COMPARISONS AMONG EXTERNAL RESISTIVE LOADING, DRUG-INDUCED BRONCHOSPASM, AND DENSE GAS BREATHING IN CATS - ROLES OF VAGAL AND SPINAL AFFERENTS

In anesthetized cats, breathing spontaneously, increase in lung resist ance (RL) was induced by either external resistive loads (ERL) or inte rnal loading produced by dense gas breathing (sulfur hexafluoride, SF6 ) or serotonin (5-HT)-induced bronchoconstriction. The 3 test agents w ere used in each animal. Arterial blood gases were maintained in the n ormal range. Ventilatory and cardiovascular responses were studied in 3 groups of animals: intact, vagotomized, or spinalized at C8 level, a condition that preserved diaphragmatic afferents. In intact or spinal animals, ERL as well as SF6 inhalation lengthened the inspiratory and /or the expiratory periods, whereas 5-HT injections elicited rapid sha llow breathing. The changes in ventilatory timing with either type of load were not observed in vagotomized cats. In all animals, ERL breath ing or 5-HT injections increased the moving-time average of diaphragma tic EMG measured at constant time (Edi 0.1 and 0.5 secs), but this was not observed during SF6 inhalation, a condition in which the magnitud e of RL increase was less than in the 2 other situations. The changes in systemic arterial blood pressure and/or cardiac frequency were most ly associated with 5 HT-induced bronchoconstriction. They persisted in spinalized cats, but were not observed or reversed in vagotomized one s. These observations demonstrate that vagal afferents play a major ro le in the changes in ventilatory timing and cardiovascular function in response to both external or internal moderate resistive loading. The existence of Edi changes in the 3 groups of cats suggests also that d iaphragmatic afferents, preserved in both situations, are involved in this response.