Daily Patterns of Secretion of Neurohypophysial Hormones in Man: Effect of Age Mary L
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Experimental Physiology (1998), 83, 409-418 Printed in Great Britain DAILY PATTERNS OF SECRETION OF NEUROHYPOPHYSIAL HORMONES IN MAN: EFFECT OF AGE MARY L. FORSLING*, H. MONTGOMERYt, D. HALPINtt, R. J. WINDLE AND D. F. TREACHERt Departments of Obstetrics and Gynaecology and t Medicine, United Medical and Dental Schools, St Thomas's Campus, Lambeth Palace Road, London SE] 7EH, UK (MANUSCRIPT RECEIVED 8 DECEMBER 1997, ACCEPTED 14 JANUARY 1998) SUMMARY The neurohypophysial hormone vasopressin contributes to control of urine output and, while urine flow shows a clear daily rhythm, there has been debate as to whether this is true of neuro- hypophysial hormones. A study was performed on fifteen adult males, with a mean age of 25 years, over a 24 h period, nine blood samples being taken at regular intervals for the determination of neurohypophysial hormones and indices of fluid balance. Samples were taken via an indwelling cannula so that sleep was undisturbed. A daily variation in the plasma concentrations of oxytocin and vasopressin was demonstrated with concentrations reaching a nadir in the late afternoon. Concentrations of both hormones peaked at 02.00 h. Vasopressin concentrations were inversely correlated with packed cell volume, indicating that the altered hormone release was affecting fluid retention. Consistent with this was the observation that the relationship of plasma osmolality to vasopressin depended on the time of day. To determine the effect of ageing, a similar study was performed on nine healthy elderly subjects with a mean age of 70 years. The nocturnal peak of vasopressin was markedly attenuated, while oxytocin release was similar to that in the younger group. These observations confirm the existence of a daily rhythm in the plasma concentrations of neurohypophysial hormones and indicate that the amplitude of the vasopressin change decreases with age. INTRODUCTION Changes in circulating vasopressin concentrations have obvious implications for fluid balance, but recent evidence suggests that oxytocin may also play a role in sodium balance in some species (Verbalis, Mangione & Stricker, 1991; Windle & Forsling, 1991). To understand pathological changes in fluid balance, as for example in nocturia, it is necessary to establish normal daily variations in circulating concentrations of vasopressin and possibly oxytocin and their relationship to changes in indices of fluid balance. Many of the circadian changes are predictive in nature serving to switch on a function of importance for anticipated demands, so that if vasopressin does contribute to the nocturnal fall in urine flow in man, as suggested by the experiments of Papper & Rosenbaum (1952), one would expect to see an increase in plasma vasopressin concentrations, unrelated to plasma osmolality, in the early part of the night. t Present address: Department of Respiratory Medicine, Royal Devon and Exeter Hospital, Exeter EX2 3DN, UK. * Corresponding author: [email protected] 410 M. L. FORSLING AND OTHERS Daily rhythms have been shown in the levels of neurohypophysial hormones in the hypo- thalamus (Noto, Hashomoto, Doi, Nakajima & Kato, 1983), pituitary (Dyball, Forsling, Patterson & Peysner, 1988) and plasma of the rat (Mohring, Kohrs, Mohring, Petri, Homsy & Haack, 1978; Greeley, Morris, Eldridge & Kizer, 1982). The plasma concentrations of the hormones increase during the hours of daylight, falling again during the hours of darkness. A daily rhythm in the secretion of vasopressin has not been demonstrated in all species studied, and in the rat its demonstration depends on a number of factors, including the timing of the samples (Wells, Windle, Peysner & Forsling, 1993); there may also be a genetic component. A daily rhythm of vasopressin secretion with a nocturnal rise has been observed in humans by some authors (George et al. 1975; Linkola, Ylikahri, Fyhrquist & Wallenius, 1978) and has provided the rationale for investigation and treatment of enuresis (Rittig, Knudsen, Norgaard, Pedersen & Djurhus, 1989). Others have questioned whether neurohypophysial hormone secretion in man shows a daily rhythm (Katz, Smith, Lock & Loeffel, 1979; Richards et al. 1987; Forsling, 1993), while Drumer et al. (1987) noted a daily rhythm in plasma vaso- pressin, but they studied recumbent subjects and posture could affect the results, altering patterns seen in normally active subjects. There is a similar debate concerning plasma oxytocin concentrations in humans. Landgraf, Hacker & Buhl (1982) described a fall in plasma concentrations during the day, although Amico Tenicel, Johnstone & Robinson, 1983) were unable to confirm these observations. Data obtained from clinical studies and from studies in the rat clearly show that the ageing process is accompanied by alterations in the ability to regulate water excretion and there is also a change in the circadian pattern of urine production. The increased susceptibility of the aged animal or human to significant derangement of water balance could result from a number of factors including