Measurement of thermodynamic parameters for norepinephrine contraction of isolated rabbit thoracic aorta
Raffa, R.B.; Aceto, J.F.; Tallarida, R.J.
Journal of Pharmacology and Experimental Therapeutics 235(3): 596-600
1985
ISSN/ISBN: 0022-3565 PMID: 3001269 Document Number: 249956
The thermodynamic quantities of change in free energy (.DELTA.G.degree.'), change in enthalpy (.DELTA.H.degree.') and change in entropy (.DELTA.S.degree.') were determined for the interaction of norepinephrine with the alpha-1 adrenoceptor of vascular smooth msucle. 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.degree. C from equiactive concentrations obtained before (A) and after (A') partial irreversible receptor blockade by phenoxybenzamine, plotted as 1/A against 1/A against 1/A' (KA = (slope-1)/intercept). The values of KA increased with temperature over the range 25-40.degree. 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.' and .DELTA.S.degree.' were determined from a van't Hoff plot of ln (KA) against 1/T. The relative magnitudes of the change in enthalpy (.DELTA.H.degree.' = -25.58 kcal mol-1) and the change in entropy (.DELTA.S.degree.' = -0.052 kcal mol-1 deg-1) suggest that the reaction between norepinephrine and the alpha-1 adrenoceptor (.DELTA.G.degree.') = -9.15 kcal mol-1) is enthalpy driven, which is consistent with radioligand binding studies of other adrenoceptor subtypes. It is concluded that alpha-1 adrenoceptors in vascular smooth muscle have higher affinities for norepinephrine at lower temperatures than at more physiological temperatures and that the thermodynamic changes are consistent with the concept that the interaction between norepinephrine and alpha-1 adrenoceptors possibly involves a conformational transition of the receptor-ligand complex. The close agreement between thermodynamic quantities obtained here and in ligand binding studies suggests that such fundamental thermodynamic quantities may be a common ground for comparison of the two methods.