Spontaneous activity of the light-dependent channel irreversibly induced in excised patches from Limulus ventral photoreceptors

Abstract

We have studied the properties of membrane patches excised from the transducing lobe of Limulus ventral photoreceptors. If patches are excised into an "internal" solution that resembles the ionic composition of the cytoplasm, channel activity is typically absent, but can be turned on by cyclic GMP (cGMP). In contrast, if patches are excised directly into sea water and subsequently examined in internal solution, they exhibit a high channel activity in the absence of any second messenger (spontaneous channel activity). Because these patches contained only light-dependent channels when examined before excision and because these spontaneous channels have properties in common with the light/cGMP-dependent channel, we believe that the spontaneously active channels represent light/cGMP-dependent channels that have been damaged by exposure to sea water, perhaps due to proteolysis activated by the high Ca2+ levels of the sea water. One type of the spontaneously active channel resembles the light/cGMP-dependent channel in open time, reversal potential, conductance states and voltage dependence. Application of micromolar Ca2+ to this channel produces a reversible decrease in the opening rate, indicating a high affinity binding site for Ca2+ on this channel. Another type of spontaneously active channel has a conductance state and reversal potential similar to the light/cGMP-dependent channel, but has apparently lost its dependence and sensitivity to Ca2+ and voltage.

Cyclic-GMP enhances light-induced excitation and induces membrane currents in Drosophila retinal photoreceptors

“…2A). One possible reason for the decline in 8-Br-cGMP induced current amplitudes at high I,, is that intracellular Caz+, greatly elevated by bright illumination (Peretz et al, 1994;Ranganathan et al, 1994), might reduce channel activity, as proposed for Limulus photoreceptors (Johnson and Bacigalupo, 1992). Another possibility is that enhancement of light responses by 8-Br-cGMP could be due to an inhibitory effect on light adaptation.…”

Unitary Recordings of TRP and TRPL Channels From Isolated Drosophila Retinal Photoreceptor Rhabdomeres: Activation by Light and Lipids

“…Because it is highly likely that the only channels residing in the lightsensitive region are the light-dependent channels, like in other photoreceptors (Johnson and Bacigalupo 1992;Nasi and Gomez 1992;Stern et al 1982), we expected that single-channel currents recorded from this region of the photoreceptor corresponded to TRP and TRPL. To test this, we compared the characteristics of the rhabdomeric channels with those previously reported of their respective macroscopic currents in whole cell voltage-clamp studies.…”

Calcium-dependent inactivation of light-sensitive channels in Drosophila photoreceptors.

“…A similar conclusion has been reached in excised patches from Limulus ventral photoreceptors, in which the light-sensitive channels have entered an irreversible spontaneously active state. The open probability of channels in such patches is reduced by application of Ca 2+ to the cytoplasmic face of the patch (Johnson and Bacigalupo, 1992).…”

“…A third potential site of feedback is at the level of the light-sensitive channels themselves, as has recently been suggested from experiments on excised patches from Limulus ventral photoreceptors (Johnson and Bacigalupo, 1992). Previously the properties of the light-sensitive channels had not been investigated in Drosophila inasmuch as they have proved inaccessible to the patch pipette, whereas the properties of the light-induced current recorded in whole-cell recordings are dominated by the kinetic and noise properties of the quantum bumps.…”