5364 Results Stimulation of ETA and ETB receptors activate 2–3 pS cation channel currents in rabbit coronary artery myocytes In initial experiments we investigated the identity of the ET-1 receptor subtype involved in activating native 2–3 pS cation channel currents. For these experiments 100 nM ET-1 was used since at these concentrations ET-1 does not activate the TRPC3/C7 conductance expressed in this preparation (see Peppiatt-Wildman et al. 2007 and Introduction) and the 2–3 pS conductance is recorded in isolation. Both ETA and ETB G-protein-coupled receptors are expressed in vascular smooth muscle (see S. N. Saleh and others J Physiol 587.22 Miwa et al. 2005) and therefore we studied the effect of selective concentrations of ETA and ETB receptor antagonists, respectively BQ-123 and BQ-788 (Davenport, 2002), on ET-1-induced native 2–3 pS channel activity in cell-attached patches from freshly dispersed coronary artery myocytes. Figure 1A, E and F shows that bath application of 100 nM ET-1 activated cation channel activity at −70 mV which had a mean peak open probability (NPo) of 0.25 ± 0.07 (n = 10) and was composed of channel openings with a unitary conductance of 2.6 pS and a reversal potential (Er) of about 0 mV. These responses were seen in approximately 90% of patches tested. Figure 1Ab illustrates that the channel current amplitude histogram of Figure 1. Stimulation of ETA and ETB receptors activates 2–3 pS cation channel currents in cell-attached patches from freshly dispersed coronary artery myocytes Aa, bath application of 100 nM ET-1 induced cation channel activity at an applied patch voltage of +70 mV. According to convention we will refer to this as −70 mV membrane potential throughout the text (see Methods). Ab, amplitude histogram of channel currents shown in Aa could be ﬁtted with the sum of four Gaussian curves indicating 1 closed and 3 multiple open levels inferring that the patch contained at least 3 channels. B and C, ET-1 evoked cation channel activity in the presence of respectively either the ETA receptor antagonist 100 nM BQ-123 or the ETB receptor antagonist 100 nM BQ-788. D and F, ET-1-induced cation channel activity was blocked in the presence of a mixture of 100 nM BQ-123 and 100 nv BQ-788 at −70 mV. E, I–V relationship of cation channel currents evoked by ET-1 (open circles), ET-1 in the presence of BQ-123 (open squares) and ET-1 in the presence of BQ-788 (ﬁlled squares) showing that they all had a unitary conductance of 2.6 pS and Er of about 0 mV. Each point represents at least n = 6. F, mean data showing neither BQ-123 nor BQ-788 inhibited ET-1-evoked channel activity when applied separately. However when the antagonists were added together ET-1-evoked cation channel activity was abolished. Each value is the mean of 10 patches. C 2009 The Authors. Journal compilation C 2009 The Physiological Society