Age related decreases in thermoregulation and cardiovascular function in horses.
BACKGROUND
Older horses have an increased risk of hyperthermia due to impaired cardiovascular function. While many studies have investigated thermoregulation in horses during exercise, none have investigated the effects of ageing.


OBJECTIVE
To test the hypothesis that there is a difference in thermoregulation during exercise and plasma volume (PV) in young and old horses.


METHODS
Study 1: 6 young (Y, 7.7 ± 0.5 years) and 5 old (O, 26.0 ± 0.8 years) unfit Standardbred mares (507 ± 11 kg, mean ± s.e.) ran on a treadmill (6% grade, velocity calculated to generate a work rate of 1625 watts) until core temperature reached 40 °C. Core (CT), skin (ST), rectal temperature (RT) and heart rate (HR) were measured every min until 10 min post exertion. Packed cell volume (HCT), lactate (LA) and plasma protein (TP) were measured in blood samples collected before, at 40 °C and every 5 min until 10 min post exercise. Sweat loss was estimated using bodyweight. Study 2: Plasma volume was measured in 26 young (8.2 ± 0.7 years) and 8 old (26.6 ± 0.7 years) Standardbred mares (515 ± 12 kg) using Evans Blue dye. Pre-exercise blood (rBV) and red cell (rRCV) volumes were calculated using PV and HCT. Data analysis utilised repeated measures ANOVA and t tests and data are expressed as mean ± s.e.


RESULTS
Old horses reached 40 °C faster (998 ± 113 vs. 1925 ± 259 s; P < 0.05) with a greater HR at 40 °C (184 ± 6 vs. 140 ± 5 beats/min; P < 0.05) and greater sweat losses (P < 0.05). Heart rate did not differ (P > 0.05) post exercise. Age did not alter (P > 0.05) CT, ST, RT, LA, HCT or TP. Plasma volume was greater in Y vs. O horses (P < 0.05, 28.5 ± 1.4 vs. 24.1 ± 1.6 l) as was rBV (41.3 ± 2.0 vs. 35.3 ± 2.3 l) and rRCV (13.3 ± 0.6 vs. 11.1 ± 0.8 l).


CONCLUSIONS
Ageing compromises the ability to handle the combined demand of exercise and thermoregulation in part due to decreased absolute pre-exercise PV.
Equine Science Center, Department of Animal Sciences, Rutgers - the State University of New Jersey, New Brunswick, USA. mckeever@aesop.rutgers.edu
K H
KH
McKeever
T L
TL
Eaton
S
S
Geiser
C F
CF
Kearns
R A
RA
Lehnhard
D000375:Aging / Q000502:physiology*; D000818:Animals; D001798:Blood Proteins / Q000502:physiology; D001833:Body Temperature Regulation / Q000502:physiology*; D003681:Dehydration; D005260:Female; D006339:Heart Rate / Q000502:physiology*; D006400:Hematocrit; D006736:Horses / Q000502:physiology*; D019344:Lactic Acid / Q000097:blood; D010805:Physical Conditioning, Animal / Q000502:physiology; D010953:Plasma Volume; D013997:Time Factors
D001798:Blood Proteins; D019344:Lactic Acid