The cell-attached and excised patch configurations of the patch clamp technique were used to characterize Ca(2+)-activated maxi-K+ channels in freshly-isolated Müller glial cells. The cells were dissociated from postmortem adult human and porcine retinas. The maxi-K+ channels in Müller cells of both species display a single channel conductance of 175 pS in cell-attached and inside-out patches (125/110 mM K+). The channels are activated by membrane depolarization and by elevation of intracellular Ca2+. In the presence of 10(-5), 10(-4), and 10(-3) M intracellular free Ca2+, the half-activation voltages are +7.2, -26.6, and -47.5 mV, respectively. The half-activation-[Ca2+] at +10 mV is 8.1 microM, and the Hill coefficient of Ca2+ binding is 1.7, Ba2+ exerts a voltage-dependent block of the open-state probability. The maxi-K+ channels of Müller cells are activated by raising of the intracellular pH as well as by Mg2- ions at the cytosolic face of the channels. Phosphorylation of the channel after cytosolic addition of the catalytic subunit of a cAMP dependent protein kinase in the presence of Mg-ATP caused a shift of the activation curve to negative membrane potentials. Between -40 and -80 mV membrane potentials, the open-state probability rose to 190.3% of the control value (100%) after phosphorylation of the channel. Therefore, phosphorylation enhances sensitivity of the channels to Ca2+ and voltage. The maxi-K+ channels may provide a link between second messenger systems and membrane conductance of retinal Müller cells and may have an important function in repolarization of the Müller cell membrane and, therefore, in the maintainance of the retinal spatial K+ buffering mechanisms.