Modulation of type A K+ current inDrosophila larval muscle by internal Ca2+; effects of the overexpression of frequenin

The calcium-dependent modulation of type A K⁺ current (I _A) has been investigated using a two-electrode voltage clamp on larval muscle cells ofDrosophila. It was found that the amplitude ofI _A increases when Ca²⁺]₀ is changed from 0.2 mM to 2 mM. The increase inI _A amplitude is not due to overlap with the Ca²⁺-dependent fast K⁺ current,I _CF, since it is observed also inslo ¹ mutants, which are deficient for this current. This effect is not due to Ca²⁺-dependent shifts in the steady-state activation/inactivation kinetics. The phenomenon is probably due to elevations in internal calcium since it is abolished by Ca²⁺ channel blockers and promoted by caffeine (5 mM) if added in the absence of external calcium. This calcium effect was dose-dependent since it was not observed in the presence caffeine plus 2 mM calcium in the bath nor for values of Ca²⁺]₀ above 4 mM. The Ca²⁺-dependent modulation ofI _A is absent inV7, a mutation that causes overexpression of frequenin, a recoverin-like Ca²⁺-binding protein which stimulates guanylyl cyclase 31]. One possible explanation for the loss ofI _A modulation in theV7 mutation is that the excess of frequenin alters intracellular cGMP-dependent metabolic pathways responsible for the internal calcium homeostasis.