Brain fluid calcium concentration and response to acute hypercalcaemia during development in the rat.

1. In vivo measurements of plasma, cerebrospinal fluid (CSF) and brain interstitial fluid (ISF) ionic Ca2+ concentrations ([Ca2+]) have been made in anaesthetized rats aged between 19 days gestation and adult using calcium-selective microelectrodes. Total calcium concentration ([ Ca]) has also been determined in plasma and CSF samples by atomic absorption spectrometry. 2. Under control conditions, plasma, CSF and ISF [Ca2+] showed a small, but significant, decrease with age. Plasma and CSF, but not ISF, showed a transient hypocalcaemia at birth. After birth there were no significant differences between plasma, CSF and ISF [Ca2+], except in adult rats where CSF [Ca2+] was significantly lower than plasma [Ca2+]. The age-related changes in CSF and ISF [Ca2+] were small and it is uncertain as to whether they may have any functional significance. 3. Under control conditions, plasma and CSF [Ca] also declined with age. The fall in plasma [Ca] paralleled the changes in plasma [Ca2+]. The decrease in CSF [Ca] was steeper than that in [Ca2+] and indicated a higher proportion of protein-bound or complexed calcium in the CSF of young when compared to old rats. 4. Acute plasma hypercalcaemia was induced by intramuscular injections of calcium gluconate and consequent changes in CSF or ISF [Ca2+] were monitored in vivo. There was very weak regulation of CSF and ISF [Ca2+] at 21 days gestation, which may reflect placental control over fetal plasma calcium. Soon after birth there was good regulation in both ISF and CSF [Ca2+]. CSF [Ca] was also measured during hypercalcaemia in samples from post-natal rats and there was similar regulation to that in CSF [Ca2+]. 5. It is concluded that under control conditions, during rat development, CSF and ISF [Ca2+] closely follow changes in plasma [Ca2+], but that soon after birth homeostatic mechanisms develop to prevent large fluctuations in brain fluid calcium.