Mechanisms subserving the physiological nocturnal relative hypoprolactinemia of healthy older men: Dual decline in prolactin secretory burst mass andbasal release with preservation of pulse duration, frequency, and interpulse interval - A general clinical research center study
A. Iranmanesh et al., Mechanisms subserving the physiological nocturnal relative hypoprolactinemia of healthy older men: Dual decline in prolactin secretory burst mass andbasal release with preservation of pulse duration, frequency, and interpulse interval - A general clinical research center study, J CLIN END, 84(3), 1999, pp. 1083-1090
Increasing age is accompanied by decrements in randomly obtained, fasting,
or frequently sampled serum PRL concentrations. The precise neuroendocrine
mechanisms underlying such relative hypoprotactinemia in aging are incomple
tely understood. In the present study, we sampled blood at 2.5-min interval
s overnight in 11 young (aged 21-34 yr) and 8 older (aged 62-72 yr) healthy
men for subsequent chemiluminescence-based assay of serum PRL concentratio
ns. The mean (+/- SEM) serum PRL concentration was significantly reduced at
4.3 +/- 0.78 mu g/L in older men compared with 9.5 +/- 1.2 mu g/L in young
volunteers (P = 0.0049). PRL concentrations correlated with serum testoste
rone (r = 0.473; P = 0.041), dehydroepiandrosteroen sulfate (r = + 0.455, P
= 0.05), and insulin-like growth factor I (r = 0.494; P = 0.032) levels. D
econvolution analysis was used to evaluate combined pulsatile and basal mod
es of PRL secretion. In older men, discrete PRL secretory bursts were marke
d by a significantly (2.4-fold) attenuated mass of hormone secreted per bur
st (amount of PRL secreted per unit distribution volume), viz. 1.6 +/- 0.23
(older) vs. 3.9 +/- 0.57 mu g/L (young; P < 0.01). In contrast, PRL secret
ory burst frequency, interpulse interval, and pulse duration were invariant
of age. Concomitantly, basal PRL secretion was reduced by 2-fold in older
subjects, namely to 0.00030 +/- 0.00027 (older) vs. 0.00065 +/- 0.0002 mu g
/L/min(young; P < 0.01). The amount of total PRL secretion that was pulsati
le averaged 82 +/- 5.3% in young and 99 +/- 0.13% in older men (P = 0.012),
indicating preferential loss of the basal mode of PRL release in aging.
Assuming that basal PRL secretion mirrors functional pituitary lactotroph c
ell secretory mass, whereas pulsatile PRL release reflects effective (net)
intermittent hypothalamic drive to responsive lactotroph cells, then our re
sults suggest both an attrition in lactotroph cell mass and an impoverishme
nt of net positive hypothalamic (agonistic) input to lactotrophs in older m
en. Given the multiple roles of PRL reported in experimental animals (e.g.
on the one hand to support immune function and adrenal androgen biosynthesi
s and on the other hand to activate intraprostatic growth factors), we sugg
est that the nocturnal relative hypoprolactinemia observed in healthy aging
men may have both adaptive and maladaptive clinical implications to target
tissues.