Role of endothelin receptors, calcium and nitric oxide in the potentiation by endothelin-1 of the sympathetic contraction of rabbit ear artery during cooling

1 To examine further the potentiation by endothelin-1 on the vascular response to sympathetic stimulation, we studied the isometric response of isolated segments, 2 mm long, from the rabbit central ear artery to electrical field stimulation (1-8 Hz), under different conditions, at 37 degrees C and during cooling (30 degrees C). 2 Electrical stimulation produced frequency-dependent contraction, which was reduced (about 63% for 8 Hz) during cooling. At 30 degrees C, but not at 37 degrees C, endothelin-1 (1, 3 and 10 nM) potentiated the contraction to electrical stimulation in a dose-dependent way (from 43+/-7% to 190+/-25% for 8 Hz). 3 This potentiation by endothelin-1 was reduced by the antagonist for endothelin ETA receptors BQ-123 (10 mu M) but not by the antagonist for endothelin ETB receptors BQ-788 (10 mu M). The agonist for endothelin ETB receptors IRL-1620 (0.1 mu M) did not modify the contraction to electrical stimulation. 4 The blocker of L-type Ca2+ channels verapamil (10 mu M l(-1)) reduced (about 72% for 8 Hz) and the unspecific blocker of Ca2+-channels NiCl2 (1 mM) practically abolished (about 98%), the potentiating effects of endothelin-1 found at 30 degrees C. 5 Inhibition of nitric oxide synthesis with N-G-nitro-L-arginine (L-NOARG, 0.1 mM) increased the contraction to electrical stimulation at 30 degrees C more than at 37 degrees C (for 8 Hz, this increment was 297+/-118% at 30 degrees C, and 66+/-15% at 37 degrees C). Endothelium removal increased the contraction to electrical stimulation at 30 degrees C (about 91% for 8 Hz) but not at 37 degrees C. Both L-NOARG and endothelium removal abolished the potentiating effects of endothelin-1 on the response to electrical stimulation found at 30 degrees C. 6 These results in the rabbit ear artery suggest that during cooling, endothelin-1 potentiates the contraction to sympathetic stimulation, which could be mediated at least in part by increasing Ca2+ entry after activation of endothelin ETA receptors. This potentiating effect of endothelin-1 may require the presence of an inhibitory tone due to endothelial nitric oxide.