Comparative Biochemistry and Physiology Part C 134 (2003) 385–395 Circadian variations of plasma renin activity (PRA), aldosterone and electrolyte concentrations in plasma in pregnant and non-pregnant goats Ewa Skotnicka* Department of Animal Physiology, Agricultural University of Szczecin, Poland Received 19 August 2002; received in revised form 11 January 2003; accepted 13 January 2003 Abstract The aim of this study was to estimate and analyse circadian variations of the renin–angiotensin–aldosterone system (RAA) activity in blood of goats and the influence of late pregnancy on the circadian variations of RAA system. The study was carried out on a group of 17 non-pregnant and 9 pregnant goats. The animals were kept in uniform environmental conditions, (9 h lighty15 h darkness). Blood samples were collected seven times over a period of 24 h, every 4 h. Plasma renin activity (PRA), plasma aldosterone (PA), sodium, potassium and chloride concentrations were determined. PRA and PA of both groups changed during 24 h, with the highest values in the dark phase and with higher RAA system activity (especially during the night) in the pregnant goats. In the non-pregnant goats, no circadian changes in PRA and PA were observed. The circadian changes in PRA and PA found in pregnant goats had acrophases at 06:27 h and 01:13 h, respectively. Plasma electrolyte concentrations in both groups of goats also changed during 24 h. These results suggest that circadian changes of potassium concentration in plasma of goats during late pregnancy may be one of the main factors affecting the RAA system. 䊚 2003 Elsevier Science Inc. All rights reserved. Keywords: Biological rhythms; Circadian variations; Goats; Plasma aldosterone concentration; Plasma electrolyte concentrations; PRA; Pregnant; RAA system 1. Introduction Circadian variations are changes in the intensity of physiological or biochemical processes over approximately 24 h (approx. 1 solar day) (Mick and Jouvet, 1994; Miller, 1993). The length of the cycles and the timing of particular phases may be Abbreviations: A I, angiotensin I; A II, angiotensin II; JG, renal juxtaglomerular; PA, plasma aldosterone; PRA, plasma renin activity; RAA, renin–angiotensin–aldosterone system *Corresponding author. Present address: University of Szczecin, Faculty of Natural Sciences, Department of Biochemistry, 3a Felczaka St., 71-412 Szczecin, Poland. Tel.: q 48-91-444-1550; fax: q48-91-444-1550. E-mail address: [email protected] (E. Skotnicka). modified by the changes in daylight and in alternating phases of day-time activity and sleep of animals (Miller, 1993). Circadian variation has been observed in many physiological variables. Renal excretion of salt and water in various species of mammals is known to show clear patterns over the 24-h cycle (Bultasova et al., 1986; Muszczynski et al., 1996; Stoynev et al., 1982; Voogel et al., 2001). One might also expect to find parallel variations in the secretory or plasma concentration pattern of the hormones controlling water–electrolyte metabolism. Among these hormones, the renin–angiotensin–aldosterone (RAA) system has a well-established role in salt and water excretion, the volume of water 1532-0456/03/$ - see front matter 䊚 2003 Elsevier Science Inc. All rights reserved. PII: S 1 5 3 2 - 0 4 5 6 Ž 0 3 . 0 0 0 0 7 - 3 386 E. Skotnicka / Comparative Biochemistry and Physiology Part C 134 (2003) 385–395 spaces and acid–base balance (Goldfarb and Novich, 1994; Phillips et al., 1993; Murlow, 1999). The activity of the RAA system is controlled by protease renin produced in the renal juxtaglomerular (JG) cells (Della Bruna et al., 1996). Renin interacts with angiotensinogen in the circulation to form angiotensin I (Ang I)—a decapeptide with the low biological activity (Goldfarb and Novich, 1994). After removal of dipeptide His–Leu from Ang I in the lung by the action of kinase II (EC 188.8.131.52), i.e. angiotensin-converting enzyme (ACE), angiotensin II (Ang II-octapeptide) is formed (Erdos and Skidgel, 1990). Ang II is then transported in the circulation to effector target sites (blood vessels, kidney, adrenal