Forskolin and phorbol esters decrease the same K⁺ conductance in cultured oligodendrocytes

Summary

Cultured ovine oligodendrocytes (OLGs) express a number of voltage-dependent potassium currents after they attach to a substratum and as they begin to develop processes. At 24–48 hours following plating, an outward potassium current can be identified that represents a composite response of a rapidly inactivating component and a steady-state or noninactivating component. After 4–7 days in culture, OLGs also develop an inward rectifier current. We studied the effects of forskolin and phorbol 12-myristate 13-acetate (PMA) on OLG outward currents. These compounds are known to alter the myelinogenic metabolism of OLGs. PMA, an activator of protein kinase C (PK-C), has been shown to enhance myelin basic protein phosphorylation while forskolin acting on adenylate cyclase, and thereby increasing cAMP, inhibits it. Both forskolin and PMA increase the phosphorylation of 2′3′-cyclic nucleotide phosphodiesterase, an OLG/myelin protein. We found that forskolin decreased the steady-state outward current at 120 mV by 10% at 100nm, and by 72% at 25 μm from a holding potential of −80 mV. The time course of inactivation of the peak currents was decreased, affecting both the fast and slow time constants. There was no significant change in the steady-state parameters of current activation and inactivation. The effect of forskolin was attenuated when the adenylate cyclase inhibitor adenosine (2mm) was present in the intracellular/pipette filling solution. The results of PMA experiments were similar to those obtained with forskolin. Whereas the amplitude of the currents in the presence of PMA was reduced by 28% at 1.5nm and 60% and 600nm, the decay phase of the peak currents was less affected. The PMA effect could still be seen when the intracellular Ca²⁺ was reduced to ≤10nm with 5mm BAPTA, but was inhibited when the cells were pre-exposed to 50 μm psychosine, a PK-C inhibitor. It is postulated that the potassium currents in OLG can be physiologically modulated by two distinct second-messenger systems, perhaps converging at the level of a common phosphorylated enzyme or regulatory protein.