Ion channels in spinal cord astrocytes in vitro. I. Transient expression of high levels of Na+ and K+ channels.

1. Na+ and K+ channel expression was studied in cultured astrocytes derived from P--0 rat spinal cord using whole cell patch-clamp recording techniques. Two subtypes of astrocytes, pancake and stellate, were differentiated morphologically. Both astrocyte types showed Na+ channels and up to three forms of K+ channels at certain stages of in vitro development. 2. Both astrocyte types showed pronounced K+ currents immediately after plating. Stellate but not pancake astrocytes additionally showed tetrodotoxin (TTX)-sensitive inward Na+ currents, which displayed properties similar to neuronal Na+ currents. 3. Within 4-5 days in vitro (DIV), pancake astrocytes lost K(+)-current expression almost completely, but acquired Na+ currents in high densities (estimated channel density approximately 2-8 channels/microns2). Na+ channel expression in these astrocytes is approximately 10- to 100-fold higher than previously reported for glial cells. Concomitant with the loss of K+ channels, pancake astrocytes showed significantly depolarized membrane potentials (-28.1 +/- 15.4 mV, mean +/- SD), compared with stellate astrocytes (-62.5 +/- 11.9 mV, mean +/- SD). 4. Pancake astrocytes were capable of generating action-potential (AP)-like responses under current clamp, when clamp potential was more negative than resting potential. Both depolarizing and hyperpolarizing current injections elicited overshooting responses, provided that cells were current clamped to membrane potentials more negative than -70 mV. Anode-break spikes were evoked by large hyperpolarizations (less than -150 mV). AP-like responses in these hyperpolarized astrocytes showed a time course similar to neuronal APs under conditions of low K+ conductance. 5. In stellate astrocytes, AP-like responses were not observed, because the K+ conductance always exceeded Na+ conductance by at least a factor of 3. Thus stellate spinal cord astrocyte membranes are stabilized close to EK as previously reported for hippocampal astrocytes. 6. It is concluded that spinal cord pancake astrocytes are capable of synthesizing Na+ channels at densities that can, under some conditions, support electrogenesis. In vivo, however, AP-like responses are unlikely to occur because the cells' resting potential is too depolarized to allow current activation. Thus the absence of electrogenesis in astrocytes may be explained by two mechanisms: 1) a low Na-to-K conductance ratio, as in stellate spinal cord astrocytes and in other previously studied astrocyte preparations; or, 2) as described in detail in the companion paper, a mismatch between the h infinity curve and resting potential, which results in Na+ current inactivation in spinal cord pancake astrocytes.