THE EFFECT OF β-ADRENORECEPTOR ANTAGONISTS ON THE INHIBITION OF PENDULAR MOVEMENTS OF RABBIT ILEUM PRODUCED BY PERIARTERIAL SYMPATHETIC NERVE STIMULATION AND SOME SYMPATHOMIMETIC AMINES

• 1 The effects of propranolol and of the selective β₁ and β₂-adrenoreceptor blocking drugs atenolol and ICI 118,551 were determined on the inhibitory responses of isolated segments of rabbit ileum to noradrenaline, isoprenaline and salbutamol and to periarterial sympathetic nerve stimulation.
• 2 Responses to isoprenaline (0.04–10.24 μM) and salbutamol (1.4–89.6 μM) were blocked by propranolol in concentrations up to 5.0 and 12.8 μM, respectively. Responses to sympathetic nerve stimulation were reduced but responses to noradrenaline (0.03-1.92 μM) were unaffected by propranolol in concentrations up to 10.0 and 5.0 μM, respectively.
• 3 Atenolol in concentrations up to 30.0 μM blocked responses to isoprenaline (0.04-2.56 μM) but did not affect responses to noradrenaline, salbutamol or sympathetic nerve stimulation in concentrations up to 3.0,3.0 and 1.0 μM, respectively. However, when responses to noradrenaline and sympathetic nerve stimulation were reduced by phentolamine (1.0 μM), atenolol then produced further reductions.
• 4 Responses to isoprenaline (0.04-2.56 μM) and salbutamol (1.4–89.6 μM) were blocked by ICI 118,551 in concentrations up to 0.5 μM. Responses to sympathetic nerve stimulation were reduced but responses to noradrenaline were unaffected by ICI 118,551 in concentrations up to 0.01 and 0.3 μM, respectively.
• 5 Salbutamol (0.1 μM.) increased the inhibitory response to sympathetic nerve stimulation and this effect was blocked by ICI 118,551 (0.01 μM).
• 6 It was concluded that blockade of β₂-adrenoreceptors, presumably located on sympathetic nerve terminals, decreases the release of transmitter noradrenaline and that blockade of β₁-adrenoreceptors, presumably located in longitudinal smooth muscle cells, reduces the response to transmitter noradrenaline when α-adrenoreceptors are also blocked.