Expression of potassium channels during postnatal differentiation of rabbit Muller glial cells

The postnatal maturation of Muller glial cells from immature radial glial c ells is accompanied by specific changes in the activity of distinct types o f K+ channels, as shown by whole-cell and cell-attached records on freshly isolated cells from retinae of young (postnatal days 1-30, P1-P30) and adul t rabbits. (i) The density of inwardly rectifying currents, providing the m ain K+ conductance in adult Muller cells, was very low (0.8 pA/pF) from P1 to P6 but increased rapidly thereafter until a relatively stable level of 1 1.0 pA/pF was established at P17. (ii) Transient (A-type) K+ currents were expressed in all immature cells at a high density (9.6 pA/pF). After P12, b oth the percentage of cells with A-type currents and the peak amplitudes of the currents (2.8 pA/pF) declined. (iii) Delayed rectifying K+ currents de veloped slowly until after P30. (iv) The postnatal maturation of radial gli al cells was accompanied by a strong decrease in the activity of large-cond uctance, Ca2+-activated K+ channels, the open probability of which (measure d at the resting membrane potential) decreased from 0.69 at P2-4 to 0.06 at P13-14. The developmental decrease of the activity of Ca2+-activated K+ ch annels is assumed to be mainly caused by alteration of the resting membrane potential which developed from low values (-49 mV) at P1-6 to high adult v alues (-84 mV) after P13. The activity of each distinct type of K+ channel investigated is differently modulated by developmental regulation. This may reflect different functional requirements of immature and mature Muller ce lls.