Acetylcholine activates inwardly rectifying potassium channels (IK.ACh) in the heart through muscarinic receptor binding and activation of pertussis-toxin-sensitive G proteins. Experiments showing that only the beta gamma-subunit (G beta gamma) activates IK.ACh (ref. 4) were challenged by reports that only the activated alpha-subunit (G alpha) was effective. Here we examine IK.ACh regulation using purified brain and recombinant G-protein subunits. Six recombinant G beta gamma-subunits activated IK.ACh with apparent half-maximal activation concentrations of 3-30 nM. Activation of IK.ACh by recombinant G alpha-GTP gamma S was observed, but this was probably due to release of GTP gamma S from the protein. Importantly, IK.ACh activity elicited by GTP gamma S was inhibited by purified brain and recombinant G alpha-GDP, suggesting that native G beta gamma plays a major role in this pathway. We conclude that G beta gamma is a primary regulator of IK.ACh activity.

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The eyes of B u l k express a circadian rhythm in optic nerve impulse frequency. Bulla also exhibit a circadian rhythm in locomotor behavior. In order to determine what role the ocular oscillators play in controlling rhythmic behavior, the eyes were removed and locomotor activity was recorded during free runs and while entrained to light-dark cycles. We find that eye removal leads to a disorganization of the free-running locomotor rhythm, suggesting that the ocular oscillators are involved in timing this behavior. Eyeless Bulla in the presence of light cycles exhibit locomotor rhythmicity but there is a switch from predominantly nocturnal to diurnal activity. While diurnal behavior in eyeless Bulla reveals the presence of extraocular photoreceptors, we presently find no evidence that a circadian pacemaker mediates this diurnal activity.

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