Modulation of intracellular Ca++ in cultured astrocytes by influx through voltage‐activated Ca++ channels

Abstract

Fura‐2 and indo‐1 fluorescence measurements were used to examine intracellular Ca⁺⁺ concentration ([Ca⁺⁺]_i) and its modulation by voltage‐activated influx in murine cortical astrocytes in primary cell culture. Extracellular K⁺ was increased from 5 to 50 mM to depolarize cells to determine if Ca⁺⁺ influx through voltage activated Ca⁺⁺ channels could alter [Ca⁺⁺]_i. In confluent 4 to 6 weeks in vitro astrocyte cultures 50 mM K⁺ increased [Ca⁺⁺]_i 3‐4‐fold (from 150 nM up to 550 nM); this increase was blocked by nifedipine and enhanced by BayK 8644 indicating that influx was through L‐type channels. However, in 1 to 2 weeks in vitro astrocyte cultures, high K⁺ reduced [Ca⁺⁺]_i. L‐type channels were apparently present in these cells because high K⁺ in combination with BayK 8644 increased [Ca⁺⁺]_i. Following pretreatment of 1 to 2 weeks in vitro astrocytes with dibutyryl cAMP (dbcAMP) high K⁺ increased [Ca⁺⁺]_i in the absence of BayK 8644 indicating enhanced activity of Ca⁺⁺ channels in agreement with previous voltage‐clamp studies. Ca⁺⁺ influx through voltage‐activated channels in cultured cortical astrocytes can substantially increase [Ca⁺⁺]_i and these channels can be dynamically modulated by dihydropyridines. Immature astrocytes may express ‘silent’ or inactive Ca⁺⁺ channels or have a much lower number of channels.