Plasma samples were obtained after centrifugation of trunk blood collected at sacrifice of six flight and six ground control rats. A follow-up experiment with a group of sixteen rats was performed to test whether halothane anesthesia or the decapitation procedure affected renin release in either the flight or control rats. Plasma Renin Concentrations (PRC) were estimated by measuring the conversion of rat renin substrate to angiotensin I using radioimmunoassay.

Results:

No significant differences were found in PRC between flight and control animals, although, the flight mean was lower. Neither halothane anesthesia nor decapitation produced a significant increase in PRC above the levels measured in conscious control rats. Due to the twelve-hour delay between re-entry and sampling, all renin measured was probably secreted in the postflight period, so the PRC of flight rats do not indicate levels during flight, but only the levels obtained by flight animals during the twelve-hour recovery period. In summary, it was predicted that under conditions of microgravity, the tonic secretion of atriopeptin III may cause contraction of the extracellular fluid compartment by its diuretic and natriuretic actions. In addition, through its ability to inhibit renin secretion and interfere with the stimulation of aldosterone secretion, atriopeptin III may concomitantly depress the opposing influences of the renin-angiotensin-aldosterone that defend the extracellular fluid compartment.