The effects of (+)- and (-)-propranolol on 3H-transmitter efflux in guinea-pig atria and the presynaptic beta-adrenoceptor hypothesis.
The effects of isoprenaline and of the (+)- and (-)-isomers of propranolol on the stimulation-induced overflow of 3H-transmitter was assessed in guinea-pig atria to evaluate the hypothesis of presynaptic beta-adrenoceptors. 2 Isoprenaline (1.2 x 10(-8) M) enhanced the efflux of tritium at 2 and 5 Hz with 100 pulses and did so to a similar extent at both frequencies. 3 The (-)-isomer of propranolol (1.0 x 10(-7) M) blocked the enhancing effect of isoprenaline but did not by itself modify transmitter efflux. 4 The (+)-isomer of propranolol, almost devoid of beta-adrenoceptor blocking properties, was also effective at 1.0 x 10(-7) M in blocking the enhancement of tritium efflux by isoprenaline. 5 The (-)-isomer of propranolol (1.0 x 10(-7) M) blocked almost entirely the inotropic response to isoprenaline (3 x 10(-7) M) but even 3.0 x 10(-6) M (+)-propranolol was ineffective in antagonizing the beta-adrenoceptor-mediated contractile responses to the catecholamine. 6 It is concluded that the presynaptic site of isoprenaline action does not show the requisite stereo-specificity of beta-adrenoceptors and that a 'non-specific' action of the antagonist probably accounts for its reduction of the effect of isoprenaline.
D000818:Animals; D005260:Female; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D009206:Myocardium / Q000378:metabolism*; D018377:Neurotransmitter Agents / Q000378:metabolism*; D009638:Norepinephrine / Q000378:metabolism; D011433:Propranolol / Q000494:pharmacology*; D011941:Receptors, Adrenergic / Q000502:physiology*; D011943:Receptors, Adrenergic, beta / Q000502:physiology*; D013237:Stereoisomerism; D013569:Synapses / Q000378:metabolism
D018377:Neurotransmitter Agents; D011941:Receptors, Adrenergic; D011943:Receptors, Adrenergic, beta; D011433:Propranolol; D007545:Isoproterenol; D009638:Norepinephrine