Changes in cardiovascular responsiveness to dopexamine and β₁- and β₂-adrenoceptor function after the chronic treatment of β-adrenoceptor antagonists and agonists in anaesthetized dogs

1 The studies were designed to investigate the changes in responsiveness to β-adrenoceptor agonists induced by chronic administration of β-adrenoceptor antagonists and agonists. 2 Dose-response curves for dopexamine, isoprenaline, noradrenaline and impromidine on heart rate, blood pressure and myocardial contractility (dP/dt:integrated isometric tension) were obtained in untreated dogs and compared to those measured in dogs which had been pretreated with propranolol (8 mg kg^-1 day^-1), atenolol (6 mg kg^-1 day^-1), isoprenaline (0.5 μg kg^-1 min^-1) or noradrenaline (0.5 μg kg^-1 min^-1) for 14 days. 3 Propranolol pretreatment significantly enhanced the inotropic and chronotropic responses to isoprenaline. There were noticeable, but non-significant increases in inotropic sensitivity to noradrenaline and dopexamine, especially at higher dose levels. In the atenolol treatment group, there were significant increases in inotropic responses to dopexamine and isoprenaline and in depressor responses to isoprenaline. 4 Thus, chronic administration of propranolol increased responses mediated by both β₁- and β₂-adrenoceptors, whereas, atenolol selectively enhanced the inotropic responsiveness to dopexamine, which is mediated mainly by β₂-adrenoceptors. 5 Isoprenaline pretreatment caused a significant decrease in inotropic sensitivity to dopexamine, isoprenaline and noradrenaline and a significant reduction in chronotropic responses to dopexamine. The tendency to reduced depressor responses to isoprenaline and dopexamine failed to reach significance. Pretreatment with noradrenaline decreased only the inotropic response to noradrenaline. 6 Thus, chronic isoprenaline treatment reduced the responsiveness of both β₁- and β₂-adrenoceptors, while chronic noradrenaline infusion only reduced β₁-adrenoceptor-mediated responses. 7 There was no significant change in any of the dose-response curves to impromidine after any β-adrenoceptor antagonist or agonist treatment. This indicates that there was no non-specific alteration in responsiveness and that the observed changes were likely to be associated with specific alterations in β-adrenoceptor number or function.