Spontaneous, ligand‐independent activity of the cGMP‐gated ion channels in cone photoreceptors of fish.

Abstract

1. We studied the electrical conductance of membrane patches detached from the outer segment of single cone photoreceptors isolated from striped bass retina. 2. Only a single class of ion channels exists in the plasma membrane of the cone outer segments; they are gated by cytoplasmic cGMP and select cations over anions, but distinguish poorly among cations. In the absence of added cGMP and of divalent cations, however, membrane patches detached from the outer segments exhibit a small conductance that ideally selects cations over anions, but distinguishes poorly between Na+ and Li+. 3. The cGMP-independent conductance does not arise from the effect of residual cGMP that may remain associated with the detached membrane, because treatment of the patch with cGMP-specific phosphodiesterase does not affect this conductance. 4. The cGMP-independent conductance is pharmacologically indistinguishable from that activated by cGMP Ca2+ and L-cis-diltiazem block both conductances at comparable concentrations and with similar quantitative characteristics. 5. We analysed the noise of Ca2+-or L-cis-diltiazem-dependent macroscopic currents both in the presence and in the absence of cGMP. In the presence of cGMP, the power density spectrum of the noise is well fitted by the sum of two Lorentzian components. The same function with similar corner frequencies fits the noise of the cGMP-independent currents. However, the total power in the current fluctuations is smaller in the absence of cGMP than in its presence; also, the ratio of the zero frequency asymptotes of the low over the high frequency components, Sj(0)/Sh(0), is larger in the absence of cGMP than in its presence. In contrast, the noise of the current through the leak conductance between electrode and membrane patch does not exhibit a Lorentzian component over the frequency tested. 6. The maximum value of the cGMP-dependent conductance over all membrane patches sampled varies over a range of about 10-fold. Over this range, however, there is a linear relationship between the cGMP-dependent and cGMP-independent conductances. The sum of our results suggest that the cGMP-independent conductance arises from the spontaneous activity of the ion channels ordinarily gated by cGMP. The cGMP-independent conductance is about 4 5 % of that measured under saturating cGMP concentrations. 7. In the absence of divalent cations and cGMP, the spontaneous channel activity sustains a mean (± S.D.) membrane conductance of 0-18 + 0-11 nS in the detached patches. This predicts an outer segment conductance in the intact cell that is about 500-fold larger in magnitude than the conductance actually measured in physiological solutions under bright light, when the cytoplasmic cGMP concentration is expected to be nil. This difference cannot be explained by the known effect of divalent cations on the unitary conductance of cGMP-gated channels alone. Divalent cations, therefore, must also control the probability of channel opening and, probably, prevent spontaneous channel activity.In a number of sensor...

Molecular cloning, functional expression and chromosomal localization of a human homolog of the cyclic nucleotide‐gated ion channel of retinal cone photoreceptors

“…The brief openings were also detected in the absence of cGMP, but not in mock-transfected cells. These may have resulted from a very low residual cGMP concentration in the bath solution in our experiments, or from spontaneous channel openings which have been described for the native cone channel in fish [45]. At +60 mV, the single-channel conductance is ,-~ 38 pS for the long openings.…”

Cyclic Nucleotide-Gated Ion Channels

“…One of the three predictions of the model of Monod et al (288) is satisfied by experiment, and two are not. In fact, CNG channels show ligand-independent openings (200,323,348,349,393). The spontaneous open probability (P sp ) is significantly lower for A1 than for A2 channels [P sp ϭ 1.25 ϫ 10 Ϫ4 for a chimeric channel comprising the A1 sequence and an olfactory A2 pore, P sp ϭ 1.5 ϫ 10…”

Allosteric proteins after thirty years: The binding and state functions of the neuronalα7 nicotinic acetylcholine receptors

“…For G-protein coupled receptors, constitutive mutants demonstrate that a conformational transition can occur in the absence of the triggering ligand [58]. In addition, for cyclic nucleotide-gated channels evidence of spontaneous openings has recently been punished [59,60]. However, a further complication for the ligand- Experientia 52 (1996), Birkh/iuser Verlag, CH-4010 Basel/Switzei-land Reviews .…”