@article {Raffa596, author = {R B Raffa and J F Aceto and R J Tallarida}, title = {Measurement of thermodynamic parameters for norepinephrine contraction of isolated rabbit thoracic aorta.}, volume = {235}, number = {3}, pages = {596--600}, year = {1985}, publisher = {American Society for Pharmacology and Experimental Therapeutics}, abstract = {The thermodynamic quantities of change in free energy (delta G degree{\textquoteright}), change in enthalpy (delta H degree{\textquoteright}) and change in entropy (delta S degree{\textquoteright}) were determined for the interaction of norepinephrine with the alpha-1 adrenoceptor of vascular smooth muscle. Specifically, a standard isolated rabbit thoracic-aorta preparation was used to examine the effect of temperature on norepinephrine-induced isometric tension development. Dissociation constants (KA) for norepinephrine were determined at several temperatures over the range 25-40 degrees C from equiactive concentrations obtained before (A) and after (A{\textquoteright}) partial irreversible receptor blockade by phenoxybenzamine, plotted as 1/A against 1/A{\textquoteright} (KA = (slope-1)/intercept). The values of KA increased with temperature over the range 25-40 degrees C, indicating that the affinity of norepinephrine for the alpha-1 adrenoceptor is an inverse function of temperature over this range. From these results, the thermodynamic quantities delta H degree{\textquoteright} and delta S degree{\textquoteright} were determined from a van{\textquoteright}t Hoff plot of In (KA) against 1/T. The relative magnitudes of the change in enthalpy (delta H degree{\textquoteright} = -25.58 kcal mol-1) and the change in entropy (delta S degree{\textquoteright} = -0.052 kcal mol-1 deg-1) suggest that the reaction between norepinephrine and the alpha-1 adrenoceptor (delta G degree{\textquoteright} = -9.15 kcal mol-1) is enthalpy driven, which is consistent with radioligand binding studies of other adrenoceptor subtypes.(ABSTRACT TRUNCATED AT 250 WORDS)}, issn = {0022-3565}, URL = {https://jpet.aspetjournals.org/content/235/3/596}, eprint = {https://jpet.aspetjournals.org/content/235/3/596.full.pdf}, journal = {Journal of Pharmacology and Experimental Therapeutics} }