Measurement of cardiac parameters
Contractions were measured isometrically via Dynamometer UF1 transducers (sensitivity range, 559 mN) connected to Lectromed 5230 preamplifiers (Letchworth, Hertfordshire). The beating rate of right atria was measured with a Lectromed 5250 ratemeter preamplifier. These were housed within a MT8P preamplifier unit which relayed signals to a MT6 thermal-pen-recorder giving an output on heat sensitive paper. Each channel was calibrated such that a full scale deflection of 20 mN could be observed. Time to peak tension was measured from the onset of electrical stimulation to the peak of the contraction (Penefsky, 1994). The effective refractory periods (ERP) of left atria, right papillary muscles, and ventricular strips were measured using a modification of the extra stimuli method (Reuter and Heeg, 1971).