Interactive effect of acetylcholine and norepinephrine on the sinoatrial node of the trained rat.

We examined the chronotropic influence of acetylcholine and norepinephrine, separately and in combination, on right atrial preparations from trained and sedentary rats. Atria from trained rats had a lower intrinsic rhythm, and a lower sinoatrial rhythm at low concentrations of acetylcholine and norepinephrine (10(-7) and 10(-8) M) than atria from sedentary animals. At higher concentrations, both groups achieved identical atrial rates. Training appears to modify intrinsic sinoatrial rate without affecting the responses of the pacemaker to either acetylcholine, norepinephrine, or their combination.