Journal of Applied Physiology, Vol 62, Issue 1 82-90, Copyright © 1987 by American Physiological Society
Isovolumetric and isobaric rabbit tracheal contraction in vitro
R. H. Moreno, J. C. Hogg, T. M. McLean and P. D. Pare
Isovolumetric and isobaric tracheal smooth muscle (TSM) contraction were
studied in vitro in a preparation of the whole rabbit trachea. Eight
tracheae from New Zealand White rabbits were excised and mounted at a fixed
length in an organ bath. Electrical field stimulation (EFS) was performed
in isovolumetric and isobaric conditions at varying transmural pressures
(TMP). Supramaximal stimulation with methacholine was done at 0 TMP. Active
change in pressure (delta P) with EFS showed a peak at 3.1 +/- 1.06 cmH2O
TMP during inflation and at 4.1 +/- 1.18 cmH2O TMP during deflation (mean
+/- SE). Active delta P decreased at higher or lower TMP. Active change in
volume with EFS showed a peak at 3.2 +/- 1.26 cmH2O TMP during inflation
and at 1.8 +/- 0.98 cmH2O TMP during deflation. A decrease in response was
also observed at higher and lower TMP. From these data, we concluded that
TSM is at optimal length (Lmax) at TMP of 2-3 cmH2O. Maximal TSM shortening
with supramaximal stimulation with methacholine was 32% Lmax. This figure
is considerably smaller than the 80% shortening found in unloaded strips of
TSM. We conclude that rabbit TSM length is close to Lmax at TMP similar to
those found at functional residual capacity and that the loads that the
muscle has to overcome probably contribute to the limited shortening
observed in situ.
