Abstract
1. The present experiments investigate the effects of phenoxybenzamine and propranolol, singly and in combination, on plasma catecholamine levels in sheep receiving a three-hour adrenaline infusion. 2. Five groups of five anaesthetized sheep were studied for a period of 3 h each. One group acted as a control and received only a saline (0.9% w/v NaCl solution) infusion. A second group received a constant infusion of adrenaline (2 mug kg body weight-1 min-1). A third group received a similar adrenaline infusion, having been premedicated with phenoxybenzamine (1 mg/kg body weight). A fourth group recieved a similar adrenaline infusion following premedication with (+/-)-propranolol hydrochloride (7 mug/kg body weight). The fifth group received the adrenaline infusion following premedication with both the alpha- and beta-blocker in the above doses. 3. Plasma catecholamines were measured on blood samples taken at seven intervals before during and following the infusion. 4. Control animals receiving only a saline infusion remained physiologically and biochemically stable throughout the experimental period. 5. Adrenaline infusion in animals not receiving adrenoceptor blocking drugs caused a rise in plasma adrenaline levels from a low basal value of 1 mug/litre to a maximum level of 19.8 mug/litre. Animals premedicated with phenoxybenzamine exhibited a similar response. 6. Animals premedicated with propranolol before the infusion of adrenaline did not demonstrate as marked a rise of plasma adrenaline levels as the two previous groups. The maximum mean plasma adrenaline level recorded in this group was 6.88 mug/litre. 7. Animals premedicated with phenoxybenzamine and propranolol before the infusion of adrenaline showed only a small rise in plasma adrenaline levels compared with animals receiving adrenaline infusion alone. The maximum mean plasma adrenaline level in the group was only 3.43 mug/litre. 8. The studies demonstrate that by an unknown mechanism beta-adrenoceptor blockade with (+/-)-propranolol, either alone or in combination with phenoxybenzamine, lowers the plasma adrenaline response evoked by adrenaline infusion.
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