Tissue preparation
Rats were administered 1000 IU kg-1 of sodium heparin by an intraperitoneal injection. After 15 minutes, they were stunned by a blow to the head, exsanguinated, and hearts were excised. Isolated atria, ventricular strips (≤2mm in width) dissected longitudinally towards the apex of the heart, and right papillary muscles were prepared and suspended in 30 ml organ baths, containing (in mM), NaCl 119; KCI 3.8; MgS04 1.18; KH2PO41.18; NaHCO3 25; CaCl­2 1.9 and D-Glucose 10.0, gassed with 95% O2, 5% CO­2 (BỌC medical gases, Guildford), and maintained at 37°C. Each preparation was subjected to a resting diastolic tension of 10 mN and stimulated with square wave pulses of 5 ms duration at a frequency of 1 Hz via a Grass S48 or S88 stimulator (Quincy, Massachusetts). Tissues were stimulated at twice threshold voltage ≤15 V. Right atria were allowed to equilibrate such that spontaneous, rhythmic beating occurred. In all cases, tissues were washed periodically throughout the stabilisation period.