ACh-induced depolarization (D response) in D cells markedly decreases as the external Na + is reduced. However, when Na + is completely replaced with Mg ++, the D response remains unchanged. When Na + is replaced with Tris(hydroxymethyl)aminomethane, the D response completely disappears, except for a slight decrease in membrane resistance. ACh-induced hyperpolarization (H response) in H cells is markedly depressed as the external CI-is reduced. Frequently, the reversal of the H response; i.e., depolarization, is observed during perfusion with Cl--free media. In cells which show both D and H responsessuperimposed, it was possible to separate these responses from each other by perfusing the cells with either Na+-free or Cl--free Ringer's solution. High [K +] 0 often caused a marked hyperpolarization in either D or H cells. This is due to the primary effect of high [K +] 0 on the presynaptic inhibitory fibers. The removal of this inhibitory afferent interference by applying Nembutal readily disclosed the predicted K + depolarization. In perfusates containing normal [Na+] 0, the effects of Ca ++ and Mg++ on the activities of postsynaptic membrane were minimal, supporting the current theory that the effects of these ions on the synaptic transmission are mainly presynaptic. The possible mechanism of the hyperpolarization produced by simultaneous perfusion with both high [K +] o and ACh in certain H cells is explained quantitatively under the assumption that ACh induces exclusively an increase in C1-permeability of the H membrane. I N T R O D U C T I O NT h e ionic permeability changes during synaptic excitation at the neuromuscular junction of the frog were first considered by F a t t and K a t z (20) a n d del Castillo and K a t z (14). T h e y inferred from the conductance changes which they observed that the excitatory transmitter m a y cause the subsynaptic m e mbrane to become nonspecifically permeable not only to Na + but also to K +

The hormone-induced differentiation of an active Na+ transport across larval bullfrog skin during metamorphosis was investigated in vitro and in vivo. In in vitro experiments, EDTA-treated larval dorsal skin from which apical cells were removed was used. Even in the absence of thyroid hormone, corticoids induced the differentiation. Although aldosterone was the most potent hormone, hydrocortisone or corticosterone was also effective. Prolactin inhibited the corticoid-induced differentiation. The differentiation of the transport system coincided almost exactly with the appearance of adult features of the epidermis, namely, the epidermis at 7 days carried the human blood group antigen A, a specific molecular marker of adult-type bullfrog epidermis. The transport system appeared to develop in cells that had been newly generated from basal cells. On the contrary, in in vivo experiments, the effect of amiloride on the short-circuit current of the skin of tadpoles raised in the presence of aldosterone was very small, suggesting that a mechanism exists to inhibit the ability of aldosterone to induce the differentiation of the transport system in vivo.

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