Applied Physiology

Eur J Appl Physiol (1990) 60:191-193

European Journal of

and Occupational Physiology © Springer-Verlag 1990

The effects of immersion and exercise on prolactin during pregnancy Vern L. Katz 1, Robert McMurray 2, Craig D. Turnbull 3, Michael Berry 2, Chris Bowman 1, Robert C. Cefalo 1 Departments of 1 Obstetrics and Gynecology, Division of Materrnal and Fetal Medicine, of 2 Physical Education, Division of Exercise Physiology, and 3 Biostatistics, University of North Carolina, Chapel Hill, North Carolina, USA Accepted November 22, 1989

Summary. Prolactin is an i m p o r t a n t h o r m o n e during p r e g n a n c y , affecting mother, fetus, a n d a m n i o t i c fluid volume. I m m e r s i o n is k n o w n to affect prolactin levels significantly. To determine the effect o f immersion a n d exercise o n the prolactin response during p r e g n a n c y , we e x a m i n e d s e r u m prolactin levels at 15, 25, a n d 35 weeks' gestation a n d 10 weeks post partum. Twelve wom e n c o m p l e t e d 20 min land rest, 20 min i m m e r s i o n in 3 0 ° C water to the xiphoid, a n d 20 m i n exercise in the water at 60% 17o. . . . . Resting prolactin levels were 1.91+0.32, 4.55_+0.5, a n d 5.85_+0.27 n m o l - 1 - 1 _+ s t a n d a r d error o f the m e a n at 15, 25, a n d 35 weeks' gestation, respectively. P o s t p a r t u m lactating w o m e n h a d a resting m e a n prolactin level o f 3 . 9 5 + 1 . 6 versus 0.22___0.4 n m o l - 1 - 1 in non-lacting w o m e n . Prolactin levels declined significantly during i m m e r s i o n even after correction f o r dilution b y p l a s m a v o l u m e shifts. The i m m e r s i o n response was inversely related to the d u r a t i o n o f p r e g n a n c y with 29%, 22%, a n d 12% d r o p s during 15-, 25- a n d 35-week trials, respectively. C o m p a r e d to rest, exercise prolactin levels r e m a i n e d depressed d u r i n g the 15th a n d 25th week trials~ We h y p o t h esize that i m m e r s i o n in water c a u s e d prolactin levels to decline. Key words: Prolactin - P r e g n a n c y - Exercise - I m m e r sion

Introduction T h i s investigation e x a m i n e d the changes in prolactin levels during p r e g n a n c y a n d the p u e r p e r i u m , during immersion a n d i m m e r s e d exercise. Prolactin is involved in water a n d o s m o r e g u l a t i o n in m o s t vertebrates (Loretz a n d H o w a r d 1982). Prolactin is also involved in amniotic fluid homeostasis in primates (Josimovich et al. Offprint requests to: L. Katz, CB # 7570, 214 MacNider Building, Maternal and Fetal Medicine, Department of Ob-Gyn, University of North Carolina, Chapel Hill, NC 27599-7570, USA

1977; Leontic 1979; Ross et al. 1983). Thus, fluid shifts m a y affect prolactin levels. I m m e r s i o n induces significant intravascular v o l u m e shifts (Epstein 1984). Prolactin has b e e n f o u n d to increase after exercise during p r e g n a n c y ( R a u r a m o et al. 1982). Since i m m e r s i o n a n d i m m e r s e d exercise (including water aerobics a n d swimming) are b e c o m i n g increasingly p o p u l a r m o d e s o f maintaining fitness during p r e g n a n c y , we designed a study to e x a m i n e multiple physiological parameters. Related results o f this study have b e e n r e p o r t e d previously (Katz et al. 1988, 1990; M c M u r r a y et al. 1988, 1988). N o detrimental effects on the mother, fetus, or p r e g n a n c y have been found.

Methods The study was conducted at the University of North Carolina and was approved by the Committee for the Protection of Human Subjects. Informed consent was given by all subjects. Specific details of the method have been published elsewhere (Katz et al. 1988; McMurray et al. 1988). Twelve women were studied at 15, 25, and 35 weeks' gestation and 10 weeks post partum. Specific characteristics of the subjects include: age 23-36 years, mass prepregnancy 50-68 kg, height 1.58-1.76 m. Six women did not exercise regularly, five walked less than 5 km a day, one jogged regularly, and three were nulliparous. Studies were conducted 2 h postprandial and consisted of: 20 min lateral supine rest; 20 min immersion to the xiphoid at 30 ° C; 20 min immersed exercise on a modified ergometer at 60% I?o.... ; and 20 min recovery in the lateral supine position. A submaximal exercise test was performed within a week of each exercise trial to calculate 60% 1Io Conversion from land 60% I?o.... to water 60% I?o.... was .made using the method of Morlock and Dressendorfer (!974). Vo.... was measured every 5 min during the exercise and pedal frequency was adjusted to maintain appropriate exertion levels. Open-circuit spirometry was used for measurements of oxygen uptake. Blood was drawn for analysis after each 20-min period. Prolactin was measured using a radioimmunoassay kit (Serono Diagnostics, Braintree, Mass.). Measurements were made in triplicate and averaged. Intra-assay variation was less than 5%. Sensitivity was 0.64 nmol.1-1. The procedure of Lilliefors (1967) demonstrated that prolactin values were not normally distributed; thus Wilcoxon's signed-rank procedure was used to test for statistical significance (Wilcoxon 1945). Two-tailed tests were used due to the varied directions of . . . .

