T. Wang et al., Autonomic control of heart rate during forced activity and digestion in the snake Boa constrictor, J EXP BIOL, 204(20), 2001, pp. 3553-3560
Reptiles, particularly snakes, exhibit large and quantitatively similar inc
rements in metabolic rate during muscular exercise and following a meal, wh
en they are apparently inactive. The cardiovascular responses are similar d
uring these two states, but the underlying autonomic control of the heart r
emains unknown. We describe both adrenergic and cholinergic tonus on the he
art during rest, during enforced activity and during digestion (24-36h afte
r ingestion of 30% of their body mass) in the snake Boa constrictor. The sn
akes were equipped with an arterial catheter for measurements of blood pres
sure and heart rate, and autonomic tonus was determined following infusion
of the beta -adrenergic antagonist propranolol (3mg kg(-1)) and the muscari
nic cholinoceptor antagonist atropine (3 mg kg-1).
The mean heart rate of fasting animals at rest was 26.4 +/- 1.4 min(-1), an
d this increased to 36.1 +/- 1.4 min(-1) (means +/- S.E.M.; N=8) following
double autonomic block (atropine and propranolol). The calculated cholinerg
ic and adrenergic tones were 60.1 +/- 0.3% and 19.8 +/- 2.2%, respectively.
Heart rate increased to 61.4 +/- 1.5 min(-1) during enforced activity, and
this response was significantly reduced by propranolol (maximum values of
35.8 +/-1.6 min(-1)), but unaffected by atropine. The cholinergic and adren
ergic tones were 2.6 +/- 2.2 and 41.3 +/- 1.9 % during activity, respective
ly. Double autonomic block virtually abolished tachycardia associated with
enforced activity (heart rate increased significantly from 36.1 +/- 1.4 to
37.6 +/- 1.3 min(-1)), indicating that non-adrenergic, non-cholinergic effe
ctors are not involved in regulating heart rate during activity. Blood pres
sure also increased during activity.
Digestion was accompanied by an increase in heart rate from 25.6 +/- 1.3 to
47.7 +/- 2.2 min(-1) (N=8). In these animals, heart rate decreased to 44.2
+/- 2.7 min-1 following propranolol infusion and increased to 53.9 +/- 1.8
min-1 after infusion of atropine, resulting in small cholinergic and adren
ergic tones (6.0 +/- 3.5 and 11.1 +/- 1.1 %, respectively). The heart rate
of digesting snakes was 47.0 +/- 1.0 min(-1) after double autonomic blockad
e, which is significantly higher than the value of 36.1 1.4 min-1 in double
-blocked fasting animals at rest. Therefore, it appears that some other fac
tor exerts a positive chronotropic effect during digestion, and we propose
that this factor may be a circulating regulatory peptide, possibly liberate
d from the gastrointestinal system in response to the presence of food.