Chronotropic and dromotropic effects of atropine and hyoscine methobromide in unanaesthetized dogs.

Atrial pacing at progressively increasing frequencies was performed in 12 awake dogs through electrodes implanted aseptically in the right atrium and led out through the skin of the neck. Heart rate and the Wenckebach point (the minimum pacing frequency for which a second degree A-V block first appeared) were measured by electrocardiography. The responses of the sino-atrial and auriculo-ventricular nodes to atropine and hyoscine methobromide were characteristically bimodal. Slowing of the heart rate and atrioventricular conduction appeared with the lower doses of atropine (6.25 micrograms kg-1 i.v) and hyoscine methobromide (0.4 micrograms kg-1 i.v.); at doses of atropine 12.5 micrograms kg-1 i.v. and hyoscine methobromide 1.56 micrograms kg-1 i.v., only atrioventricular conduction was slowed; an acceleration of heart rate and atrioventricular conduction occurred in response to the higher doses of both drugs. These results show, in the awake dog, that the effects of atropine and hyoscine methobromide on heart rate and atrioventricular conduction are entirely comparable when appropriate doses are used, and reveal a difference in the responsiveness of the sino-atrial and auriculo-ventricular nodes to these drugs.

Comparative electrophysiologic effects of bepridil, verapamil and diltiazem in conscious and anesthetized dogs.

The electrophysiologic effects of bepridil (10 mg/kg), verapamil (0.15 mg/kg) and diltiazem (0.6 mg/kg) were studied in the chronically instrumented conscious and pentobarbital-anesthetized dog. Sinus node automaticity assessed by the corrected sinus node recovery time and atrio-ventricular conduction assessed by the Wenckebach phenomenon were evaluated by atrial pacing via electrodes placed in the wall of the right atrium, and exteriorized in the neck region. Nineteen dogs were studied and groups of 6 dogs were used for each experimental session. The calcium channel blocking drugs were administered by slow i.v. infusion (15 min). Effects were measured over 45-60 min and compared with the pretreatment value. In conscious dogs, heart rate was initially markedly increased by bepridil and diltiazem and only slightly increased by verapamil. These chronotropic responses were reversed to bradycardia by diltiazem only. Corrected sinus node recovery time measured at the end of infusion was decreased by verapamil and diltiazem and was unchanged by bepridil. Lengthening of corrected sinus node recovery time was observed with verapamil at the end of the experiment. All 3 calcium channel blocking agents produced negative dromotropic responses. To determine to what extent electrophysiologic effects of pentobarbital were involved in the cardiac responses measured in the anesthetized dog, pentobarbital was administered prior to injection of each calcium channel blocking agent. Pentobarbital produced positive chronotropic and dromotropic effects which were attenuated by the 3 calcium channel blocking agents. The reduction of corrected sinus node recovery time induced by pentobarbital was diminished by bepridil and diltiazem and unchanged by verapamil. Pentobarbital anesthesia thus has important electrophysiologic implications on the effects of calcium channel blocking agents on chronotropic and dromotropic variables on the heart.