(+/-)-Pindolol has beta 2-adrenoceptor antagonist but not agonist action on the costo-uterine muscle of the rat.
1. The effects of isoprenaline and (+/-)-pindolol on rat isolated costo-uterine muscle have been compared. 2. Isoprenaline produced reproducible concentration-dependent inhibition of contractions, and maximal doses (less than 0.1 mumol/l) produced mean inhibition of 87, 94 and 97% of field, carbachol and potassium-stimulated preparations, respectively. 3. (+/-)-Pindolol, when effective, produced inhibition only in concentrations greater than its pA2 value (9.87) as an antagonist of isoprenaline; mean maximal effects were less than 60% of those produced by isoprenaline. 4. It is concluded that (+/-)-pindolol is a potent antagonist, but has only variable agonist action, at the beta 2-adrenoceptors of the rat costo-uterine muscle.