title: An apparent preferential antagonism by practolol of the positive inotropic responses of guinea-pig isolated atria to isoprenaline.
abstract: The positive inotropic response of guinea-pig isolated atria to isoprenaline was antagonized to a greater extent than the positive chronotropic response, when measured at 38 degrees C as the reduction of the responses to a single concentration of isoprenaline. This method avoided increasing the the concentration of agonist in the presence of practolol and any difference in the proportion metabolized affecting the blockade. This observation was substantiated from the vertical displacement of dose-response curves from four concentrations of isoprenaline added sequentially. No difference between the antagonism of rate and tension was obtained when the antagonism was measured as the horizontal displacement of complete dose-response curves constructed sequentially or cumulatively. The resultant pA2 values from cumulative dose-response curves were identical, indicating that the beta-adrenoceptors subserving these responses are the same. The rate and tension curves were not, however, superimposable; rate lay to the left. By lowering the bath temperature to 30 degrees C they were brought together and the selective antagonism was no longer observed. This phenomenon was therefore attributed to the separation of rate and tension curves at 38 degrees C, the selected concentration of isoprenaline occupying different positions on these curves.
mesh_terms: D000818:Animals; D004305:Dose-Response Relationship, Drug; D005260:Female; D006168:Guinea Pigs; D006325:Heart Atria; D066298:In Vitro Techniques; D007545:Isoproterenol / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D008297:Male; D009200:Myocardial Contraction / Q000187:drug effects*; D011217:Practolol / Q000494:pharmacology*; D011941:Receptors, Adrenergic / Q000187:drug effects*; D011943:Receptors, Adrenergic, beta / Q000187:drug effects*; D013268:Stimulation, Chemical
chemical_list: D011941:Receptors, Adrenergic; D011943:Receptors, Adrenergic, beta; D007545:Isoproterenol; D011217:Practolol