150
D. J. RANDALL
choline has been shown to slow the heart (Johansen et al., 1966), and it
has been assumed that the cardiac vagus contains cholinergic fibers
(Randall, 1966). Some investigators, however, have been able to produce
cardioacceleration by vagal stimulation ( see chapter by Campbell, this
volume), and recently adrenergic fibers have also been found to innervate the trout heart (Yamauchi and Burnstock, 1968; Gannon
and Burnstock, 1969).
The lamprey heart is innervated by a branch of the vagus nerve. This
nerve has not been stimulated directly, but stimulation of the medulla
oblongata causes cardioacceleration ( Augustinsson et al., 1956). The
effects of nervous stimulation are similar to those produced by the
application of acetylcholine on the heart. Acetylcholine has a positive
chronotropic ( increased rate of contraction) but a negative inotropic
(decreased force of contraction) effect on the lamprey heart (Falck
et al., 1966). The effects of acetylcholine cannot be explained in terms
of the release of catecholamines from stores within the heart or of the
action of these compounds on P-adrenergic receptors known to be present
in the lamprey heart (Falck et aZ., 1966). The effects of acetylcholine
are not blocked by atropine, but they are blocked by curare. The lamprey
heart, like most other fish, appears to receive a cholinergic nervous
supply; the receptor sites, however, are different. The lamprey cholinergic
receptors are blocked by curare, whereas those in teleosts and elasmobranchs are blocked by atropine.
A rise in dorsal aortic blood pressure in teleosts causes a bradycardia (Mott, 1957) which can be blocked by atropine (Randall and
Stevens, 1967). Irving et aZ. (1935) have shown that increases in blood
pressure increase the level of afferent activity in nerves innervating the
branchial arches of the dogfish. Recently, Laurent ( 1967) has recorded
chemoreceptor and baroreceptor discharges from nerves innervating the
pseudobranch in teleosts. Dorsal aortic pressure varies little in the intact
animal, and the pseudobranch may play a role in regulating heart rate,
via the level of vagal tone, to maintain a constant dorsal aortic pressure.
The level of vagal tone to the heart vanes between species and is
altered, within a single species to a variable extent, by a large number
of parameters. Bradycardia has been observed in response to light
flashes, mechanical vibrations, salinity changes, atmospheric pressure
changes, anoxia, removal from water, and touch (see Randall, 1968, for
references). The exact nature of the response varies from fish to fish,
some fish slow their heart in response to almost any stimulus, whereas
others are more selective in terms of the stimuli required to evoke bradycardia. Most of the above parameters probably slow the heart by increasing the level of vagal tone. The functional significance of many of the
D. J. RANDALL
choline has been shown to slow the heart (Johansen et al., 1966), and it
has been assumed that the cardiac vagus contains cholinergic fibers
(Randall, 1966). Some investigators, however, have been able to produce
cardioacceleration by vagal stimulation ( see chapter by Campbell, this
volume), and recently adrenergic fibers have also been found to innervate the trout heart (Yamauchi and Burnstock, 1968; Gannon
and Burnstock, 1969).
The lamprey heart is innervated by a branch of the vagus nerve. This
nerve has not been stimulated directly, but stimulation of the medulla
oblongata causes cardioacceleration ( Augustinsson et al., 1956). The
effects of nervous stimulation are similar to those produced by the
application of acetylcholine on the heart. Acetylcholine has a positive
chronotropic ( increased rate of contraction) but a negative inotropic
(decreased force of contraction) effect on the lamprey heart (Falck
et al., 1966). The effects of acetylcholine cannot be explained in terms
of the release of catecholamines from stores within the heart or of the
action of these compounds on P-adrenergic receptors known to be present
in the lamprey heart (Falck et aZ., 1966). The effects of acetylcholine
are not blocked by atropine, but they are blocked by curare. The lamprey
heart, like most other fish, appears to receive a cholinergic nervous
supply; the receptor sites, however, are different. The lamprey cholinergic
receptors are blocked by curare, whereas those in teleosts and elasmobranchs are blocked by atropine.
A rise in dorsal aortic blood pressure in teleosts causes a bradycardia (Mott, 1957) which can be blocked by atropine (Randall and
Stevens, 1967). Irving et aZ. (1935) have shown that increases in blood
pressure increase the level of afferent activity in nerves innervating the
branchial arches of the dogfish. Recently, Laurent ( 1967) has recorded
chemoreceptor and baroreceptor discharges from nerves innervating the
pseudobranch in teleosts. Dorsal aortic pressure varies little in the intact
animal, and the pseudobranch may play a role in regulating heart rate,
via the level of vagal tone, to maintain a constant dorsal aortic pressure.
The level of vagal tone to the heart vanes between species and is
altered, within a single species to a variable extent, by a large number
of parameters. Bradycardia has been observed in response to light
flashes, mechanical vibrations, salinity changes, atmospheric pressure
changes, anoxia, removal from water, and touch (see Randall, 1968, for
references). The exact nature of the response varies from fish to fish,
some fish slow their heart in response to almost any stimulus, whereas
others are more selective in terms of the stimuli required to evoke bradycardia. Most of the above parameters probably slow the heart by increasing the level of vagal tone. The functional significance of many of the