The ␤␥ subunits of GTP-binding proteins (G ␤␥ ) activate the muscarinic K ؉ channel (K ACh ) in heart by direct binding to both of its component subunits. K ACh channels can also be gated by internal Na ؉ ions. Both activation mechanisms show dependence on hydrolysis of intracellular ATP. We report that phosphatidylinositol 4,5-bisphosphate (PIP 2 ) mimics the ATP effects and that depletion or block of PIP 2 retards the stimulatory effects of G ␤␥ subunits or Na ؉ ions on channel activity, effects that can be reversed by restoring PIP 2 . Thus, regulation of K ACh channel activity may be crucially dependent on PIP 2 and phosphatidylinositol signaling. These striking functional results are in agreement with in vitro biochemical studies on the PIP 2 requirement for G ␤␥ stimulation of G protein receptor kinase activity, thus implicating phosphatidylinositol phospholipids as a potential control point for G ␤␥ -mediated signal transduction.

Direction-selective retinal ganglion cells show an increased activity evoked by light stimuli moving in the preferred direction. This selectivity is governed by direction-selective inhibition from starburst amacrine cells occurring during stimulus movement in the opposite or null direction. To understand the intrinsic membrane properties of starburst cells responsible for direction-selective GABA release, we performed whole-cell recordings from starburst cells in mouse retina. Voltage-clamp recordings revealed prominent voltagedependent K ϩ currents. The currents were mostly blocked by 1 mM TEA, activated rapidly at voltages more positive than Ϫ20 mV, and deactivated quickly, properties reminiscent of the currents carried by the Kv3 subfamily of K ϩ channels. Immunoblots confirmed the presence of Kv3.1 and Kv3.2 proteins in retina and immunohistochemistry revealed their expression in starburst cell somata and dendrites. The Kv3-like current in starburst cells was absent in Kv3.1-Kv3.2 knock-out mice. Current-clamp recordings showed that the fast activation of the Kv3 channels provides a voltage-dependent shunt that limits depolarization of the soma to potentials more positive than Ϫ20 mV. This provides a mechanism likely to contribute to the electrical isolation of individual starburst cell dendrites, a property thought essential for direction selectivity. This function of Kv3 channels differs from that in other neurons where they facilitate highfrequency repetitive firing. Moreover, we found a gradient in the intensity of Kv3.1b immunolabeling favoring proximal regions of starburst cells. We hypothesize that this Kv3 channel gradient contributes to the preference for centrifugal signal flow in dendrites underlying direction-selective GABA release from starburst amacrine cells

The mechanism of the action of acetylcholine (ACh) on the L-type calcium current (ICa,L) was examined using a whole-cell voltage-clamp technique in single sino-atrial myocytes from the rabbit heart. ACh depressed basal ICa,L at concentrations in the range 0.05-10 microM, without previous beta-adrenergic stimulation. The ACh-induced reduction of ICa,L was reversed by addition of atropine, indicating that muscarinic receptors mediate it. Incubation of cells with a solution containing pertussis toxin led to abolition of the ACh effect, suggesting that this effect is mediated by G proteins activated by muscarinic receptors. Dialysis of cells with protein kinase inhibitor or 5'-adenylyl imidodiphosphate, inhibitors of the cAMP-dependent protein kinase, decreased basal ICa,L by about 85% and suppressed the effect of ACh. The ACh effect was also absent in cells dialysed with a non-hydrolysable analogue of cAMP, 8-bromo-cAMP. The results suggest that, in basal conditions, a large part of the L-type calcium channels should be phosphorylated by protein kinase A stimulated by a high cAMP level correlated with a high adenylate cyclase activity. The depressing effect of ACh on ICa,L may occur via inhibition of the high basal adenylate cyclase activity leading to a decrease of cAMP-dependent protein kinase stimulation and thus to a dephosphorylation of calcium channels.