Age-related increases in diaphragmatic maximal shortening
velocity.
Powers, Scott K., David Criswell, Robert A. Herb, Haydar Demirel, and
Stephen Dodd.
Dept. of Exercise and Sport Sciences and Physiology, Center for
Exercise Science, University of Florida, Gainesville, Florida
32611
APStracts 2:0400A, 1995.
Recent evidence demonstrates that aging results in an increase in fast
(type IIb) myosin heavy chain (MHC) in the rat diaphragm. It is
unknown if this age-related change in fast MHC influences
diaphragmatic maximal shortening velocity (Vmax). Therefore, we
tested the hypothesis that aging is associated with an increase in
diaphragmatic Vmax and that the increase in Vmax is highly correlated
with the percent of type IIb MHC. In vitro contractile properties
were measured using costal diaphragm strips obtained from young (4
month old; N= 8) and old (24 month old; N = 8) male F-344 rats.
Diaphragmatic maximal tetanic specific force production (specific Po)
was 14.5% lower (P<0.05) in the old animals compared to the
young (23.0+/- 0.4 vs. 19.7+/- 0.8 N/cm2). In contrast, diaphragmatic
Vmax was significantly (P<0.05) higher in the old animals
compared to the young (5.5+/-.1 vs. 4.4+/-.3 L.s-1). Although the %
type IIb MHC was significantly higher (+ 14%; P<0.05) in old
animals compared to young, the correlation between Vmax and % type
IIb MHC was relatively low (r = .50: P>0.05). These data support
the hypothesis that an age-related increase in diaphragmatic Vmax
occurs, however, factors in addition to type IIb MHC are involved in
regulating diaphragmatic Vmax. Interestingly, although aging resulted
in a decrease in diaphragmatic maximal specific Po, power output at
all muscle loads was maintained in the old animals due to the
increase in diaphragmatic shortening velocity.
Received 6 March 1995; accepted in final form 31 August 1995.
APS Manuscript Number A254-5.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 September 1995.