Abstract
Stimulation of left atrial receptors causes a diuresis partly through a blood borne agent. Malpighian tubules of Rhodnius prolixus can detect a blood borne agent during the diuresis. It has been suggested that the agent is ADH but this is in dispute. In the present investigation, vasopressin (1-300 mu i.u. cm-3) was found to have no effect on Malpighian tubule secretion when added to plasma, Rhodnius Ringer and Rhodnius Ringer + 5-HT. In addition, it was shown that incubation with sodium thioglycollate of plasma samples obtained from anaesthetized dogs, before and during stimulation of atrial receptors by distension of a balloon in the lumen of the left atrium, did not abolish the differences between test and control plasma samples detected by the Malpighian tubules. It is concluded that differences in plasma detected by the Malpighian tubules and related to the diuresis which results from left atrial stimulation are not attributable to changes in concentration of ADH.
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