Blood pressure (PA) and heart rate (HR) were measured in the conscious, resting toad, Bufo marinus. Treatment with bretylium (an adrenergic neurone blocking agent), alone or in combination with phentolamine and propranolol (adrenoceptor antagonists) did not alter PA or HR significantly. Atropine caused a small but significant increase in HR but had no effect on PA. The experiments indicate a cholinergic cardio-inhibitory tone but give no evidence for an adrenergic pressor tone at rest. Treadmill exercise caused a rapid increase in PA and HR which was sustained throughout the exercise period. This response was partly psychogenic. The concentration of plasma catecholamines increased during exercise and was high enough to affect organs that were included in an extracorporeal blood circuit with the exercising animal. Bretylium treatment revealed an initial hypotension, presumably due to work hyperaemia, followed by a hypertension which was reduced compared to controls. The tachycardia was delayed but HR eventually reached control levels. Additional treatment with phentolamine and propranolol did not further affect the PA response, but significantly reduced the tachycardia reached during exercise. It is concluded that the cardiovascular responses to exercise involve adrenergic nerve fibres causing hypertension and an initial rapid tachycardia. Circulating catecholamines seem to be the major cause of the sustained tachycardia.

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