Identification and characterization of presynaptic and postsynaptic beta adrenoreceptors in the longitudinal smooth muscle/myenteric plexus of dog ileum.

The subcellular distribution of [125I]cyanopindolol (125I-CYP) binding in membrane fractions derived from the longitudinal smooth muscle/myenteric plexus (LSM/MP) of dog ileum followed a bimodal pattern with selective enrichments in the purified synaptosomal (S2) fraction and the purified smooth muscle plasma membrane (M2) fraction. The half-maximal saturation of binding sites in both fractions occurred at 10 pM [125I]CYP with maximal binding capacities, in femtomoles per milligram of protein, 70 +/- 10 (S2 fraction) and 110 +/- 25 (M2 fraction). Approximately 80% of binding sites in either membrane fraction exhibited a high affinity for beta-2 selective antagonist ICI 118,551 (Ki = 1.4 +/- 0.4 nM in S2 and 1.8 +/- 0.6 nM in M2; mean +/- S.D. n = 3), whereas remaining 20% of binding sites had a low affinity for this agent (Ki = 2.6 +/- 1.5 microM in S2 and 0.55 +/- 0.2 microM in M2). The beta-1-selective antagonist ICI 89,406 had a high affinity for approximately 20% of binding sites (Ki = 12 +/- 7 nM in S2 and 4 +/- 2 nM in M2) and a low affinity for approximately 80% of binding sites (Ki = 0.34 +/- 0.16 microM in S2 and 0.42 +/- 0.24 microM in M2). The displacement potencies of beta-adrenergic agonists followed the order (-)-isoproterenol greater than salbutamol (-)-epinephrine greater than (-)-norepinephrine in each membrane preparation. The inclusion of 0.1 mM 5'-guanylylimidodiphosphate into assay medium resulted in a decreased affinity of receptors for agonists and increased values of Hill coefficients.(ABSTRACT TRUNCATED AT 250 WORDS)