Affinities of full agonists for cardiac beta-adrenoceptors calculated by use of in vitro desensitization.

Positive chronotropic and inotropic responses of guinea-pig isolated spontaneously beating right atria and paced left atria to beta-adrenoceptor agonists were recorded. Cumulative concentration-response curves for the rate and tension responses to isoprenaline, orciprenaline, terbutaline and fenoterol were obtained before incubation of the tissues with isoprenaline (10(-6) M), the tissues were then washed during 1 h to remove isoprenaline before constructing a post-incubation curve to the same agonist. Incubation with isoprenaline for 4 h caused parallel rightwards shifts to the curves, but extending the incubation to 8 h caused additional depression of the rate (85.2 +/- 5.4%) and tension (68.9 +/- 2.3%) maxima. Incubation with isoprenaline for 4 h only shifted the curves for orciprenaline to the right but depressed the post-incubation maximum rate and tension responses to terbutaline (74.8 +/- 1.5 and 33.8 +/- 2.5%) and fenoterol (85.6 +/- 5.5 and 76.0 +/- 5.1%). Thus incubation with isoprenaline for 4 h induced desensitization of the cardiac beta-adrenoceptor which was revealed as a loss in sensitivity to both isoprenaline and other beta-adrenoceptor agonists. The reduced maxima were attributed to a loss of beta-adrenoceptors and exhaustion of the receptor reserve. Those experiments displaying the reduced maxima were used for calculation of dissociation constants (KA) values by the method of Furchgott (1966) for irreversible antagonism. The values for the right atria were 57 (19-170)nM, 29 (8.4-100)microM and 13 (1.9-8.4)microM and for the left atria, 89 (15-510)nM, 25 (9.9-64)microM and 18 (7.7-42)microM for isoprenaline (8 h incubation), terbutaline and fenoterol respectively.(ABSTRACT TRUNCATED AT 250 WORDS)