Effects of stress and chronic propranolol infusion on cardiac beta-receptor function in the Maudsley Reactive and Non-Reactive rat strains.

The effects of beta-adrenoceptor blockade on left ventricular beta-receptor characteristics were evaluated in the Maudsley Reactive (MR/Har) and Non-Reactive (MNRA/Har) rat strains. After propranolol infusion for one week administered by subcutaneously implanted osmotic minipumps, differences in beta-adrenoceptor-agonist interactions between strains were assessed by comparison of isoproterenol competition for [125I]iodocyanopindolol (ICYP) binding sites in crude left ventricular homogenate preparations from both strains. Propranolol exposure had no significant effect on the binding parameters investigated (Bmax, Kd, % receptors in high affinity state, IC50). In contrast to previous observations in naive animals, these experiments revealed no strain differences in the sham-implanted Maudsleys in any of the left ventricular beta-adrenoceptor parameters investigated. We hypothesize that the surgical procedures and/or presence of the plastic inserts acted as stressors in the Maudsley strains and thus altered beta-adrenoceptor function. This hypothesis is supported by the observation that exposure to another stress (footshock) also eliminated strain differences in ventricular beta-receptor binding variables.