Permanent depression of plasma cGMP during long-term space flight

The purpose of this study was to investigate plasma concentrations of cycli c guanosine monophosphate (cGMP) and atrial natriuretic peptide (ANP) durin g and after real and simulated space flight. Venous blood was obtained 3 mi n after the beginning and 2 min after the lower body negative pressure mane uver in two cosmonauts preflight (supine), inflight, and postflight (supine ) and in five other subjects before, at the end, and 4 days after a 5-day h ead-down tilt (-6 degrees) bed rest. In cosmonaut 1 (10 days in space), pla sma cGMP fell from preflight 4.3 to 1.4 nM on flight day 6, and was 3.0 nM on the fourth day after landing. In cosmonaut 2 (438 days in space), it fel l from preflight 4.9 to 0.5 nM on on flight day 3, and stayed <0.1 nM with 5, 9, and 14 months in space, as well as on the fourth day after landing. T hree months after the flight his plasma cGMP was back to normal (6.3 nM). C osmonaut 2 also displayed relatively low inflight ANP values but returned t o preflight level immediately after landing. In a ground-based simulation o n five other persons, supine plasma cGMP was reduced by an average of 30 % within 5 days of 6<degrees> head-down tilt bed rest. The data consistently demonstrate lowered plasma cGMP with real and simulated weightlessness, and a complete disappearance of cGMP from plasma during, and shortly after lon g-duration space flight.