Subsequent studies showed no change in cortisol levels in men. In the latter study, subjects had a 15-day pre-bed-rest period during which they were permitted to adapt to the protocol and environment. Similarly, spaceflight studies of humans showed unchanged cortisol levels after an initial adjustment period. Table 1 summarizes the results of studies of male reproduction utilizing ground-based models of simulated hypogravity. In humans, 6° head-down tilt bed rest for 60-120 days altered sperm morphology and reduced the number of active spermatozoa. This finding has important implications because in humans even slight reductions in sperm numbers compromise reproductive capacity.
Ground-based models used to simulate hypogravity have facilitated the interpretation of spaceflight studies. In ground-based simulation models, stress effects can be controlled, which has important implications for research on reproduction because stress responses are known to adversely affect reproductive function. For example, in animals and humans increased cortisol excretion associated with stress can result in reduced serum T levels. Thus, it has been difficult to ascertain whether the reproductive changes are due to stress or to hypogravity associated with spaceflight.
This finding contrasts with those of other reports of reduced T in response to spaceflight. The use of urine samples may provide a more comprehensive analysis of effects on the hypothalamic-pituitary-gonadal (HPG) axis. Clearly, more studies are required before definitive statements can be made regarding effects of spaceflight on male reproductive hormones.
The testes are important organs of the male reproductive system with the dual function of spermatogenesis and steroidogenesis. Testosterone (T), the principal androgen, maintains reproductive organ function and stimulates sperm production in adult males. In most spaceflight studies, reduced testes weight and T levels were reported (Table 1). Reduction in T following spaceflight is not always accompanied by reduced spermatogenesis. Reduction in T levels may not be great enough or the duration may not be long enough to affect spermatogenesis.
The HLS model involves suspending rats by the tail base to produce a 30° head-down position that complements the human 6° head-down tilt utilized in bed-rest studies.
A ground-based model for studying hypergravity is centrifugation. With the use of proper controls for Coriolis effects, centrifugation allows for an infinite number of graded increases in gravitational load and is therefore applicable to a broad range of gravity-related questions. Acute centrifugation is used to mimic the hypergravity associated with launch and landing, whereas chronic centrifugation is used to study the long-term effects of increased gravity on biological systems.
There are many challenges to overcome when conducting spaceflight experiments with mammals. For example, it is difficult to distinguish whether reproductive changes are due to gravity or other conditions aboard the spacecraft, such as increased radiation, noise, isolation, disrupted circadian rhythms, and stress. Responses to decreased gravity on orbit versus increased gravity during landing are difficult to separate because subjects are not immediately recovered after flight.
Mammalian reproduction evolved within Earth’s 1-g gravitational field. Therefore, deviations from Earth’s normal gravity, i.e., hypogravity (forces < 1 g) or hypergravity (forces > 1 g), may compromise reproduction. Some interesting findings are emerging from spaceflight studies. For example, a transient but dramatic reduction in testosterone (T) has been reported during spaceflight in male rats and humans. This observation suggests that fertility may be reduced during spaceflight, because adequate levels of T are required by adult males to maintain reproductive function. Pregnancy during spaceflight has been contraindicated, leading female astronauts to suppress their fertility cycles.