Beta 2-adrenoceptors in guinea-pig atria

The occurrence of beta 2-adrenoceptors in the isolated, spontaneously beating right atrium and the electrically driven left atrium of the guinea-pig was studied. Isoprenaline was used as reference compound and procaterol as selective beta 2-agonist. Cumulative concentration response (C/R) curves were obtained with the agonists. The C/R curve of procaterol was biphasic in both preparations. Compared with isoprenaline, procaterol was a partial agonist, with a mean maximum response of 0.78 +/- 0.04 in the right atrium and 0.29 +/- 0.05 in the left atrium. The beta 2-selective antagonist ICI 118,551, 10(-7) mol litre-1, caused a small but significant shift of the C/R curve of isoprenaline to a higher concentration range in both preparations. The same concentration of the beta 2-blocker changed the shape of the C/R curve of procaterol from biphasic to monophasic by blocking the responses to low concentrations of procaterol. Practolol, a beta 1-selective antagonist, 10(-6) mol litre-1, gave a highly significant shift of the C/R curve of isoprenaline to a higher concentration range in both preparations but had no effect on the responses to low concentrations of procaterol. The effect of practolol on the responses to high concentrations of procaterol is discussed. We conclude that the guinea-pig atria may contain beta 2-adrenoceptors mediating positive chronotropic and inotropic effects.