1. A method is described for altering the ionic composition of the alveolar (residual) milk in the goat mammary gland. Transepithelial (blood-milk) potential difference (PD) was recorded before and after composition was changed. 2. When [Na+] and [K+] were varied while [Na+ + K+] and [Cl-] were maintained constant, there were no significant changes in PD. 3. When [Na+] was varied, with [K+] and [Cl-] constant, there was a linear relation between PD and log [Na+], with a change of 18 mV per tenfold change in concentration. 4. When [K+] was varied, with [Na+] and [Cl-] constant, there was a linear relation between PD and log [K+], with a change of 24 mV per tenfold change. 5. In view of the lack of discrimination between Na+ and K+ in the first experiment, the sum of [Na+] and [K+] was considered. In the experiments in which [Na+] or [K+] were varied, there was a change of 37 and 38 mV, respectively, per tenfold change in [Na+ + K+]. The quantitative effects on PD of changing [Na+] or [K+] when expressed as changes in [Na+ + K+] were not significantly different. 6. When [Cl-] was varied, with [Na+] and [K+] constant, there was a linear relation between PD and log [Cl-], with a change of 22 mV per tenfold change. 7. The results are discussed in relation to the types of ion channel that might be present in the apical membrane of the secretory cell and in relation to hypotheses on the mechanisms of ion transport in the mammary gland.(ABSTRACT TRUNCATED AT 250 WORDS)
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