Differential effect of selective beta 1 and nonselective beta-adrenoceptor blockade on epinephrine and atropine...

Sympathetic stimulation with epinephrine (EPI) combined with parasympathetic blockade with atropine was studied in 10 healthy volunteers premedicated with placebo or three different beta-adrenoceptor blockers: atenolol (62.5 micrograms/kg, beta 1-selective), propranolol (62.5 micrograms/kg, nonselective), and pindolol (7.5 micrograms/kg, nonselective with intrinsic sympathomimetic activity, ISA). EPI infusion (0.06 microgram/kg/min) after placebo increased heart rate (HR) and systolic blood pressure (SBP) and decreased diastolic BP (DBP). Pretreatment with atenolol reduced the HR increase, and caused similar changes in BP. In contrast, pretreatment with propranolol and pindolol decreased HR and increased BP. Combined EPI and atropine (15 micrograms/kg) after placebo increased HR by 40% without causing BP changes. Similar HR changes were observed after administration of all beta-adrenoceptor blockers, but whereas a marked pressor response was observed after propranolol and pindolol a blunted response was observed after atenolol. Propranolol and pindolol reduced myocardial oxygen demand estimated by the HR x BP product after EPI, but this response was abolished by atropine. Serum potassium decreased from 3.9 +/- 0.2 to 3.2 +/- 0.3 mM after EPi and atropine. This effect was less after atenolol, and potassium increased after premedication with propranolol and pindolol. Our results show that nonselective beta-adrenoceptor blockade has a favorable effect on potassium homeostasis and oxygen demand parameters during EPI infusion but causes a marked pressor response, contrary to a beta 1-selective agent, during combined sympathetic stimulation and parasympathetic blockade. They also highlight the importance of the vasodilator cholinergic system as a defense mechanism in such situations.