Sensitivity (pD
2 value, negative logarithm of the molar concentration producing the half maximum response) and affinity (pK
A value, negative logarithm of dissociation constant) of norepinephrine were determined in 6 arteries from rabbits. A positive correlation was found in the pD
2 and pK
A values of norepinephrine. The slope was not significantly different from 1. The pD
2 and pK
P (a negative logarithm of dissociation constant of a partial agonist) values of tizanidine, an α
1-partial agonist, were also determined. There were positive correlations between the pD
2 and pK
P values of tizanidine and also between the two pD
2 values of norepinephrine and tizanidine. The slopes were not significantly different from 1. These results suggest that the regional differences in pD
2 values of norepinephrine and tizanidine in the arteries are partly due to the affinity and suggest that both drugs interact with one recognition site in the α
1-adrenoceptors. The dissociation constants, KD values, and the maximum binding sites, B
max, for [
125I]-HEAT were also estimated by Scatchard analysis of the specific binding of [
125I]-HEAT to the membrane fractions from rabbit arteries. The KD values for [
125I]-HEAT were also identical. However, B
max varied considerably among rabbit arteries. There was a positive correlation between the logarithm of B
max and the pD
2 values for norepinephrine. The present results suggest that the regional difference in the pD
2 values for norepinephrine in rabbit arteries is due to variations in the affinities to the α
1-adrenoceptors as well as the receptor densities.
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