4. THE CIRCULATORY SYSTEM
151
heart rate changes is not clear. Usually only rate changes have been
measurcd. A decrease in heart rate does not necessarily indicate a decreascid cardiac output. In the trout, bradycardia which develops during
hypoxia is offset by an increase in stroke volume in such a way that only
minor changes in cardiac output are observed.
The level of vagal tone to the heart oscillates in phase with the
breathing cycle in many fish and tends to inhibit the heart as the mouth
opens ( Randall, 1966). Satchel1 ( 1961) has suggested that this sinus
arrhythmia serves to correlate maximum flows of blood and water at
the rcqiiratory surface.
Swimming in freshwater teleosts is associated with a decrease in
vagal tone and an increase in heart rate. The magnitude of the increase
is related to the level of vagal tone prior to activity. Large heart rate
changes indicate a high level of vagal tone before exercise, and vice
versa. \Yhen Chondrichthyes or some marine teleosts are forced to swim,
cxercise is associated with an increase in vagal tone and a bradycardia,
which disappears if the activity is prolonged ( Randall, 1968). The bradycardia may not occur during spontaneous swimming.
2. ANEURAL REGULATION
Starling’s law of the heart is a basic aneural cardiac control mechanism. This law states that the energy of contraction is a function of the
initial length of the muscle fiber. The greater the end diastolic volume,
the greater will be the force of contraction and therefore the stroke
volume. The venous input pressure to the heart usually determines end
diastolic volume.
Catecholamines are known to increase both the rate (positive chronotropic effect) and myocardial contractility (positive inotropic effect) of
mammalian hearts. The increased myocardial contractility results in a
larger stroke volume being ejected from an unchanged end diastolic
volume, i.e., there is greater systolic emptying of the heart. Both the
positive inotropic and chronotropic responses result from the action of
catecholamincs on P-adrenergic receptor sites in the heart. Catecholamines, therefore, alter the nature of the Starling relationship between
end diastolic volume and stroke volume by their action on p-adrenergic
receptors in the heart.
The fish heart appears to obey Starling’s law. The in situ aneural hagfish heart responds to increased filling by increasing its force of contraction (Chapman et al., 1963). The isolated in vitro trout heart, Salmo
gairdneri, also obeys Sartling’s law ( Bennion, 1968). As the input pressure
is raised both stroke volume and apparent stroke work (calculated by
151
heart rate changes is not clear. Usually only rate changes have been
measurcd. A decrease in heart rate does not necessarily indicate a decreascid cardiac output. In the trout, bradycardia which develops during
hypoxia is offset by an increase in stroke volume in such a way that only
minor changes in cardiac output are observed.
The level of vagal tone to the heart oscillates in phase with the
breathing cycle in many fish and tends to inhibit the heart as the mouth
opens ( Randall, 1966). Satchel1 ( 1961) has suggested that this sinus
arrhythmia serves to correlate maximum flows of blood and water at
the rcqiiratory surface.
Swimming in freshwater teleosts is associated with a decrease in
vagal tone and an increase in heart rate. The magnitude of the increase
is related to the level of vagal tone prior to activity. Large heart rate
changes indicate a high level of vagal tone before exercise, and vice
versa. \Yhen Chondrichthyes or some marine teleosts are forced to swim,
cxercise is associated with an increase in vagal tone and a bradycardia,
which disappears if the activity is prolonged ( Randall, 1968). The bradycardia may not occur during spontaneous swimming.
2. ANEURAL REGULATION
Starling’s law of the heart is a basic aneural cardiac control mechanism. This law states that the energy of contraction is a function of the
initial length of the muscle fiber. The greater the end diastolic volume,
the greater will be the force of contraction and therefore the stroke
volume. The venous input pressure to the heart usually determines end
diastolic volume.
Catecholamines are known to increase both the rate (positive chronotropic effect) and myocardial contractility (positive inotropic effect) of
mammalian hearts. The increased myocardial contractility results in a
larger stroke volume being ejected from an unchanged end diastolic
volume, i.e., there is greater systolic emptying of the heart. Both the
positive inotropic and chronotropic responses result from the action of
catecholamincs on P-adrenergic receptor sites in the heart. Catecholamines, therefore, alter the nature of the Starling relationship between
end diastolic volume and stroke volume by their action on p-adrenergic
receptors in the heart.
The fish heart appears to obey Starling’s law. The in situ aneural hagfish heart responds to increased filling by increasing its force of contraction (Chapman et al., 1963). The isolated in vitro trout heart, Salmo
gairdneri, also obeys Sartling’s law ( Bennion, 1968). As the input pressure
is raised both stroke volume and apparent stroke work (calculated by