gland), where it interacts with specific receptors to exert its action (Goldfarb and Novich, 1994), among other things playing an important role in aldosteronogenesis. Ang II interacts with the AT1 receptors type in adrenal zona glomerulosa cells, which increases synthesis and release of aldosterone (Bottari et al., 1993). Aldosterone in turn, acts mainly on the nephrone proximal tubules (Mutos, 1995), leading an increase in sodium reabsorption and the excretion of potassium and hydrogen into the tubule (Mutos, 1995; Todd-Turla et al., 1993). Water-electrolyte metabolism undergoes significant modifications in pregnancy (Chapman et al., 1999; Koehler, 1993; Sturgiss et al., 1996). It is connected with the different activities of regulation systems and the change in intensity of metabolic processes (Sturgiss et al., 1996; Chapman et al., 1999). We may assume that the RAA system activity also undergoes certain changes in pregnancy. Circadian changes in the activity of the RAA system are not well known, especially during pregnancy. Except for reports in humans and rodents, these changes have not been established in mammals (Chiang et al., 1994; Kawasaki et al., 1990; Lemmer et al., 2000; Voogel et al., 2001). The circadian variation in the activity of the RAA system is poorly known in ruminants, especially in goats (quadruped animals with a permanent horizontal posture). The aim of the present study was to estimate and analyse circadian variations in plasma renin activity (PRA) and in plasma aldosterone (PA)— parameters of the circulating renin–angiotensin– aldosterone (RAA) system activity in goats—and whether these findings are similar to those previously reported in other species. Moreover, the influence of late pregnancy on the circadian variations of RAA system was examined. 2. Materials and methods 2.1. Animals The study was carried out in January and February. The studies were performed on 26 healthy goats (75% of the Polish White Breed) aged 2–3 years. The animals were divided into two study groups, depending on their physiological state. First group (ns17), non-pregnant, non-lactating goats; and second group (ns9), goats in late pregnancy, 2 weeks before parturition, first parturition. The animals of both groups were kept under controlled environmental conditions (temperature and lighting). The study was carried out in light conditions natural for the season of the year: 9 h lighty15 h darkness with lights off from 07.30 to 16.30 h. The period from 07.30 to 16.30 h will be referred to as the day phase and that from 16.30 to 07.30 h as the light phase. For 10 days prior to the studies and also during the studies, the animals of particular study groups were kept in two separate parts of the room. They were fed according to the standards (barley grain, 600 gyday; beetroot pulp, 400 gyday; water, hay and salt-lick to taste). The dry food was given at 09.00 h. Before the experiment began, the external jugular vein of animals was catheterized to enable blood sampling without stress. 2.2. Analytical procedure Blood was collected from the external jugular vein, seven times in 24 h (at 16.00, 20.00, 24.00, 04.00, 08.00, 12.00 and 16.00 h). Then, the scheme was repeated. While collecting blood in the dark phase, a red spotlight was used (max. wavelengths600 nm, 3 lux). Blood samples (5 ml) were collected into tubes containing heparine (250 I.U. Heparine, Heparinum Jelfa, Poland, to plasma electrolyte analysis) or EDTA (2 mgyml of blood, to PRA and plasma aldosterone concentrations analysis), depending on the requirements of the analytical methods. The samples centrifuged at approximately 2000=g for 30 min in a refrigerated centrifuge to recover the plasma. Plasma samples were stored at a temperature of y20 8C until analysis. E. Skotnicka / Comparative Biochemistry and Physiology Part C 134 (2003) 385–395 2.3. Biochemical procedure Plasma renin activity (PRA) was determined by RIA Kit (code 12964, BIODATA Diagnostics S.p.A., Italy) for angiotensin I generation. The sensitivity of the PRA assay was 0.15 ngymlyh and intra- and interassay coefficients of variations were 7.8% and 8.2%, respectively. Aldosterone was measured by RIA Kit (code 12254, BIODATA Diagnostics S.p.A, Italy), with intra- and interassay coefficients of variation of 5.4% and 6.6%, respectively. The sensitivity of the PRA assay was 6 pgyml. Plasma sodium and potassium concentrations were measured by flame photometry using a Flapho-40 photometer. Plasma chloride concentration was determined by potentiometric titration using a Spexon-100 Chlorimeter. 