The β-blocking activity of flestolol was established during increasing isoprenaline infusions and during graded physical exercise in conscious, chronically instrumented dogs. After a control cycle, flestolol was infused at three doses (1, 2.67, and 10 μg/kg/min). Flestolol has an extremely short half-life, demonstrated by an 83% loss of effect within 25 min. Thus, flestolol allows easy titration of the effect, which might be a valuable property for its use in the treatment of critically ill patients. Due to the difference between pure β-adrenergic stimulation and the much more complex regulation of circulation during exercise, the hemodynamic response to flestolol elicited marked differences between both set ups. Flestolol shifted the dose-response curves of isoprenaline-induced changes in heart rate, positive left ventricular dp/dtmax and diastolic arterial pressure dose dependently to the right, while its main effect during exercise was a decrease in positive left ventricular dp/dtmax. Thus, testing of β-adrenoceptor blockers using isoprenaline-induced tachycardia leads to an overestimation of potency and therefore is not appropriate to predict the clinical efficacy of these drugs to prevent stress- or exercise-induced in-creases in heart rate and hence myocardial oxygen-demand. © 1990 Raven Press, Ltd., New York.
