AJP - Regulatory, Integrative and Comparative Physiology, Vol 264, Issue 2 262-R267, Copyright © 1993 by American Physiological Society
Autonomic regulation of heart rate and blood pressure in hemorrhaged toads
J. Michalicek and G. Campbell
Department of Zoology, University of Melbourne, Parkville, Victoria, Australia.
Mean arterial blood pressure (P(a)) and heart rate (HR) were studied in
normal conscious cane toads (Bufo marinus) or in autonomically blocked
animals treated variously with bretylium, atropine, phentolamine, and
propranolol. Partial or total autonomic blockade had no effect on resting
values of the variables tested, with the exception that atropine treatment
of bretylium-treated toads caused an increase of heart rate by 5.3 +/- 1.3
beats/min. In graded hemorrhage, aliquots of blood (1.68 ml/100 g body wt)
were removed at 60-min intervals. There was extensive plasma recruitment,
and blood could be removed up to 10.08 ml/100 g without inducing
cardiovascular shock. Each hemorrhage caused an initial transient decrease
in P(a) and increase in HR followed by recovery toward prehemorrhage
values. Repeated hemorrhage caused a progressive rise in basal HR and fall
in basal P(a). The effects of drug treatments suggest that transient
increases in HR may be caused by the release of adrenal catecholamines. In
addition, hemorrhage may increase sympathetic tone so that basal HR becomes
elevated, which, together with sympathetic vasoconstriction, limits the
progressive fall of basal P(a).
