Effect of I1-imidazoline receptor activation on responses of hypoglossal and phrenic nerve to chemical stimulation.
Sedation elicited by some centrally acting antihypertensive agents may interfere with respiratory control, and by selectively inhibiting upper airway dilating muscle activity it may facilitate obstructive sleep apnea. Autoradiographic studies with [125I]p-iodoclonidine in the presence of 10 microM epinephrine to block alpha 2-adrenergic sites or 100 nM moxonidine to mask I1-imidazoline sites show that both I1- as well as alpha 2-sites are localized in putative chemosensory areas of the rostral ventrolateral medulla in the cat. We sought to determine the effect of activating I1 and alpha 2-receptors on central chemosensitivity by using moxonidine as a selective I1 agonist, clonidine as a mixed I1/alpha 2 agonist, SK&F-86466 as a specific alpha 2-antagonist, and efaroxan as a mixed I1/alpha 2 antagonist. We recorded responses of phrenic, hypoglossal, and cervical sympathetic nerve activities to progressive hypercapnia after hyperventilation to apnea. Moxonidine (3-100 micrograms/kg i.v.) caused dose-dependent decreases in tonic cervical sympathetic nerve activity and blood pressure, but had no effect on the CO2 threshold (after 30 or 100 micrograms/kg moxonidine, phrenic nerve activity reappeared at 5.8 +/- 0.2% CO2 versus 5.6 +/- 0.3% CO2 in control). Following moxonidine, the slope of the steep portion of the CO2 response tended to increase (10.3 +/- 1.8 versus 7.3 +/- 0.9). Peak phrenic nerve activity was comparable to control at 7.5% CO2 (20 +/- 2 U in control) and at 9.5% CO2 (30 +/- 3 versus 27. +/- 2 U). Similarly, the response of hypoglossal and inspiratory phasic cervical sympathetic nerve activity to a progressive CO2 rise was not affected by moxonidine. By contrast, clonidine in the same doses decreased CO2 sensitivity, because the CO2 threshold was elevated from 5.3 +/- 0.5% to 6.7 +/- 0.4% (p < 0.001). The slope of the CO2 response was decreased from 9.7 +/- 1.9 to 7.4 +/- 1.3 (p = 0.05). Peak phrenic nerve activity was reduced at 7.5% CO2 (11 +/- 5 versus 25 +/- 2 U; p < 0.05) and at 9.5% CO2 (21 +/- 4 versus 33 +/- 2 U; p = 0.06). Clonidine selectively inhibited the response of hypoglossal nerve activity to CO2. The depressive effects of clonidine were reversed by alpha 2-blockade with SK&F-86466 (0.5 or 1 mg/kg). Inspiratory phasic cervical sympathetic nerve activity increased after SK&F-86466 in parallel with phrenic and hypoglossal nerve activity, but the tonic component of cervical sympathetic nerve activity and blood pressure increased only transiently.(ABSTRACT TRUNCATED AT 400 WORDS)