The isolated gallbladder is a good model system to study basic aspects of the mechanisms of salt transport by epithelia such as renal proximal tubule, small intestine and choroid plexus. In this project, the gallbladder of the salamander Necturus maculosus will be employed, because of its structural and functional simplicity and large cell size, for electrophysiologic and flux studies designed to ascertain the mechanisms of ion transport at the single cell membrane level and the regulation of such processes. Answers to the following main questions will be sought: 1) At the apical (luminal) membrane: (a) whether there is net bicarbonate uptake and what is the mechanism involved, (b) what are the precise mechanisms for the long-term sodium requirement for chloride transport and vice versa, (c) whether there are uphill potassium transport mechanisms, and (d) whether the potassium conductive pathway is voltage-sensitive. 2) At the basolateral membrane: (a) whether chloride transport can be quantitatively accounted for by potassium chloride cotransport, (b) the pharmacologic properties of the cotransporter, (c) the coupling ratio of the sodium pump and its kinetic properties, (d) whether the potassium conductive pathway is voltage-sensitive, and (e) the mechanisms of proton and base transport. 3) Concerning the mechanisms of regulation of ion transport, we will study: (a) the mechanism of the stimulation of NaCl absorption by bicarbonate, (b) the possibility of a role of intracellular calcium on regulation of ion transport at apical and basolateral membranes, and the mechanisms of regulation of the intracellular calcium activity, (c) the role of intracellular pH in the regulation of ion transport at the two cell membranes, and (d) the mechanisms of the effects of cyclic nucleotides on ion transport. 4) Two feasibility studies will also be carried out: (a) we will ascertain whether the recently developed patch-clamp technique can be applied to apical and basolateral membranes, and (b) we will study whether intracellular microelectrode and patch-clamp methods can be employed to study ion transport processes in cultured cells obtained from rabbit kidneys. The results of these investigations are expected to provide unique, significant information on the mechanism of ion transport by gallbladder and other epithelia, which have similar functions. A better understanding of these processes will be useful, in a practical sense, because of the importance of epithelial transport in the regulation of volume and composition of body fluids.
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