The Effects of Prolonged Beta-adrenoceptor Blockade on Heart Weight and Cardiac Intracellular Potentials in Rabbits

Estimates were made in vivo in rabbits of the relative beta-receptor blocking potency and duration of action of propranolol and practolol. In further experiments groups of litter mates were injected twice daily with approximately equi-active amounts of propranolol or practolol, or with saline, for several weeks. The heart weights of the treated animals were significantly lower than those of the controls, the water contents were higher, and the dry weight differences were highly significant; -16.8% after 2 mg/kg bd propranolol for six weeks and -33.8% after 10 mg/kg practolol. (The treated animals grew less rapidly than the controls; when corrected for body weight these figures were -11.9% and -20.4%, respectively.) In the practolol group, but not the propranolol group, the duration of the atrial intracellular potentials was prolonged. There was no evidence that the prolonged treatment with either drug had a negative inotropic effect, or reduced positive inotropic responses to isoprenaline.