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 su rae muscle of anesthetized cats are influenced by the pattern of the c ontraction. To address this, three different types of muscle contracti on were evoked: (1) a high tension, continuous tetanic (HT-CT) contrac tion; (2) a moderate tension, continuous tetanic (MT-CT) contraction; and (3) high tension, intermittent tetanic (HT-IT) contractions. The d uration of each contraction period was 60 sec. The MT-CT and HT-IT con tractions increased minute volume (VE; 19 +/- 4% and 15 +/- 5%, respec tively) within the first 15 sec. These increases were the result of ri ses in breathing frequency and tidal volume. However, only the MT-CT c ontraction increased phrenic activity (pVE) in the first 15 sec. By co ntrast, ventilation and phrenic nerve activity failed to increase with in the first 15 sec of the HT-CT contraction. If fact, 'tidal' phrenic activity (pVT; -14 +/- 5%) decreased during the first 5 sec, and ther e was a tendency for tidal volume (VT; -8 +/- 5%), VE (-8 +/- 6%), and pVE (-16 +/- 8%) to fall. These data suggest that stimulation of musc le afferent fibers by static contraction can initially inhibit phrenic nerve activity, provided the activation is sustained and of sufficien t intensity.