Beta-adrenoreceptors of the cat nictitating membrane

1. The relaxant effects of isoprenaline, salbutamol, adrenaline and noradrenaline on the partially contracted isolated nictitating membrane of the cat were determined in the absence and the presence of alpha-adrenoreceptor blockade. 2. It was possible to demonstrate significant relaxant effects of isoprenaline and salbutamol in the absence as well as in the presence of alpha-adrenoreceptor blockade. However, the relaxant effects of adrenaline and noradrenaline could not be demonstrated in the absence of alpha-adrenoreceptor blockade. 3. Molar concentrations (EC30) of isoprenaline, salbutamol and adrenaline causing 30% relaxation of the tone did not significantly differ from each other; EC30 of each of these agents was significantly less than that of noradrenaline. 4. Beta 1-Adrenoreceptor antagonist metoprolol was less potent than beta 2-antagonist H 35/25 in antagonizing the effect of salbutamol; metoprolol was more potent than H 25/35 in antagonizing noradrenaline. Both these agents were effective antagonists of isoprenaline; however, metoprolol and H 25/35 in combination produced greater antagonism of isoprenaline than did each antagonist separately. 5. It is concluded that the cat nictitating membrane possesses both beta 1- and beta 2-adrenoreceptors which are responsible for the relaxant effects of sympathomimetic agents. A study of these receptors is, however, complicated by concomitant stimulation of alpha-adrenoreceptors.