[Effect of nifedipine on cardiac chronotropism in conscious dogs]

The effect of nifedipine (NIF) was studied in intact conscious dogs with and without autonomic blockade. Maximal decrease of systolic arterial pressure (delta P, in mmHg), maximal increase of heart rate (delta F, in beats/min), the ratio delta F/delta P, and the time elapsed between delta F and delta P (delta t) were measured after the acute administration of the drug. Two intravenous doses of NIF (50 and 150 micrograms/kg) were administered either alone or after sympathetic and parasympathetic blockade. Group Ia: 50 micrograms/kg of NIF alone. The delta F/delta P and delta T were 3.8 +/- 0.4 and 11.3 +/- 2.2 sec respectively. Group Ib: 150 micrograms/kg of NIF alone. The delta F/delta P and delta T were 2.4 +/- 0.2 and 15 +/- 3 sec respectively. Group IIa: 2 mg/kg of propranolol + 150 micrograms/kg of NIF. The delta F/delta P and delta T were 0.84 +/- 0.4 and 16 +/- 5 sec respectively. Group IIIa: 0.2 mg/kg of atropine + 2 mg/kg of propranolol + 50 micrograms/kg of NIF. In this group left systolic ventricular pressure decreased about 14 mmHg, but the heart rate (HR) was not modified. Group IIIb: 0.2 mg/kg of atropine + 2 mg/kg of propranolol + 150 micrograms/kg of NIF. In this group, left systolic ventricular pressure decreased about 18 mmHg but the HR was not modified. We conclude that acute administration of NIF decreases systolic ventricular pressure and increases HR. The reflex tachycardia is partially abolished by 2 mg/kg of propranolol, and completely abolished by sympathetic and parasympathetic blockade.(ABSTRACT TRUNCATED AT 250 WORDS)