside-out configurations. Nitroglycerin in a 10 μM concentration increased the amplitude and intensified oscillations of outward transmembrane current induced by step depolarization. In this case, the threshold of activation of the current (–40 mV) did not change. If the potential was +70 mV, the transmembrane current in the presence of NG increased, as compared with the control, by 32.6 ± 19.4% (n = 6), on average. Simultaneous addition of 10 μM NG and 1 mM tetraethylammonium chloride (TEA), a blocker of KCa channels, to the external solution at the potential of +70 mV decreased the amplitude of outward transmembrane current with respect to the control by 25.2 ± 11% (n = 6) and suppressed oscillations of this current. In the series of experiments carried out in the outside-out configuration (concentration of K+ ions in the external solution was 5.9 mM), we calculated the conductance of a single KCa channel, which was approximately 150 pS. In the case where the potential was equal to +40 mV, 1 mM TEA suppressed completely the current through unitary KCa channels. In the series of experiments performed in the cell-attached configuration, 100 μM NG to a considerable extent intensified the activity of unitary high-conductance KCa channels by increasing the probability of the channel open state (P0), on average, by 80 ± 1%, as compared with the control. In this case, NG did not influence the conductance of single KCa channels. We concluded that the NO donor NG increases the amplitude of outward transmembrane current in SMC of the rabbit main pulmonary artery by stimulation of the activity of TEA-sensitive high-conductance KCa channels. Our experiments carried out on single KCa channels demonstrated that the activating effect of NG on KCa channels is realized at the expense of an increase in the P0 of these channels, but not of a change in the conductance of single channels.