Respiratory modulation by vagal afferents in pigs.

We have studied the role of pulmonary vagal afferents and baroreceptors of the aortic arch on control of breathing in anaesthetized pigs. As in other species (dog, cats and rabbits) the depth and rate of breathing was found to be controlled by a volume-related vagal feed-back loop from pulmonary stretch receptors. Rapid shallow breathing following histamine aerosol and phenyldiguanide i.v. (which are likely to stimulate pulmonary vagal irratant and J receptors) occurred through a decrease in the activation threshold of the inspiratory cut-off mechanism without altering the matching of inspiratory to expiratory time. These results are also similar to those found in cats and rabbits. Histamine caused an increase in pulmonary resistance and a decrease in compliance: these effects were partially vagally mediated and partially due to a direct stimulation of smooth muscles by the drug. It seems, from relating the respiratory to the vascular systemic effect of histamine administration, that pigs have a greater respiratory response than cats or rabbits. A stepwise decrease in blood pressure through haemorrhage under iso-PCO2 and iso-PO2 conditions was found to affect the matching of the expiratory to the inspiratory time without affecting the activation threshold of the inspiratory cut-off mechanism.