The potential difference across the rectal mucosa during depressive illness and lithium therapy

The potential difference across the rectal mucosa (rectal p.d.) is generated by the active transport of sodium across the mucosa, and it is sensitive to the action of aldosterone. The rectal p.d. values of depressive patients on no treatment, tested whilst depressed or after recovery, were found to be similar to those of control subjects, indicating that sodium transport across the rectal mucosa and the activity of aldosterone were normal in these patients. This contrasts with previous reports of abnormalities of sodium transport and of aldosterone levels in manic-depressive patients. Manic-depressive patients taking lithium carbonate as a prophylactic agent were found to have significantly elevated rectal p.d. values when normothymic. Patients who had become depressed whilst taking lithium, and in whom prophylaxis had therefore failed, were found to have normal rectal p.d. values. Lack of elevation of rectal p.d. in response to lithium administration may be a characteristic of patients who fail to respond to lithium prophylaxis.