166
D. J . RANDALL
in the systemic circulation of elasmobranchs is decreased (Hanson,
1967). Acetylcholine has a negative chronotropic effect on teleost and
elasmobranch hearts but acts as a cardiac stimulant in the lamprey (Falck
et al., 1966). Methacholine, succinylcholine, pilocarpine, neostigniine,
and d-tubocurarine all have similar but less marked effects than acetylcholine, causing bradycardia and fall in blood pressure in the eel
( Chan, 1967).
B. Atropine
Atropinization increases heart rate in both elasmobranchs ( Johansen
et al., 1966) and teleosts (Randall, 1966), blocking thc effect of efferent
activity in cholinergic fibers innervating the heart. Atropine injected into
the vascular system increases heart rate in both the resting and active
dogfish causing an increase in ventral aortic systolic blood pressure hut
a decrease in peak ejcction velocity and stroke volume of the heart
(Johansen et al., 1966).
C. Catecholamines
Both adrenaline and noradrenaline increase vcntral and dorsal aortic
blood pressure and may either incrcasc, dccrease, or have no cffcct on
heart rate when injected into the intact fish (Hanson, 1967; Randall and
Stevens, 1967). A decrease in heart rate is the result of reflex vagal inhibition on the heart, probably resulting from a rise in blood pressurc,
and can be blocked by atropine. The direct effect of catecholamines on
the heart is stimulatory. Adrenaline has a positive chronotropic and
inotropic effect on the lamprey, teleost, and elasmobranch heart ( Johansen et al., 1966; Falck et al., 1966; Bennion, 1968); but it has little or
no effect on the hagfish heart (Bloom et al., 1961). Adrenaline and
noradrenaline dilate the vessels in the teleost and elasmobranch gill
(Keys and Bateman, 1932; Ostlund and Fiingc, 1962) decrease the rcsistance to flow through the gills, and incrcasc blood flow in thc sccondary
lamellae ( Steen and Kruysse, 1964 ) . Phenoxybenzamine, an a-adrcnergic
receptor blocking agent, diminishes the effect of catecholamines on the
circulation, Isoprenaline, a p-adrencrgic stimulant, decreases thc resistance to blood flow through the gills (Randall, 1967). Chan ( 1967) suggested that isoprenaline dilated vessels in the systemic circulation of the
eel. Hanson ( 1967) concluded that catecholamines increase systcmic
resistance to flow in elasmobranchs. Kcys and Bateman (1932) found
D. J . RANDALL
in the systemic circulation of elasmobranchs is decreased (Hanson,
1967). Acetylcholine has a negative chronotropic effect on teleost and
elasmobranch hearts but acts as a cardiac stimulant in the lamprey (Falck
et al., 1966). Methacholine, succinylcholine, pilocarpine, neostigniine,
and d-tubocurarine all have similar but less marked effects than acetylcholine, causing bradycardia and fall in blood pressure in the eel
( Chan, 1967).
B. Atropine
Atropinization increases heart rate in both elasmobranchs ( Johansen
et al., 1966) and teleosts (Randall, 1966), blocking thc effect of efferent
activity in cholinergic fibers innervating the heart. Atropine injected into
the vascular system increases heart rate in both the resting and active
dogfish causing an increase in ventral aortic systolic blood pressure hut
a decrease in peak ejcction velocity and stroke volume of the heart
(Johansen et al., 1966).
C. Catecholamines
Both adrenaline and noradrenaline increase vcntral and dorsal aortic
blood pressure and may either incrcasc, dccrease, or have no cffcct on
heart rate when injected into the intact fish (Hanson, 1967; Randall and
Stevens, 1967). A decrease in heart rate is the result of reflex vagal inhibition on the heart, probably resulting from a rise in blood pressurc,
and can be blocked by atropine. The direct effect of catecholamines on
the heart is stimulatory. Adrenaline has a positive chronotropic and
inotropic effect on the lamprey, teleost, and elasmobranch heart ( Johansen et al., 1966; Falck et al., 1966; Bennion, 1968); but it has little or
no effect on the hagfish heart (Bloom et al., 1961). Adrenaline and
noradrenaline dilate the vessels in the teleost and elasmobranch gill
(Keys and Bateman, 1932; Ostlund and Fiingc, 1962) decrease the rcsistance to flow through the gills, and incrcasc blood flow in thc sccondary
lamellae ( Steen and Kruysse, 1964 ) . Phenoxybenzamine, an a-adrcnergic
receptor blocking agent, diminishes the effect of catecholamines on the
circulation, Isoprenaline, a p-adrencrgic stimulant, decreases thc resistance to blood flow through the gills (Randall, 1967). Chan ( 1967) suggested that isoprenaline dilated vessels in the systemic circulation of the
eel. Hanson ( 1967) concluded that catecholamines increase systcmic
resistance to flow in elasmobranchs. Kcys and Bateman (1932) found