Activation of chloride conductance in pig jejunal brush border vesicles

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Abstract
Requirements for the activation of Cl conductance have been investigated in pig jejunal brush border vesicles. The stability of ATP as a substrate for protein kinase activity, the stability of the phosphoprotein product of protein kinase action, and the choice of buffer system used for vesicle preparation were studied as variables which affected the outcome of in vitro activation attempts. Arsenate was selected as the most effective agent in protecting ATP from hydrolysis by the phosphatase activity in this vesicle system. Brush border vesicle protein appeared to prevent the accumulation of phosphoprotein in a cAMP-dependent protein kinase reaction, and vesicle protein only had phosphate acceptor activity when KF was added as a presumptive inhibitor of phosphoprotein phosphatase. A Cl conductance response to a potassium gradient and valinomycin was present in vesicles prepared in buffers containing tetramethylammonium. Cl− conductance activity was not increased in this system by the addition of ATP, dibutyryl cyclic AMP, and cyclic AMP-dependent protein kinase. There was no Cl conductance response to a potassium gradient in vesicles buffered with imidazolium-acetate. Incorporation of ATP, AsO 4 3− , and F− into these nonconductive vesicles by homogenization, followed by addition of dibutyryl cAMP, produced substantial conductance activity. Maximal activation of Cl− conductance was obtained with vesicles prepared in imidazolium-acetate buffering, using precautions to stabilize ATP and phosphoprotein prior to conductance measurements.