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192 Table 1. Mean (SEM) and median serum prolactin levels, adjusted for hydration status; results are reported in nmol.1-1

Pregnancy period 15 weeks 25 weeks 35 weeks

Rest

Immersion

Exercise

Recovery

n Mean Median n Mean Median n Mean Median

12 1.91 (0.32) 1.55 12 4.55 (0.5) 4.32 11 5.85 (0.27) 5.91

12 1.36 (0.23) 1.05* 12 3.55 (0.41) 3.50" 10 5.09 (0.23) 5.05*

12 1.50 (0.27) 1.18"* 12 3.91 (0.45) 3.59* 10 5.77 (0.32) 5.73

12 1.27 (0.18) 1.18* 12 3.77 (0.45) 3.27* 10 5.95 (0.55) 5.59

n Mean Median n Mean Median

5 3.95 (1.6) 2.09 7 0.22 (0.4) 0.19

4 2.77 (1.3) 1.23 5 0.20 (0.01) 0.19

4 2.14 (1.0) 0.77 5 0.19 (0.01) 0.18

4 1.64 (0.77) 0.91 5 2.0 (0.01) 0.18

Postpartum

Lactating: Non-lactating:

*P___0.01 (two-sided) for Wilcoxon's statistic when compared to rest ** P = 0.03 (two-sided) for Wilcoxon's statistics when compared to rest

response of prolactin reported by other investigators. The data were analyzed both before and after adjusting for the dilutional effects of plasma volume change. Prolactin level - (% change of plasma volume x prolactin level) plasma volume changes have been reported previously (Katz et al. 1988). This adjustment was performed to ensure that changes in prolactin represented true and not dilutional changes.

Results

Twelve women completed the study, although one did not participate in the 35-week trial for reasons unrelated to the study. Prolactin levels decreased significantly at each gestational age during immersion by 29%, 22%, and 12% at 15, 25, and 35 weeks, respectively. Prolactin remained significantly decreased during exercise at 15 and 25 weeks and post partum but not at 35 weeks. Serum prolactin values adjusted for hydration status are reported in Table 1. Data for the postpartum period should be interpreted with caution because of the small numbers in the two subgroups, lactating and non-lactating. Prolactin levels significantly decreased with immersion, exercise, and recovery in both subgroups.

Discussion

This study showed that prolactin levels were significantly depressed by immersion throughout gestation and post partum. The physiological cause of the prolactin decline may lie within the hypothalamic-pituitary system or the peripheral circulation. Hormones affected by immersion, which may in turn affect prolactin levels, include: angiotensin II, a stimulator o f prolactin secretion, and dopamine, an inhibitor o f prolactin release (Yen 1986; Dufy-Barbe et al. 1982). Dopamine levels have been found to increase in men during im-

mersion (Krishna et al. 1983). The plasma volume expansion of immersion induces a significant decrease in the renin-angiotensin system in pregnant and non-pregnant individuals (Doniec-Ulman et al. 1987; Epstein 1984). In this study, calculated plasma volume increased 4% during immersion. Plasma volume declined 4%, compared to resting levels, during exercise (Katz et al. 1988). After 20 min recovery, plasma volume was similar to resting levels. Although the calculated changes in plasma volume are small, we felt it would be more accurate to adjust for hydration status in evaluating changes in prolactin levels. Prolactin controls osmolarity and the transmembranous flow of water in many vertebrate species, including fish migrating from salt water to fresh water, chicks in the shell, and tadpoles (Loretz and H o w a r d 1982; Yen 1986; Murphy et al. 1986). Although prolactin has not been shown to be important in adult h u m a n osmoregulation, it does appear to be an important factor in fetal osmoregulation. In primates, prolactin acts to maintain homeostasis to protect the amniotic fluid osmolarity. Prolactin binds to the placenta and amnion (McCoshen et al. 1982; Yen 1986; Leontic et al. 1979; Ross et al. 1983; Josimovich et al. 1977) and influences diffusional flow of water across amniotic membranes. We postulate (as have others) that teleologically the maternal organism, in her attempt to maintain physiological control, senses increased intravascular volume and suppresses prolactin secretion (Coruzzi et al. 1988). During immersion, exercise, and recovery, plasma osmolarity did not change despite plasma volume shifts (Katz et al. 1989). Osmolarity may have been maintained during the plasma volume expansion o f immersion through an increase in atrial natiuretic factor, which has been noted in other investigations o f pregnant women (Doniec-Ulman et al. 1987). Alternatively,

193 prolactin may have played a role in maintaining osmolarity. Rauramo et al. (1982) examined prolactin levels in pregnancy after land exercise at approximately 60% Vo. . . . . They found that prolactin increased significantly during the recovery period after exercise. In our study, at 15 and 25 weeks' gestation prolactin levels were lower both during exercise and after the 20-min recovery period. At 35 weeks' gestation, there was no increase in prolactin levels during exercise or during the recovery period. The immersion effect, which enabled our subjects to maintain a plasma volume similar to resting levels during their exercise, may have led to the lack o f an elevation of prolactin levels, in contrast to the findings of Rauramo et al. Alternatively, the immersion effect may have dominated the exercise effect. In non-pregnant women, prolactin does not always increase with exercise (DeMeirleir et al. 1985; Loucks and Horvath 1984; Chang et al. 1986). W o m e n may need to exercise at a critical exertion level, perhaps above the anaerobic threshold, in order to increase prolactin (Chang et al. 1986; DeMeirleir et al. 1985). This study found a decline in prolactin levels during immersion and continued suppression during immersed exercise. The decline in prolactin may be influenced by plasma volume shifts a n d / o r hormones related to those shifts. This would be in keeping with the evolutionary role of prolactin as a volume and osmoregulator in the aquatic or larval stages of vertebrates. Though plasma volume and osmolarity were maintained by physiological controls, the shifts in intravascular fluid may have affected prolactin.

References

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