Effects of temperature on inotropic responses of isolated canine atria under spontaneous or electrically paced rhythm

Effects of temperature on atrial contractility were investigated on isolated, blood-perfused canine atrial preparations which were spontaneously beating or constantly paced at 2 Hz and 2.5 Hz. The contrctility was increased by cooling from 37 degrees C to 27 degrees C in spontaneously beating hearts (the rate decreased from 100 to 56 beats/min) but in the paced atrium the increase of contactile force was followed by a slight decrease under 29 degrees C usually with pulsus alternans. The durations of shortening and relaxation of the developed tension were increased in parallel with cooling of temperature. The rate of shortening was slightly increased by cooling in both spontaneous rate and constant rate. On the other hand, the rate of relaxation was not so influenced in spontaneously beating hearts but slightly increased in paced atria but not significantly. The frequency-force relationship showed the positive staircase phenomenon (2 to 3.5 Hz) at 37 degrees C. However, at lower temperature the positive staircase was not clear and rather negative staircase appeared (1--3 Hz).