Hormonal responses of metoclopramide-treated subjects experiencing nausea or emesis during parabolic flight.

Metoclopramide was orally administered (10 or 20 mg) to 22 subjects, 75 min before parabolic flight. Serum levels of ACTH, EPI, NE, and vasopressin (AVP) were unaltered by metoclopramide. AVP and ACTH (1.2 and 36 pg.ml-1) were elevated 77 and 3.8-fold (92.3 and 135 pg.ml-1) following emesis, after 40 parabolas (68.7 and 140 pg.ml-1) and landing (8.7 and 79 pg.ml-1). Seven subjects displaying no nausea and no emesis demonstrated smaller elevations (8.2 and 2.2-fold). Of 15 vomiting subjects, 7 reported no nausea and had lower elevations of AVP with faster recoveries. These findings are consistent with Rowe's suggestion (1979) that nausea may correlate with AVP release. Inhibition of AVP release by fluid shifts during microgravity might account for our findings and astronaut-reported episodes of vomiting without nausea. Elevations in EPI followed emesis or exposure to 40 parabolas whether emesis occurred or not. Only emesis elevated NE (578 to 840 pg.ml-1).