title: Reduction of intrinsic sinoatrial frequency and norepinephrine response of the exercised rat.
abstract: Physical training is associated with a reduction of intrinsic sinoatrial activity; the present study examined the role of the parasympathetic nervous system in this reduction. Six groups of rats were studied for 10 weeks: inactive control; treadmill exercised; parasympathetic receptor blockade with atropine; exercise plus atropine; parasympathetic receptor stimulation with carbachol; and exercise plus carbachol. In vivo ISF (cardiac frequency 20 min after injection of propranolol and atropine) was measured at 3-week intervals. At the end of 10 weeks the right atrium was excised, in vitro measurements were made of ISF, and chronotropic dose-response curves to acetylcholine and norepinephrine were established. In vivo, ISF was reduced with time, the greatest reduction being found in the exercise plus atropine group; the treadmill-exercised and the atropine-treated groups also had a greater reduction than the control group. In vitro, no differences were observed in acetylcholine responses. The maximum norepinephrine chronotropic response was reduced in the treadmill-exercised and the exercise plus atropine groups. The maximum norepinephrine-induced frequency correlated with the in vitro ISF (r = 0.75). Thus, ISF was reduced with training, but this effect was independent of parasympathetic activity. The properties of the sinoatrial node which set ISF also influenced the maximum norepinephrine response.
mesh_terms: D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D001285:Atropine / Q000494:pharmacology; D001835:Body Weight; D002217:Carbachol / Q000494:pharmacology; D006321:Heart / Q000187:drug effects; D006339:Heart Rate / Q000187:drug effects; D066298:In Vitro Techniques; D008297:Male; D009206:Myocardium / Q000378:metabolism; D009638:Norepinephrine / Q000494:pharmacology*; D009929:Organ Size / Q000187:drug effects; D005082:Physical Exertion; D011188:Potassium / Q000378:metabolism; D051381:Rats; D012849:Sinoatrial Node / Q000502:physiology*; D012964:Sodium / Q000378:metabolism; D013997:Time Factors
chemical_list: D001285:Atropine; D002217:Carbachol; D012964:Sodium; D000109:Acetylcholine; D011188:Potassium; D009638:Norepinephrine