INHIBITORY MECHANISM OF CO2 INHALATION ON SLOWLY ADAPTING PULMONARY STRETCH RECEPTORS IN THE ANESTHETIZED RABBIT

The inhibitory effects of CO2 on slowly adapting pulmonary stretch rec eptors (SARs) were studied before and after administration of acetazol amide, a carbonic anhydrase inhibitor, or nifedipine, a calcium channe l blocker, in anesthetized, artificially ventilated rabbits after vagu s nerve section. CO2 inhalation (maximal tracheal CO2 concentration ra nging from 7.2% to 9.5%) for approximately 60 sec decreased the recept or activity during both inflation and deflation. The magnitude of decr eased receptor activity during deflation became more pronounced than t hat seen during inflation. Acetazolamide treatment (20 mg/kg) diminish ed the inhibitory responses of slowly adapting pulmonary stretch recep tors to CO2 inhalation, which were not significantly influenced by pre treatment with nifedipine (1 mg/kg). Furthermore, CO2 inhalation befor e and after vagal denervation had no effect on total lung resistance a nd dynamic lung compliance, in another series of experiments, the stai ning to determine the presence of carbonic anhydrase enzymatic reactio n was not found in the smooth muscle of either extrapulmonary or intra pulmonary bronchi. These results suggest that CO2-induced inhibition o f slowly adapting pulmonary stretch receptors is not related to the ch ange in bronchomotor tone.