2.4. Statistical analysis The mean values and standard deviations were calculated. The statistical analysis (in STATISTICA for Windows PL v.5.1) was based on the model for the assay with repeated measurements. The calculation for the plasma renin activity and aldosterone concentration were performed on the transformed data w(ypslog (yq2)x. Also, the variation analysis model with the repeatability of measurements was used. Two groups of goats (pregnant and non-pregnant) and the time of the measurement as the repeatable factor were established. The main effects (the group of goats and the time of measurement) and the time of interaction were tested. Also, the differences between the mean values of the non-pregnant and pregnant animals and the differences among the measurements within the groups were analysed. In order to confirm the existence of circadian variations and to determine acrophases and rhythm periods, all the time series were analysed in CHRONOS v.1.0, a program used in an analysis of chronobiological experiments, in order to determine acrophases and rhythm period. Acrophase (⭋) is phase equal maximal function value in relation to midnight (ts00.00 h); rhythm period (T) is the length of time between the two subsequent maximal values of a given function (in a circadian variations Ts 24"3 h). All the results were presented in tables (Tables 1 and 2) and graphically (Fig. 1). 3. Results In the non-pregnant group PRA ranged between 0.75 and 0.94 ngymlyh, reaching the highest val- 387 ues in the dark phase of the photoperiod (Table 1, Fig. 1a). The difference between the maximum (08.00 h) and the minimum value (20.00 h) was statistically significant (PF0.01). PRA of the nonpregnant goats was not characteristic of circadian variation. PRA of the pregnant goats varied during the 24 h period and ranged from 0.96 to 1.21 ngy mlyh) (Table 2, Fig. 1a). The highest PRA was observed in the second half of the dark phase and in the beginning of the light phase of the photoperiod (24.00, 04.00 and 08.00 h). The lowest value was observed during the day and in the beginning of the dark phase (12.00, 16.00 and 20.00 h). Significant differences were determined between the PRA measured at 04.00 and 08.00 h, and the levels at 12.00 (PF0.05), 16.00 (PF0.01) and 20.00 h (PF0.05). PRA in the pregnant goats was characteristic of circadian variations, with the period of the rhythm 25.55 h and the acrophase at 06.27 h, in contrast with the changes during the 24 h period in PRA of the non-pregnant goats. PRA of the pregnant goats was as also higher at all measurement times, compared to PRA of the non-pregnant goats (Fig. 1a). The PA concentration in the non-pregnant goats ranged from 21.40 pgyml (12.00 h) to 28.79 pgy ml (24.00 h) (Table 1, Fig. 1b). PA concentration of the non-pregnant goats had not characteristic of circadian variation. The PA concentration in pregnant goats during the 24 h ranged from 21.06 to 38.25 pgyml, increased during the night, reaching its maximum at 04.00 h, and decreased during the day, reaching its minimum at 12.00 h. The observed changes were statistically significant (PF0.05) and were characteristic of circadian variations (Table 2, Fig. 1b). The period of the rhythm was 22.53 h and the acrophase was at 01.13 h. Moreover, the concentration of PA in the pregnant goats was higher at night in comparison with the non-pregnant goats (04.00 h, PF0.01). During the day, however, the concentration of that hormone was similar in both groups (Fig. 1b). The concentration of sodium in plasma of the non-pregnant goats ranged from 136.27 to 140.18 mmolyl (Table 1). The highest concentration of this electrolyte was observed