The inhibitory effect of ouabain on the response of slowly adapting pulmonary stretch receptors to hyperinflation in the rabbit

The combined effects of ouabain (Na+-K+ ATPase inhibitor) and hyperinflation (inflation volume = three tidal volumes) on slowly adapting pulmonary stretch receptors (SARs) were studied before and after administration of nifedipine (an L-type Ca2+ channel blocker) and KB-R7943 (a reverse-mode Na+-Ca2+ exchanger blocker) in anesthetized, artificially ventilated rabbits after bilateral vagotomy. Before ouabain administration, hyperinflation stimulated SAR activity. After 20 min of ouabain administration (30 μg/kg) the SARs increased discharge rates in normal inflation. Under these conditions, hyperinflation initially stimulated SAR activity but subsequently inhibited the activity at peak inflation. Additional administration of 60 μg/kg ouabain (total dose = 90 μg/kg) caused a further stimulation of SAR activity, but 20 min later both normal inflation and hyperinflation resulted in a greater inhibition of the receptor activity. The hyperinflation-induced SAR inhibition in the presence of ouabain (30 μg/kg) was not significantly altered by administration of either nifedipine (2 and 4 mg/kg) or KB-R7943 (1 and 3 mg/kg). In another series of experiments, we further examined the combined effects of ouabain and hyperinflation in veratridine (a Na+ channel opener, 40 μg/kg)-treated animals. After recovery from the veratridine effect on SAR activity, which vigorously stimulated the receptor activity, ouabain treatment (30 μg/kg) that silenced the receptor activity at peak inflation greatly inhibited hyperinflation-induced SAR stimulation. These results suggest that hyperinflation-induced SAR inhibition in the presence of ouabain may be related to a Na+ overload, but not to a Ca2+ influx via activation of L-type Ca2+ channels, in the SAR endings.