Gender differences in patterns of HPA axis response to challenge: Macarthur studies of successful aging

Previous research has suggested that women may have greater and more prolonged hypothalamic-pituitary-adrenal response to challenge at older ages. Data on patterns of ACTH and cortisol responses to a "naturalistic" driving simulation challenge were examined to test the hypothesis that older women (aged 70-79 years) would show greater response than a comparable group of older men. Analyses of mean ACTH or cortisol responses in terms of maximal increase, area under the curve and repeated measures ANOVA did not reveal significant gender differences. By contrast, analyses of the joint occurrence of ACTH and cortisol responses above the respective sample medians indicated that women were significantly more likely to respond to the challenge with elevated ACTH and cortisol responses. The lack of significant gender difference in the analyses of mean scores resulted from the sensitivity of this measure of typically (or average response) to outliers in the data, thereby substantially distorting the report of typical (or average) responses for the male population. Analyses of the temporal patterns of ACTH and cortisol response in terms of the stability of the subject's relative rank at each sampling time indicated that: (1) there was considerable stability of rankings based on the initial cortisol response (though not ACTH) through the recovery period; and (2) women exhibited greater instability than men, being more likely to exhibit shifts in rank from high to low over time. These findings suggest that: (1) as a group, older women are more likely to show larger responses than comparably healthy older men to a "naturalistic" driving simulation challenge; (2) there is considerable stability in patterns of cortisol response from initial response at 20 min through the recovery period, indicating that those showing greater initial response continue to experience higher levels of cortisol during the post-challenge recovery period; and (3) instability is more common among women than men.