Plasma renin activity and levodopa in Parkinson's disease.

The effect of posture and levodopa therapy on plasma renin activity (PRA) was studied in patients with Parkinson's disease. The PRA was measured with the patient supine and upright before and during levodopa administration. Prior to therapy the mean values with the patients supine and upright were significantly lower than those of normal subjects. While they were receiving levodopa, the mean PRA when the patients were supine remained low while the mean value when the patients were upright became normal. There was an exaggerated response of PRA to upright posture with levodopa therapy. Possible relationship of these findings to the underlying mechanisms of Parkinson's disease and its treatment are discussed.