at 20.00 h, the lowest at 12.00 h. The differences were statistically significant (PF0.01). In plasma of the pregnant goats the concentration of sodium ranged from 130.11 to 137.44 mmolyl (Table 2). The highest concentration of this electrolyte was observed between 12.00 and 16.00 h. The differences in the values 388 Time 16:00 A 20:00 B 24:00 C 4:00 D 8:00 E 12:00 F 16:00 G Significance level PF0.05 PRA 0.84"0.48 0.75"0.32 0.86"0.37 0.92"0.35 0.94"0.38 0.75"0.32 0.87"0.35 (ngymlyh) Aldosterone 26.32"16.04 24.58"9.68 28.79"12.42 24.78"10.62 23.15"9.60 21.40"6.93 24.27"9.99 – (pgyml) Na (mmolyl) 138.38"4.36 140.18"3.99 138.09"4.92 138.53"2.90 138.88"4.61 136.27"2.70 137.03"1.41 B™A,C D™F,G K (mmolyl) 3.52"0.25 3.68"0.31 3.64"0.42 3.67"0.36 3.67"0.28 3.71"0.38 3.60"0.37 A™B,D,E,F Cl (mmolyl) 105.65"2.94 107.03"3.92 106.00"4.42 108.91"5.34 107.86"3.25 106.56"3.24 106.00"2.91 A™B,D,E D™C,G Bold text represents the night period. * B™D, E significant differences between PRA at 20:00 (B) and PRA at 4:00 (D), 8:00 (E) at the significance level PF0.01. Significance Rhythm Acrophase level period (T) (Ø) PF0.01 Hours Time B™D,E* 14.01 04.36 – 28.55 22.48 B™F,G 24.73 22.04 – – 57.41 23.38 06.04 04.58 E. Skotnicka / Comparative Biochemistry and Physiology Part C 134 (2003) 385–395 Table 1 Circadian variations of the plasma renin activity (PRA), aldosterone concentration, concentrations of sodium, potassium and chlorides in plasma of the non-pregnant goats (ns 17). Mean values (x), mean standard deviations ("S.D.), statistical significance of differences between the mean values at particular measurement times (A, B, C, D, E, F, G), periods and acrophases of rhythms Time 16:00 A 20:00 B 24:00 C 4:00 D 8:00 E 12:00 F 16:00 G PRA 1.02"0.42 0.96"0.34 1.15"0.39 1.21"0.38 1.18"0.42 1.03"0.38 1.05"0.45 (ngymlyh) Aldosterone 25.14"12.15 30.00"14.85 37.23"14.13 38.25"22.11 26.72"12.56 21.06"12.81 25.63"12.96 (pgyml) Na (mmolyl) 130.67"5.87 131.44"5.43 130.11"5.27 131.44"3.83 131.44"5.70 135.78"2.35 137.44"3.04 K (mmolyl) 3.52"0.36 Cl (mmolyl) 105.78"5.44 3.73"0.23 106.89"4.67 Bold text represents the night period. 3.82"0.28 109.22"3.74 3.70"0.13 105.72"5.70 3.57"0.54 104.00"4.39 3.57"0.40 105.17"3.97 3.66"0.25 107.83"4.51 Significance Significance Rhythm Acrophase level level period (⭋) PF0.05 PF0.01 T) Time Hours B™D,E F™D,E C™B,E D™A,E,G A,D,E™F,G B™G A™D,E 25.55 06.27 C™A,F,G F™B,D B™F C™F,G A™C C™F E™G 22.53 01.13 70.54 09.09 22.42 19.90 00.08 22.53 E. Skotnicka / Comparative Biochemistry and Physiology Part C 134 (2003) 385–395 Table 2 Circadian variations of the plasma renin activity (PRA), aldosterone concentration, concentrations of sodium, potassium and chlorides in plasma of the pregnant goats (ns9). Mean values (x), mean standard deviations ("S.D.), statistical significance of differences between the mean values at particular measurement times (A, B, C, D, E, F and G), periods and acrophases of rhythms 389 390 E. Skotnicka / Comparative Biochemistry and Physiology Part C 134 (2003) 385–395 Fig. 1. Circadian variations of PRA (a), aldosterone (b) and Kq (c) concentrations in blood plasma in pregnant (ns9) and non-pregnant (ns17) goats. Values are hourly means ("S.D.); (b) *significance of difference (PF0.05) between the mean values of plasma aldosterone concentration in both groups at the particular time. E. Skotnicka / Comparative Biochemistry and Physiology Part C 134 (2003) 385–395 were significant in comparison with the concentration of sodium at other times, but they did not have the features of a circadian variation. The differences between the Naq concentrations in plasma of both groups were statistically significant (PF0.01). The concentration of potassium in plasma of the non-pregnant goats was relatively stable during the whole time of the present study, ranging from 3.52 to 3.71 mmolyl (Table 1, Fig. 1c). The observed differences were not significant and they did not have the character of the circadian variations. The Kq concentration in plasma of the pregnant goats during the 24 h ranged from 3.52 to 3.82 mmoly l, with higher values at night and lower during the day (Table 2, Fig. 1c). The significant differences in concentration of this electrolyte (PF0.01) were observed between 16.00 and 24.00 h. The changes in the concentration of potassium in plasma of the pregnant group had the character of the circadian variations, with the period of the rhythm at 22.42 h and the acrophase at 00.08 h. The concentration of chlorides in plasma of the non-pregnant goats was relatively stable (Table 1). An increase in concentration of that electrolyte was observed between 04.00 and 08.00 h, with the maximum value (108.91 mmolyl) at 04.00 h. The lowest levels were observed at 16.00 h (105.65 mmolyl). The observed differences were significant (PF0.05) and they had the character of the circadian variations (the period of rhythm was 23.38 h and the acrophase at 04.58 h). The concentration of chlorides in plasma of the pregnant goats, during the 24 h, was the lowest between 04.00 and 12.00 h, with the minimum value at 08.00 h (104.00 mmolyl) (Table 2). The maximum value was observed at 24.00 h (109.22 mmolyl). The difference between those levels was statistically significant (PF0.01). The period of the rhythm was 19.90 h with the acrophase at 22.53 h. 4. Discussion In the present study, changes of the RAA system activity and of the concentration of electrolytes (Naq, Kq and Cly) in plasma during the 24 h period were found in both groups of goats. PRA in the non-pregnant goats changed significantly during the 24 h period. It increased at night, and decreased during the day, with the lowest value in the beginning of the dark phase. 391 The observed changes were not characteristic of circadian variations. The increase in PRA could be linked with the animals’ rest and the decrease in the activity of the sympathetic–adrenal medullary system in the dark phase. It is proved by the studies performed on man by Branderberger et al. (1998), Eguchi et al. (2002) and Lapinski et al. (1993) and Van Acker et al. (1993). These authors show that PRA increases at night andyor when the motoric activity phase decreases. The nocturnal oscillations in the sympathetic nervous system and decrease in blood pressure may also play a certain role in 24-h PRA variations (Branderberger et al., 1998; Sica, 1999; Van Acker et al., 1993). The results of Branderberger et al. (1998) also demonstrate that the 24-h PRA variations are not endogenous by nature, but are related to sleep processes which create the nycthemeral rhythm by increasing both the frequency and the amplitude of the oscillations. The PA concentration in the non-pregnant goats during the 24-h period was relatively stable with greater tendency at the time of rest. It was also found by the studies in humans (Bernardi et al., 1985; Janssen et al., 1992; Lapinski et al., 1993). Tendencies in changes of both PRA and PA concentration in the non-pregnant goats during the 24h period were similar. Both of the hormone concentrations in the non-pregnant goats did not have the character of the circadian variations. Circadian variation in both PRA and PA concentration has, however, occasionally been reported in humans (Chiang et al., 1994; Cugini et al., 1992; Kawasaki et al., 1990; Voogel et al., 2001) and in rats (Lemmer et al., 2000). The varying results obtained for humans in both PRA and PA concentration during the 24-h period have been explained by different methodological approaches, for example by differences in posture and activity during the experiments. Postural changes, such as head-down tilt and supine position, increase both PRA and PA concentrations in diurnally active subjects, following normal day activities and sleeping in supine position (Branderberger et al., 1998; Eguchi et al., 2002; Lapinski et al., 1993; Van Acker et al., 1993). In contrast, Chiang et al. (1994), Koopman et al. (1989) and Voogel et al. (2001) observed conversion of a daily pattern of PRA and PA concentration in persons who remained in recumbet position for 24 h. The goat represents a species that lives in a horizontal posture, and similar to other ruminants, lies down 392 E. Skotnicka / Comparative Biochemistry and Physiology Part C 134 (2003) 385–395 in sternal recumbency to prevent aspiration of regurgitated rumen contents (Kokkonen et al., 2001). The results of the present study point to absence of circadian variation in both PRA and PA concentration in non-pregnant goats, supporting the view that a sufficient stimulus is needed for renin and aldosterone release. This stimulus could be postural changes (Branderberger et al., 1998; Lapinski et al., 1993; Van Acker et al., 1993), which did not seem to occur in the goats which were posturally and physically inactive. In the present study both, PRA and PA concentration in pregnant goats also changed during the 24-h period. Moreover, the observed changes had the character of circadian variations. The pattern of PRA changes was relatively similar to the changes in plasma potassium concentrations and it is therefore possible that PRA circadian variations were the result of the rhythm in the plasma Kq concentration determining the secretion activity of the juxtaglomerular apparatus in the pregnant goats. The results of the present study suggest, moreover, that in late pregnancy in goats the circadian variation in the plasma Kq concentration may by one of the main factors stimulating aldosteronogenesis andyor secretion of aldosterone. Thomsen and Shalmi (1997) and Funder (1996) observed in humans the effect of an increase in the plasma Kq concentration on the secretion of aldosterone. Bernardi et al. (1985) suggest that the circadian variations in the concentration of this electrolyte in human blood affect not only the secretion of aldosterone but also secretion activity of the macula densa, contributing to the increase in PRA. The influence of natremia also cannot be excluded, but Weir et al. (1975) suggest that in human pregnancy the decrease in blood Naq concentration is not the main factor in the secretion of the aldosterone. PRA in the pregnant goats observed in our study was at each time point higher in comparison with PRA in the non-pregnant goats. The increased synthesis and secretion of renin in the pregnant goats could be facilitated by a lower concentration of sodium in plasma in pregnant goats. Many authors suggest that there may be also other factors responsible for higher PRA in pregnancy, as the fetus prorenin andyor renin placental transfer (Kalenga et al., 1991), estrogens (Chapman et al., 1999; Sealey et al., 1994) and prostaglandins (Chu and Beilin, 1993). It could be also due to the decrease in the activity of adrenal receptors of angiotensin II during pregnancy (Brooks and Keil, 1994; Chapman et al., 1999; Weir et al., 1975). PA concentration in the pregnant goats was higher (especially in the dark phase) in comparison with the non-pregnant goats. The higher PA concentrations in pregnant sheep (Keller-Wood, 1995), in pregnant rats (Brooks and Keil, 1994), in pregnant guinea pigs (Kalenga et al., 1991), in humans pregnancy (Chapman et al., 1999; Weir et al., 1975) were also observed. The results of the present study suggest that goats have a relatively low level of the PA concentration compared to other species. The higher values in PA concentration were observed in the non-pregnant cows, mares, guinea pigs and mice (Bardwel et al., 1978) and in humans (Bernardi et al., 1985; Chiang et al., 1994; Janssen et al., 1992; Lapinski et al., 1993; Steele et al., 1994). In the present study, the RAA system activity in the pregnant goats was higher in comparison with the non-pregnant goats. It was also observed by other authors in pregnant sheep (Gibson and Lumbers, 1996; Keller-Wood, 1995), guinea pigs (Kalenga et al., 1991) and humans (Chapman et al., 1999; Sealey et al., 1994). It is difficult to indicate the reason of the higher activity of the circulating RAA system in pregnancy because it clearly depends on many factors that do not operate individually. As reported in the literature in the case of sheep and humans, the higher activity may be connected with the altered sodium and water balance during pregnancy, and with the change in blood pressure (Chapman et al., 1999; Gibson and Lumbers, 1996; Keller-Wood, 1995). In both groups of