Association of decreased myocardial beta-receptors and chronotropic response to isoproterenol and exercise in pigs following chronic dynamic exercise.

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Abstract
The effects of chronic dynamic exercise on myocardial beta-adrenergic and muscarinic cholinergic receptors and chronotropic sensitivity to isoproterenol were studied in 5 Yucatan miniswine. Right atrial and left ventricular biopsies, heart rate responses to isoproterenol, and maximal exercise treadmill testing were obtained before and after 10-19 weeks of treadmill running. Radioligand studies using 125I-iodocyanopindolol (ICYP) and 3H-quinuclidinyl benzilate (QNB) were used to determine the number of beta-adrenergic and muscarinic cholinergic receptors. Maximal oxygen consumption increased from 52 +/- 5 to 65 +/- 7 ml/kg/min (mean +/- SD; p less than 0.02), maximal workload from 530 +/- 111 to 1,074 +/- 179 KPM/min (p less than 0.01), resting heart rate decreased from 91 +/- 13 to 62 +/- 4 beats/min (p less than 0.01), heart rate at 75% of pretraining maximal workload decreased from 253 +/- 15 to 196 +/- 12 beats/min (p less than 0.01), and maximal exercise heart rate decreased from 273 +/- 6 to 254 +/- 9 beats/min (p less than 0.01). Decreased heart rate responsiveness to adrenergic stimulation was observed following chronic exercise. Maximal isoproterenol-stimulated heart rate decreased from 225 +/- 13 to 185 +/- 28 beats/min (p less than 0.05) and the slope of the isoproterenol dose-response relation decreased from 63 +/- 16 to 40 +/- 16 (p less than 0.05). Radioligand studies revealed a decrease in beta-receptor number in the right atrium following chronic exercise (61 +/- 9 vs. 34 +/- 8 fmol/mg; p less than 0.02), but receptor number in membranes from the left ventricle did not change (60 +/- 9 vs. 62 +/- 4 fmol/mg).(ABSTRACT TRUNCATED AT 250 WORDS)