ANIMALS AND METHODS
Recording of action potentials: Guinea pigs of either sex (320 to 490 g) were killed by a blow to the neck. After excision of the heart, papillary muscles were dissected from the right ventricle. The isolated preparations were fixed in a lucite chamber (volume 1.5 mL), which was continuously superfused (1.6 to 2.1 mL/min) with a gassed (95% oxygen and 5% carbon dioxide) bathing solution of the following composition (in mmol/L): NaCl 136.9, KCl 5.4, MgCl2 1.05, NaHCO3 11.9, NaH2PO4 0.42, CaCl2 1.8, glucose 5.55; pH 7.2 to 7.4.Experiments were performed at 37±1°C. Action potentials were differentially recorded by the intracellular microelectrode technique, and were electronically differentiated for measurement of the maximum rate of rise of the action potential (Vmax). For investigation of tonic or use-dependent blockade of V max papillary muscles were continuously stimulated during the control period (30 mins) at a rate of 1 Hz. The stimulation was interrupted at the time of drug administration, and was resumed after 110 mins. V max of the first action potentials after the quiescent period was then compared with control values. Papillary muscles were again continuously driven at a rate of 1 Hz until a new steady state.
V max was reached. For evaluation of the time constant of recovery from the use-dependent block, pauses in stimulation of varying duration were applied between trains of pulses (1 Hz) of 5 mins each. The recovery time constant was estimated graphically. For recording, two microprobe systems (Model M 701, World Precision Instruments, Hamden, Connecticut, USA) and a D13 dual beam storage oscilloscope Type 5103N (Tektronix, Beaverton, Oregon, USA) were used. Photographs were taken every 5 mins with a Nihon Kohden Camera Model PC-2A (Tokyo, Japan), and action potential parameters were measured after magnification.
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