M-receptors in crude membrane preparations of pooled cardiac muscle of ICR mice were estimated by radioligand binding assay with

3

H-QNB as the labeled ligand. Each incubation tube contained 2ml. Na-K phosphate buffer (pH7.4). 0.8mg membrane protein

and various amount of

3

H-QNB. The incubation time was 30min. The bound fraction was filtered onto a glass fiber filter disc (Type 79

manufactured in Shanghai)

0.01

µM

QNB was used to estimate the non-specific binding. Scatchard plot analysis of the data obtained from 4 experiments gave a K

D

value of 0.0918±0.0136n

M

. The maxium binding capacity

the QNB dose of 50% replacement

the association rate constant and dissociation rate constant and the t

1

of dissociation due to QNB were also estimated.