Divergence of ventilatory responses to isometric contraction in anesthetized cats

The purpose of this study was to determine if the initial ventilatory and phrenic nerve responses to isometric contraction of the triceps surae muscle of anesthetized cats are influenced by the pattern of the contraction. To address this, three different types of muscle contraction were evoked: (1) a high tension, continuous tetanic (HT-CT) contraction; (2) a moderate tension, continuous tetanic (MT-CT) contraction; and (3) high tension, intermittent tetanic (HT-IT) contractions. The duration of each contraction period was 60 sec. The MT-CT and HT-IT contractions increased minute volume V̇E; 19 ± 4% and 15 ± 5%, respectively) within the first 15 sec. These increases were the result of rises in breathing frequency and tidal volume. However, only the MT-CT contraction increased phrenic activity (pV̇E) in the first 15 sec. By contrast, ventilation and phrenic nerve activity failed to increase within the first 15 sec of the HT-CT contraction. If fact, 'tidal' phrenic activity (pVT; -14 ± 5%) decreased during the first 5 sec, and there was a tendency for tidal volume (V̇T; -8 ± 5%), V̇E (-8 ± 6%), and pV̇E (-16 ± 8%) to fall. These data suggest that stimulation of muscle afferent fibers by static contraction can initially inhibit phrenic nerve activity, provided the activation is sustained and of sufficient intensity.