3. AUTONOMIC NERVOUS SYSTEMS
125
both mediating contraction of isolated spiral strips, occur in the ventral
aorta of trout and eels. And yet there is still no report of a direct effect
of autonomic nerve stimulation on the resistance of any vascular bed in
fishes. What little evidence there is goes against belief in a sympathetic
vasomotor tone. Wilber and Sudak (1960) found that there were no
compensatory cardiovascular responses to hemorrhage in Mustelus and
Squalus, while Burger and Bradley (1951) showed that a ganglionblocking drug, tetraethylammonium, and an epinephrine-blocking
drug, dibenamine, did not cause any alteration of ventral or dorsal
aortic pressure in Squalus. Randall and Stevens (1967) found that
the epinephrine-blocker, phenoxybenzamine, caused a fall of only 1-2
mm Hg in dorsal aortic pressure in coho and sockeye salmon. At the
moment it seems likely that if there is a sympathetic vasoconstrictor innervation of systemic vascular beds, it is used to regulate blood flow to
individual organs rather than to sustain a certain level of blood pressure.
Any generalized control of systemic vascular resistance is likely to be
mediated by circulating epinephrine from chromaffin cells (Randall and
Stevens, 1967).
The sympathetic innervation of the gastrointestinal tract has been
reviewed recently ( Campbell and Burnstock, 1968). Interpretation of
the available results is again made difficult by the possibility that the
observed excitatory responses to sympathetic nerve stimulation are in
reality rebound contractions caused by the stimulation of inhibitory
nerves. However, these authors concluded that a number of workers
had shown true primary contractions mediated by sympathetic nerves.
Other excitatory responses could be interpreted as rebound contractions,
and there is one clear example of an inhibitory response to posterior
splanchnic nerve stimulation in trout ( Burnstock, 195813). In summary,
these authors suggested that the sympathetic supply to all regions of
the gut in both selachians and teleosts contains a mixture of excitatory
and inhibitory nerve fibers. One may suggest that the excitatory nerve
fibers are cholinergic, for at least some of the excitatory responses are
abolished by atropine treatment ( Burnstock, 1958b). The inhibitory
nerve fibers may be adrenergic since catecholamines cause relaxation
of the gut of a number of species (see Canipbell and Burnstock, 1968)
and an adrenergic innervation of gastrointestinal muscle has been demonstrated histochemically in trout, eel, and tench (Baumgarten, 1967; Read
and Burnstock, 1968), but direct cvidence is lacking.
In addition to the adrenergic, presumably sympathetic, nerves reaching the swim bladder in the vagus trunk of teleosts (see Section 111, A, 2 ) ,
there is a direct sympathetic innervation by a branch of the splanchnic
nerve. Fange (1953) showed that if this branch is cut there is a slight
125
both mediating contraction of isolated spiral strips, occur in the ventral
aorta of trout and eels. And yet there is still no report of a direct effect
of autonomic nerve stimulation on the resistance of any vascular bed in
fishes. What little evidence there is goes against belief in a sympathetic
vasomotor tone. Wilber and Sudak (1960) found that there were no
compensatory cardiovascular responses to hemorrhage in Mustelus and
Squalus, while Burger and Bradley (1951) showed that a ganglionblocking drug, tetraethylammonium, and an epinephrine-blocking
drug, dibenamine, did not cause any alteration of ventral or dorsal
aortic pressure in Squalus. Randall and Stevens (1967) found that
the epinephrine-blocker, phenoxybenzamine, caused a fall of only 1-2
mm Hg in dorsal aortic pressure in coho and sockeye salmon. At the
moment it seems likely that if there is a sympathetic vasoconstrictor innervation of systemic vascular beds, it is used to regulate blood flow to
individual organs rather than to sustain a certain level of blood pressure.
Any generalized control of systemic vascular resistance is likely to be
mediated by circulating epinephrine from chromaffin cells (Randall and
Stevens, 1967).
The sympathetic innervation of the gastrointestinal tract has been
reviewed recently ( Campbell and Burnstock, 1968). Interpretation of
the available results is again made difficult by the possibility that the
observed excitatory responses to sympathetic nerve stimulation are in
reality rebound contractions caused by the stimulation of inhibitory
nerves. However, these authors concluded that a number of workers
had shown true primary contractions mediated by sympathetic nerves.
Other excitatory responses could be interpreted as rebound contractions,
and there is one clear example of an inhibitory response to posterior
splanchnic nerve stimulation in trout ( Burnstock, 195813). In summary,
these authors suggested that the sympathetic supply to all regions of
the gut in both selachians and teleosts contains a mixture of excitatory
and inhibitory nerve fibers. One may suggest that the excitatory nerve
fibers are cholinergic, for at least some of the excitatory responses are
abolished by atropine treatment ( Burnstock, 1958b). The inhibitory
nerve fibers may be adrenergic since catecholamines cause relaxation
of the gut of a number of species (see Canipbell and Burnstock, 1968)
and an adrenergic innervation of gastrointestinal muscle has been demonstrated histochemically in trout, eel, and tench (Baumgarten, 1967; Read
and Burnstock, 1968), but direct cvidence is lacking.
In addition to the adrenergic, presumably sympathetic, nerves reaching the swim bladder in the vagus trunk of teleosts (see Section 111, A, 2 ) ,
there is a direct sympathetic innervation by a branch of the splanchnic
nerve. Fange (1953) showed that if this branch is cut there is a slight