The dissociation constant of verapamil estimated from its effect on Ca concentration-tension curves in guinea-pig tracheal muscle.
Concentration-tension curves for calcium ions (Ca2+) were studied in indomethacin-treated guinea-pig tracheal muscle in the presence of different concentrations of carbachol in media containing 5.9 mM K+ or 40 mM K+. The effect of verapamil was investigated taking into account the steepness (the Hill coefficient) of the Ca2+ curve. When carbachol (1 microM) was added to 40 mM K+ solution, the Ca2+ concentration to produce half maximum tension (EC50) was reduced from 0.2 mM to 0.08 mM and the Hill coefficient was increased from 1.4 to 2.0, respectively. In the presence of carbachol (1 microM), the Ca2+ concentration-tension curve was not much influenced by increasing the K+ concentration from 5.9 to 40 mM K+. Verapamil (0.5 microM) shifted the Ca2+ concentration-tension curve to the right in a parallel manner under all experimental conditions, the shift being greater with curves having a smaller Hill coefficient. The dissociation constant of verapamil was not altered by carbachol when estimated from the shift of the curve if the Hill coefficient is taken into consideration. It is concluded that the relatively low susceptibility of carbachol-induced contractions to verapamil in the presence of 40 mM K+, compared with these produced by K+ alone, is not due to a decreased verapamil affinity but to improved Ca(2+)-response coupling.