Effect of vasopressin on toad bladder under conditions of zero net sodium transport

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Abstract
To determine whether vasopressin stimvilates transepithelial Na transport in toad bladder by affecting "passive" or "active" processes, the effects of the hormone were examined under conditions of zero net Na transport. A Ringer solution constituted the serosal medium, with the mucosal medium a NaCl solution. The latter was diluted with isoosmotic sucrose progressively decreasing the transport of NaCl. In the absence of parallel Na pathways, the reduction of the electrochemical potential gradient for Cl- to zero indicates zero net flow for Na as well as for Cl-. In this case, the net Na flux would be zero, both across the entire bladder and separately across the apical and basal barriers of the mucosal cells. Hence, the driving force for Na would vanish across any passive barrier. The addition of vasopressin increased the transepithelial electrical potential, but the magnitude of the response was inversely dependent on the initial electrical potential. The results suggest: an action of vasopressin only on permeability barriers to Na, a maximal electrochemical gradient equivalent to 170 mv against which Na can be actively transported, and the existence of heterogeneous parallel transepithelial pathways for Na.