Effects of (+/-)- (+)- and (-)-metoprolol, (+/-)- (+)- and (-)-pindolol, (+/-)-mepindolol and (+/-)-bopindolol on the rat left atria and portal vein.

1. The effects of (+/-)- (+)- and (-)-metoprolol, (+/-)- (+)- and (-)-pindolol, (+/-)-mepindolol and (+/-)-bopindolol on the beta 1-adrenoceptor mediated responses of the rat left atria and the beta 2-adrenoceptor mediated responses of the rat portal vein to isoprenaline have been determined. 2. Racemic and (-)-metoprolol were selective beta 1-adrenoceptor antagonists. (+)-Metoprolol was devoid of beta-adrenoceptor antagonistic activity. 3. Racemic and (-)-pindolol were potent and (+)-pindolol was a modest beta-adrenoceptor antagonist. 4. (+/-)-Mepindolol and (+/-)-bopindolol were apparently competitive antagonists of the isoprenaline beta 1-adrenoceptor mediated responses of the rat left atria but non-competitive antagonists of the isoprenaline beta 2-adrenoceptor mediated responses of the rat portal vein. 5. It is suggested that (+/-)-mepindolol and (+/-)-bopindolol are slowly dissociating beta-adrenoceptor antagonists and the non-competitive antagonism can only be detected on tissues with modest receptor reserves for maximum responses to isoprenaline.