Regulation of Ca2+-activated K+ channels by multifunctional Ca2+/calmodulin-dependent protein kinase

Activation of mesangial cells by ANG II provokes release of intracellular Ca(2+) stores and subsequent Ca(2+) influx through voltage-gated channels, events that are reflected by a large transient increase in intracellular concentration [Ca(2+)](i) followed by a modest sustained elevation in [Ca(2+)](i). These ANG II-induced alterations in [Ca(2+)](i) elicit activation of large Ca(2+)-activated K(+) channels (BK(Ca)) in a negative-feedback manner. The mechanism of this BK(Ca) feedback response may involve the direct effect of intracellular Ca(2+) on the channel and/or channel activation by regulatory enzymes. The present study utilized patch-clamp and fura 2 fluorescence techniques to assess the involvement of multifunctional calcium calmodulin kinase II (CAMKII) in the BK(Ca) feedback response. In cell-attached patches, KN62 (specific inhibitor of CAMKII) either abolished or reduced to near zero the ANG II-induced BK(Ca) feedback response. This phenomenon did not reflect direct effects of KN62 on the BK(Ca) channel, because this agent alone did not significantly alter BK(Ca) channel activity in inside-out patches. KN62 also failed to alter either the transient peak or sustained plateau phases of the [Ca(2+)](i) response to ANG II. In inside-out patches (1 mu M Ca(2+) in bath), calmodulin plus ATP activated BK(Ca) channels in the presence but not the absence of CAMKII. These observations are consistent with the postulate that CAMKII is involved in the BK(Ca) feedback response of mesangial cells, acting to potentiate the influence of increased [Ca(2+)](i) on the BK(Ca) channel or a closely associated regulator of the channel. An additional effect of CAMKII to activate a voltage-gated Ca(2+) channel cannot be ruled out by these experiments.