The effect of catecholamine infusions on myocardial blood flow, metabolic heat production and on general haemodynamics, before and after alprenolol (H56-28), in anaesthetized cats.

1. In cats anaesthetized with pentobarbitone sodium, infusions of adrenaline, noradrenaline (0.5 mug/kg per min) and isoprenaline (0.25 mug/kg per min) increased myocardial blood flow, myocardial heat production, left ventricular systolic and end-diastolic pressures, left ventricular +ve and -ve dp/dt max, and calculated cardiac output, effort and oxygen consumption. These effects (apart from the effect of noradrenaline on left ventricular systolic pressure) were markedly reduced by previous administration of alprenolol (0.5 or 1.0 mg/kg).2. Infusions of adrenaline and noradrenaline increased arterial diastolic blood pressure and isoprenaline reduced it. After alprenolol the effects of adrenaline and noradrenaline were potentiated and that of isoprenaline abolished; in some experiments isoprenaline increased arterial diastolic pressure after alprenolol. Alprenolol did not influence the increases in arterial systolic pressure which followed the administration of adrenaline and noradrenaline.3. Isoprenaline-induced tachycardia was markedly reduced and adrenaline tachycardia was converted to bradycardia after alprenolol. The bradycardia which occurred during noradrenaline infusions was unaffected.4. After blockade by alprenolol, recovery of the effects of isoprenaline on left ventricular dp/dt and on heart rate occurred more quickly than recovery of the effects on arterial diastolic pressure. This suggests that alprenolol has a greater affinity for beta(2)- than for beta(1)-adrenoceptors.5. Intravenous administration of acetylcholine decreased arterial blood pressure, left ventricular pressure and +ve and -ve dp/dt max. During recovery from these effects there was a marked increase in +ve dp/dt max. which was absent after the administration of alprenolol (0.5 mg/kg). Because this dose of alprenolol is thus able to block the effects of reflex sympathetic cardiac nerve stimulation but does not completely antagonize the effects of exogenous adrenaline on dp/dt, it is suggested that alprenolol may have some adrenergic neurone blocking activity.6. Increases in liver and myocardial blood flow and heat production produced by noradrenaline, adrenaline and isoprenaline were reduced after alprenolol.7. Isoprenaline reduced air-way resistance and this effect was abolished by alprenolol; increases in air-way resistance produced by adrenaline and nor-adrenaline were augmented. All three amines inhibited intestinal smooth muscle contractions in vivo. Only the effect of isoprenaline was reduced by alprenolol.