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.