Spontaneous changes in guinea-pig respiratory pattern during barometric plethysmography:: role of catecholamines and nitric oxide

Barometric plethysmography for unrestrained animals is a non-invasive method that allows repetitive measurements of pulmonary function, but habituation of the conscious animal to this technique has not been explored. Respiratory frequency (f(R)) and 'enhanced pause' (P-enh) were measured by barometric plethysmography for a period of 8 h in guinea-pigs. Compared with basal values, during the first hour of recording a progressive increase in P-enh (up to 25-50%) and a corresponding decrease in f(R) were recorded, followed by a relative plateau in each for up to 8 h. These changes were avoided by a 30-min pretreatment with propranolol and L-NAME (nitric oxide synthase inhibitor), with P-enh values as high as this plateau phase since the beginning of recording. Atropine, salbutamol or budesonide did not modify the progressive increment in P-enh. We concluded that catecholamines and nitric oxide are released when guinea-pigs are introduced into the plethysmographic chamber, leading to initial low P-enh values. These mediators probably diminish owing to habituation of the animal to the new environment, with an apparent progressive increment in P-enh. These spontaneous changes in P-enh and f(R) must be taken into account during barometric plethysmography in order to avoid misinterpretation of the results.