Temperature Dependence of Vasopressin Action on the Toad Bladder

Toad bladders were challenged with vasopressin at one temperature, fixed on the mucosa with 1% glutaraldehyde, and then subjected to an osmotic gradient at another temperature. Thus, the temperature dependence of vasopressin action on membrane permeability was distinguished from the temperature dependence of osmotic water flux. As the temperature was raised from 20 degrees to 38 degrees C, there was a substantial increase in the velocity of vasopressin action, but osmotic flux was hardly affected. In this range of temperature the apparent energy of activation for net water movement across the bladder amounted to only 1.2 kcal/mole, a value well below the activation energy for bulk water viscosity. It is suggested that osmotic water flux takes place through narrow, nonpolar channels in the membrane. When the temperature was raised from 4 degrees to 20 degrees C, both vasopressin action as well as osmotic water flux were markedly enhanced. Activation energies for net water movement were now 8.5 kcal/mole (4 degrees -9 degrees C) and 4.1 kcal/mole (9 degrees -20 degrees C), indicating that the components of the aqueous channel undergo conformational changes as the temperature is lowered from 20 degrees C. At 43 degrees C bladder reactivity to vasopressin was lost, and irreversible changes in selective permeability were observed. The apparent energy of activation for net water movement across the denatured membrane was 6.6 kcal/mole. Approximately 1 microosmol of NaCl was exchanged for 1 microl of H(2)O across the denatured membrane.