Influence of temperature and adrenergic stimulation on rat sinoatrial frequency

We examined the effect of temperature and adrenergic stimulation on atrial rate in the rat. Right atrial preparations were maintained for 65 min at 38.0 degrees C and then exposed to high or moderate concentrations of isoproterenol (ISO) or left as controls. Temperature was cycled through four different values between 35.6 and 42.8 degrees C and allowed to stabilize at each value before atrial rate was measured. The rate-temperature loops exhibited a modest hysteresis; higher values for rate were obtained on the ascending limb. Hysteresis was found to result from a transient overshoot of the rate response to a temperature step. The linear response of atrial rate to temperature ranged from 20.3 +/- 1.3 to 22.6 +/- 0.7 beats.min-1.degrees C-1 (mean +/- SE) for control and high ISO (P greater than 0.05). Data were analyzed by applying the Arrhenius equation and by calculating the Q10 effect. ISO, while increasing atrial rate, reduced the measures of temperature sensitivity. Q10 and mu (temperature characteristic) were 2.1 and 59.8 +/- 2.1 for control and 1.7 and 40.0 +/- 1.5 for high ISO groups, respectively. The direct effect of an increase in temperature on sinoatrial rhythm would contribute significantly to the increase in heart rate seen in exercise.