altered thirst sensation (Phillips, Johnston & Gray, 1993), altered synthesis and release of vasopressin and altered renal concentrating ability (Pavo et al. 1995). There is no loss of vasopressin-producing cells in the hypothalamic magnocellular nuclei in man (Van der Woude et al. 1995) and Helderman, Vestal, Rowe, Tobin, Andres & Robertson (1978) found no change in plasma vasopressin concentrations with ageing, although Frolkis, Golovchenkop, Medved & Frolkis (1982) report a progressive increase. The response to hypertonic saline (Raskind et al. 1995) or water loading appears to be unaffected by age (Crowe, Forsling, Rolls, Phillips, Ledingham & Smith, 1987), while the response to ortho- stasis is attenuated (Rowe, Minaker, Sparrow & Robertson, 1982). Ageing affects the circadian pattern of many functions, but no observations have been performed on this aspect of the neurohypophysial system. In order to determine whether neurohypophysial hormones show a daily rhythm under normal conditions, a study has now been performed on the daily pattern of neurohypophysial hormone release in a young group of men carrying out their normal daily activities, the changes being related to indices of fluid balance. The pattern of neurohypophysial hormone secretion was compared with that in a fully active, healthy group of elderly men. METHODS Subjects Observations were performed on fifteen healthy male subjects in the age range 22-40 years (mean 25 years) and nine healthy male subjects in the age range 60-75 (mean 70 years). All subjects were normotensive with normal renal function, with no significant past or current medical history and on no regular medication. Each was studied over a single 24 h period. The subjects spent the 36 h prior to the DAILY RHYTHMS OF NEUROHYPOPHYSIAL HORMONES 411 study following their normal daily routine at the St Thomas' Hospital and during both this period and during the study the subjects abstained from alcohol, tobacco and vigorous exercise. The meal times were standardized, and the subjects kept a diary of their activities and fluid and food intakes and were weighed. At 16.30 h a forearm vein was cannulated and after about 30 min of recumbent rest 12 ml of blood were withdrawn through the cannula. Over the following 24 h, eight further blood samples were taken (seven in the elderly subjects), each after 30 min recumbency, for the determination of packed cell volume, plasma osmolality, sodium, potassium, vasopressin and oxytocin. All subjects went to bed at 23.30 h when the lights were switched off and rose at 08.00 h the following day when lighting was restored. The investigation was approved by the local Ethical Committee and all subjects gave their written informed consent. Analyses Plasma osmolality was determined by the method of freezing point depression (Digmatic osmometer model 3D; Advanced Instruments Inc., Needham Heights, MA, USA). Sodium concentration was measured using a flame photometer (410C; Corning, Halstead, Essex, UK). Packed cell volume was determined in duplicate using heparinized microhaematocrit tubes (Hawksley & Sons, Lancing, Sussex, UK). Plasma vasopressin concentration was determined by a radioimmunoassay after prior extraction (Forsling, 1985) with SepPak C18 cartridges (Water Associates Inc, Millford, MA, USA) using the First International Standard for vasopressin (77/50 1). The lower limit of detection was 0. 12 + 0-02 pmol F'. The intra-assay coefficient of variation was 7-7 % and the interassay coefficient of variation was 11.9 % for 2-5 pmol. Oxytocin was assayed against the Fourth International Standard (76/575) as described by Windle & Forsling (1993), with an intra-assay coefficient of variation of 4.1 % and an interassay coefficient of variation of 9-0 % for 2.0 pmol 1'. Statistical analysis Results are presented as the mean + S.E.M. and values were compared using repeated measures analysis of variance. Areas under the curve were calculated using the trapezium rule as described by Altman (1991) and values in the young and elderly compared using Student's unpaired t test. Regression analysis was performed to determine the correlation between parameters. A value of P < 0 05 was taken as significant. RESULTS Fluid balance All subjects showed a similar pattern of fluid intake. In the younger group, osmolality exhibited a significant rise of 10 mosmol kg-' during the afternoon (P < 0.01) (Fig. 1). Plasma sodium concentrations peaked at the same time, although the increase was not statistically significant. Packed cell volume showed a progressive rise (P < 0-04) from 02.00 h reaching a peak at 13.00 h. The subjects were in overall fluid balance during the study, as judged by the body weight at the beginning and end of the study. The values for packed cell volume and plasma sodium in the elderly group were not significantly different from those in the younger group. In the elderly group, packed cell volume showed the same trend over the 24 h period as that in the younger group, with values increasing from 39. 1 + 2-2 % at 02.00 h to 43-3 ± 2-0 % at 12.00 h, but the change was not statistically significant; plasma sodium concentrations also showed no significant change.