Changes in plasma sodium concentration in women at the menopause

Quantitative bias analysis for epidemiological associations of perfluoroalkyl substance serum concentrations and early onset of menopause

An association between increased serum concentrations of perfluoroalkyl substances (PFAS) such as perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) and early menopause has been reported (Knox et al., 2011; Taylor et al., 2014). This association may be explained by the fact that women who underwent menopause no longer excrete PFAS through menstruation. Our objective was to assess how much of the epidemiologic association between PFAS and altered timing of menopause might be explained by reverse causality. We extended a published population life-stage physiologically-based pharmacokinetic (PBPK) model of PFOS and PFOA characterized by realistic distributions of physiological parameters including age at menopause. We then conducted Monte Carlo simulations to replicate the Taylor population (Taylor et al., 2014) and the Knox population (Knox et al., 2011). The analysis of the simulated data overall showed a pattern of results that was comparable to those reported in epidemiological studies. For example, in the simulated Knox population (ages 42-51) the odds ratio (OR) for menopause in the fifth quintile of PFOA compared to those in the first quintile was 1.33 (95% CI 1.26-1.40), whereas the reported OR was 1.4 (95% CI 1.1-1.8). Using our model structure, a substantial portion of the associations reported can be explained by pharmacokinetics.

Effect of menopause on the chemical control of breathing and its relationship with acid-base status

This study examined the role of alterations in the chemoreflex control of breathing, acid-base balance, and their interaction in postmenopausal ventilatory adaptations. A modified iso-oxic hyperoxic and hypoxic CO(2)-rebreathing procedure was employed to evaluate central and peripheral chemoreflex drives to breathe, respectively, in 15 healthy postmenopausal and 20 premenopausal women of similar age. Arterialized venous blood samples were collected at rest for the estimation of arterial Pco(2) (Pa(CO(2))) and H(+) concentration ([H(+)]), plasma strong ion difference ([SID]) and total weak acid ([A](tot)) concentrations, and serum progesterone ([P(4)]) and 17beta-estradiol ([E(2)]) concentrations. In post- compared with premenopausal women, Pa(CO(2)), [SID], and the central chemoreflex ventilatory recruitment threshold for Pco(2) (VRTco(2)) were higher, whereas [P(4)] and [E(2)] were lower (all P < 0.05), with no significant change in central or peripheral chemoreflex sensitivity, peripheral chemoreflex VRTco(2), and [A](tot). The acidifying effect of an increased Pa(CO(2)) was offset by the alkalizing effect of an increased [SID], such that [H(+)] was preserved in post- compared with premenopausal women. Pa(CO(2)) correlated positively with the central chemoreflex VRTco(2) (r = 0.67, P < 0.01), which in turn correlated positively with [SID] (r = 0.53, P < 0.01) within the pooled data. In conclusion, the relative alveolar hypoventilation and attendant arterial hypercapnia in healthy post- compared with premenopausal women could be explained, in part, by the interaction of 1) reduced central, but not peripheral, chemoreflex VRTco(2), 2) increased [SID], and 3) reduced circulating female sex steroid hormone concentrations.

Hypothesis. Biochemical changes at the menopause: possible role of the central nervous system

Some of the biochemical changes which occur at the menopause can be attributed to an increased rate of loss of bone but others cannot be explained in this way. Data, collected through literature search, are presented which suggest that the rise in plasma phosphate concentration at the menopause is not due primarily to increased breakdown of bone but rather to the following sequence of events: progesterone deficiency----respiratory hypoventilation----mild respiratory acidosis and hypoxia----compensatory metabolic alkalosis----altered carbohydrate metabolism----rise in plasma phosphate concentration. Increases in the concentration of many plasma constituents occur at the menopause, which appear to be due to fluid loss and haemoconcentration. A possible cause of these changes is a reduced secretion of vasopressin by the pituitary gland as a result of oestrogen deficiency. Increases in the fasting urinary excretion of phosphate, sodium and magnesium also occur at the menopause. These changes cannot be attributed to increased bone loss but could be due to the effects of oestrogen deficiency on circadian rhythms in the hypothalamus.

Increase in plasma lactate at the menopause and its relation to the "anion gap"

The mean plasma anion gap (Na + K + Ca + Mg)--(Cl + HCO3(-) + HPO4(2-) + protein), was significantly higher in post-menopausal women compared with pre-menopausal women (8.04 mEq/l compared with 7.03 mEq/l). This change was due, in part, to an increase in the plasma lactate concentration and to smaller increases in citrate and pyruvate concentrations. There were also changes in bicarbonate and chloride concentrations which suggested an increase pH of approximately 0.02 U. Together, these changes accounted for 46% of the increase in the "anion gap", leaving 54% unexplained. It is suggested that the rise in plasma lactate concentration may be due to mild alkalosis and that this change may account for the rise in plasma urate concentration which also occurs at the menopause.