Figure 1. Cardiorespiratory effects of αβ-MeATP.

Cardiorespiratory effects of a dose of αβ-MeATP (100 nmol i.v. at continuous vertical line) in a rat anaesthetized with pentobarbitone. In each panel the upper trace from the computerized chart recorder is arterial BP, and the lower trace is respiratory airflow (inspiration downwards, arbitrary units (a.u.), mV). Heart rate was measured by counting individual beats in the BP trace. During the control state (A) bradycardia, transient hyperventilation, vasodepression and apnoea were obtained in response to the rapid injection of αβ-MeATP. After cutting both vagus nerves at mid-cervical level (B) the bradycardia, hypotension, and apnoea were abolished, leaving hyperventilation and a vasopressor response to the same dose of agonist. Cutting both carotid sinus nerves (C) abolished hyperventilation without reducing the vasopressor response. The delayed arrhythmia seen in C differed from the reflex bradycardia observed in A, and was associated with the substantial rise in systemic blood pressure; the arrhythmia may result from activation of C fibre sympathetic cardiac afferents innervating the left ventricle (see Hainsworth, 1991).