Quinidine-like activity of atropine in rabbit atria.

Direct effects of atropine and quinidine on contractile force, overdrive suppression, and effective refractory period was studied in rabbit atria. Left atrial contractile force at normal CaCl2 concentration (2.2 mM) and the contractile force response to elevated CaCl2 (5 mM) were unchanged over 30 min. Atropine exposure (1.7 X 10(-5) M) for 30 min significantly reduced contractile force at both normal and elevated CaCl2 levels. Quinidine exposure (6.2 X 10(-6) M) for 30 min produced similar but statistically insignificant changes. At 4 h, control contractile force at 2.2 mM CaCl2 decreased to the post-atropine level, but the response to 5 mM CaCl2 was unchanged. The asystolic interval of right atria following overdrive (for 2 min at 3 rates) was increased after 30 min by 4% for controls, 14% after atropine (1.7 X 10(-5) M) and 45% after quinidine. The effective refractory period of left atria (evaluated by paired pulse stimulation) was unchanged after 30 min for controls, but increased by 20% after atropine and 45% after quinidine.