goats, the sodium, potassium and chloride concentrations in blood were relatively stable. The changes within the 24 h period ranged within the limits of the physiological norms and in the case of some parameters manifested the character of circadian variations. The observed changes in the plasma Naq concentration in the non-pregnant goats were characteristic of circadian variations with the acrophase in the dark phase. Similar results were found in anoestrous goats (Kokkonen et al., 2001), calves (Skotnicka et al., 1997) and in man (Kanabrocki et al., 1973; Koopman et al., 1989; Sothern et al., 1996). In the pregnant goats the Naq concentration in plasma was lower than in non-pregnant goats. The observed changes were not characteristic of circadian variations. Muszczynski et al. (1996) E. Skotnicka / Comparative Biochemistry and Physiology Part C 134 (2003) 385–395 also did not observed circadian variations in plasma sodium concentration in pregnant goats. In the present study, no circadian variations in the plasma Kq concentration in the non-pregnant goats were found. Also, Kokkonen et al. (2001) did not find circadian variations in plasma Kq concentration in anoestrous goats. However, circadian variations in the plasma Kq concentration were observed in calves (Skotnicka et al., 1997) and in humans (Kanabrocki et al., 1973; Bernardi et al., 1985). Moreover, Solomon et al. (1991) observed the increase in the plasma Kq concentration during the day, with the acrophase at approximately noon. According to them, the decrease in the plasma Kq concentration at night may result in the decrease in its excretion. In the pregnant goats, the circadian variations of the plasma Kq concentration were observed, with the values higher at night and lower during the day. Kanabrocki et al. (1973) observed similar results in man, with the acrophase in dark phase. Circadian variations were found in pregnant goats (Muszczynski et al., 1996), calves (Skotnicka et al., 1997) and in humans (Bernardi et al., 1985; Kanabrocki et al., 1973; Solomon et al., 1991), but the acrophases of the rhythms were in the light phase of the photoperiod. Moreover, it is also interesting that during pregnancy, there appeared circadian variations in plasma Kq concentration with similar patterns to the changes in the activity of the circulating RAA system. There is no doubt that changes in potassium concentration can cause a phase shift in vasopressin, and single unit rhythms in vitro and behavioural rhythms in vivo (Rabinovitz et al., 1986; Miller, 1993). It is not known whether such effects are due to membrane depolarisation or transmembrane transport of potassium by ionic pumps (Laming, 1989). In the present study, circadian variations in the plasma Cly concentration in non-pregnant and pregnant goats were found. The circadian variations in plasma Cly concentration was also observed by Kanabrocki et al. (1973) and Sothern et al. (1996) in man. But Muszczynski et al. (1996) with pregnant goats and Skotnicka et al. (1997) with calves did not find any circadian variations in chloride concentrations. In conclusion, the results of the present study show that PRA and PA concentration change during the 24-h study phase with achieved highest values at night, and overall higher activity of the RAA system in goats pregnant at this time. In the 393 non-pregnant goats no circadian variations were observed. These results suggest that circadian changes of potassium concentration in plasma during late pregnancy in goats may be one of the main factors affecting the RAA system. Acknowledgments The author wishes to thank Prof. W.F. Skrzypczak and Prof. K. Janus (Department of Animal Physiology, Agricultural University of Szczecin) for their help, stimulating discussions and precious remarks. The author acknowledges Dr Zbigniew Muszczynski for his technical assistance and M. Biels for his help in translation. References Bardwel, R.D., Rousel, J.D., Shaffer, L.M., Gomila, L.F., Adkinson, R.W., 1978. Factors affecting serum aldosterone levels in Holsteins. J. Dairy Sci. 61, 220–226. 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