Alterations of responsiveness to adrenoceptor agonists and calcium of non-infarcted hypertrophied muscles from rats with chronic myocardial infarction.

1. The responsiveness of hypertrophied right ventricular muscle from rats with chronic myocardial infarction (MI) was examined. Myocardial infarction was produced by ligating the left coronary artery. The positive inotropic effects of alpha- and beta-adrenoceptor agonists and calcium on the isolated non-ischaemic muscles were determined at one and four weeks after ligation, and were compared with those in sham-operated (SO) rats. 2. The contractile force of electrically-driven right ventricular papillary muscles was measured isometrically. We also determined the change of beating rate of the right atrium in response to a beta-adrenoceptor agonist. 3. Basal contractile force increased at four weeks after ligation. 4. The pEC50 values of the alpha 1-adrenoceptor agonist, methoxamine, and calcium increased at four weeks after ligation, but the pEC50 values of the beta-adrenoceptor agonist, dobutamine did not change. 5. Maximal contractile force did not change with these agonists at either one and four weeks after ligation. 6. Tissues from rats four weeks after MI exhibited slight increases in the Bmax of alpha- but not beta-receptors, without any changes of affinity. 7. The increases in the sensitivity to an alpha 1-adrenoceptor agonist and calcium may be compensatory mechanisms of non-infarcted ventricular tissue which has undergone hypertrophy in chronic MI.