{'fraction_non_alphanumeric': 0.0356906807666887, 'fraction_numerical': 0.038334434897554524, 'mean_word_length': 5.4425531914893615, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 4, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Presynaptic β‐adrenoceptors in rat atria: evidence for the presence of stereoselective β1‐adrenoceptors

1 Presynaptic β‐adrenoceptor activity was studied in rat isolated atria, previously loaded with [3H]‐noradrenaline. The stimulation‐induced release of 3H transmitter was measured in the presence of cocaine, and adrenaline was used as a facilitatory β‐adrenoceptor agonist. 2 Adrenaline (0.1 and 2 nM) increased, by about 50%, the evoked efflux of tritium. With phenoxybenzamine present, the same activity was shown with 10 nM adrenaline. 3 The β2‐selective adrenoceptor blocking drugs: IPS 339 and ICI 118 551 caused a concentration‐dependent decrease in the activity of adrenaline. Cardioselective β‐blocking drugs: acebutolol, betaxolol, nebivolol and its isomers (R 67 138 and R 67 145) also reduced dose‐dependently the agonistic action of adrenaline. The order of potency for nebivolol and its isomers was R 67 138 > nebivolol > R 67 145. The activity of pindolol was not concentration‐dependent. The inhibitory effect of acebutolol was also observed in the presence of blockade of α‐adrenoceptors. 4 The postsynaptic β‐adrenoceptor blocking activity of nebivolol and its isomers was studied in pithed rats. They reduced isoprenaline‐induced tachycardia without altering hypotensive responses. The order of potency was: R 67 138 > nebivolol > R 67 145. 5 It is concluded that in rat isolated atria, presynaptic β2‐ and β1‐adrenoceptors coexist and that facilitatory β1 ‐adrenoceptors are stereospecific.