Differential modulation of TTX-sensitive and TTX-resistant Na⁺ channels in spinal cord astrocytes following activation of protein kinase C

TTX-sensitive (TTX-S) and TTX-resistant (TTX-R) Na⁺ currents are expressed in high densities (2-8 channels/μm²) in astrocytes cultured from neonatal rat spinal cord. The two Na⁺ current types differ up to 1000-fold in their TTX sensitivity and additionally have different steady-state activation (g-V) and inactivation (h(∞)) curves. Expression of TTX-S and TTX-R Na⁺ currents is confined to morphologically distinguishable subtypes of astrocytes, allowing characterization of the two types of Na⁺ currents in isolation: stellate cells express TTX-S Na⁺ currents and flat pancake cells express TTX-R Na⁺ currents. Activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate (PMA) exhibited different effects on TTX-S and TTX-R Na⁺ currents. PMA reduced peak TTX-S Na⁺ currents by 25-60%; in contrast, PMA potentiated peak TTX-R Na⁺ currents by 60-150%. These effects developed within minutes, and were typically not reversible. PMA effects were voltage dependent, and shifted steady-state Na⁺ current activation of TTX-R and TTX- S currents by 6 and 18 mV, respectively, but without affecting their steady- state current inactivation (h(∞)). PMA treatment also changed Na⁺ current kinetics. TTX-R current activation (τ(m)) was faster and current inactivation (τ(h)) changed from a single- to a bi-exponential after PMA exposure, suggesting that PKC phosphorylation may have activated formerly quiescent Na⁺ channels. In contrast, TTX-S current activation (τ(m)) was unchanged, and current inactivation (τ(h)), on average, decreased by 50% following PMA exposure. Since these effects of PMA could be reduced or abolished by the PKC inhibitor 1-(5-isoquinolinylsulfonyl)-2- methylpiperazine (H7), it is concluded that PMA effects were mediated by activation of PKC.