Indirect effects of histamine on pulmonary rapidly adapting receptors in cats

We did experiments to determine the relative importance of lung mechanical changes during histamine induced activation of pulmonary rapidly adapting receptors (RARs). In anesthetized, open-chest, artificially ventilated cats, we recorded RAR activity and injected histamine (25-50 micrograms/kg) into the right atrium. Histamine initially increased RAR activity from 1.1 +/- 0.2 to 3.6 +/- 0.6 imp/sec (n = 30) at 15.6 +/- 0.8 sec when dynamic lung compliance (CDYN) was decreased by 29.1 +/- 1.5%. The firing pattern of RARs changed from a relatively irregular pattern to a pronounced respiratory modulation. RAR activity reached its peak (5.6 +/- 0.8 imp/sec) at 36.3 +/- 3.3 sec. The firing pattern further changed to a cardiac modulation, and the activity closely correlated with cardiac output. Comparing the initial response of RARs to histamine with the response to mechanically decreasing CDYN, we found that the activities were similar when CDYN was decreased by the same amount. Our experiments suggest that in cats the initial increase of RAR activity in response to histamine is related to lung mechanical changes, but the later increase is related to cardiovascular functions.