Hindered diffusion in excised membrane patches from retinal rod outer segments

Localization of phototransduction in Limulus ventral photoreceptors: a demonstration using cell-free rhabdomeric vesicles

Second messengers are involved in a number of cellular responses to a variety of stimuli. Diffusion of these second messengers likely will determine the speed and efficiency of such responses. Localization, particularly in large cells, would enhance the efficiency of such transduction systems by restricting the volume in which this diffusion takes place and thereby limiting the diffusion of soluble messengers. Phototransduction in Limulus ventral photoreceptors involves second-messenger systems; the volume of this cell is quite large, but the effect of a single photoexcited rhodopsin molecule is exerted over light-dependent channels localized within a very small area of the plasma membrane. In order to investigate localization of phototransduction in these photoreceptors, we have compared the light responses of small vesicles (photoballs) excised from these cells with those of the intact photoreceptors. We found that the basic kinetics of excitation and adaptation of the photoballs are essentially identical to those of the intact cell. This indicates that all of the necessary machinery for phototransduction is present and intact in the photoball and that any diffusion of second messengers that affect the normal light response of the cell must occur within a region at least as small as our photoballs (on the order of 1 micron3).

Blockage and permeation of divalent cations through the cyclic GMP-activated channel from tiger salamander retinal rods

1. Blockage and permeation of divalent cations through channels activated by guanosine 3',5'-cyclic monophosphate (cyclic GMP) were studied in membrane patches excised from retinal rods of the tiger salamander Ambystoma tigrinum by rapidly changing the ionic medium bathing the intracellular side of the excised membrane. 2. The Na+ current, observed when 110 mM-NaCl was present on both sides of the membrane patch, was reduced by the addition of 1 mM of the chloride salts of Ca2+, Mg2+, Sr2+, Ba2+ or Mn2+ to the bathing medium. The sequence of blocking potency at +60 mV was Mg2+ greater than Mn2+ approximately Ba2+ greater than Ca2+ greater than Sr2+, while at -60 mV it was Ba2+ greater than Ca2+ greater than Sr2+ greater than Mn2+ approximately Mg2+. For all divalent cations the blocking effect depended, in a complex way, on the membrane potential. 3. The blocking effect of Ca2+ and Mg2+ increased when the concentration of cyclic GMP was reduced from 100 to 5 microM. At -60 mV 1 mM-Ca2+ blocked about 34% of the Na+ current in the presence of 100 microM-cyclic GMP, while in the presence of 5 microM-cyclic GMP, 1 mM-Ca2+ blocked about 56% of the Na+ current. 4. When, in the presence of 100 microM-cyclic GMP, 110 mM-NaCl at the intracellular side was replaced by equiosmolar amounts of chloride salts of divalent cations (73.3 mM) a small outward current carried by divalent cations could be observed at large positive membrane potentials. At +60 mV the ratio between the current carried by Na+, Sr2+, Ca2+, Ba2+, Mg2+ and Mn2+ was 83.3:1.4:1:0.58:0.33:0.25. 5. In agreement with previous observations the dependence of the Na+ current on the concentration of cyclic GMP shows a clear co-operativity among cyclic GMP molecules.4+ cyclic GMP-gated channel in excised patches is similar to but not identical to the selectivity sequence of divalent cations through the channel in intact rods.

Voltage-dependent gating and block of the cyclic-GMP-dependent current in bovine rod outer segments

The properties of the cyclic-GMP-activated conductance in the plasma membrane of bovine rod outer segments were studied in excised membranes. Multiple-channel and single-channel currents were recorded by the patch-clamp technique in symmetrical NaCl solutions which were free of divalent cations. The current-voltage relationship for the current, recorded when a large population of channels was activated, exhibited outward rectification. Rectification decreased as the concentration of cyclic-GMP was increased, and the concentration of cyclic-GMP required for half maximal activation of the channel decreased with depolarization. At a concentration of 1-3 microM cyclic-GMP, single-channel activity could be observed from these excised patches. The conductance of the open channel was 6 pS and was independent of the membrane potential. These results are consistent with the interpretation that under these conditions, the mechanism responsible for the outward rectification is due to an increase in the probability of an open channel as the membrane is depolarized. The cyclic-GMP-activated current could be blocked by L-cis-diltiazem. Block was voltage and time dependent. The time constant for the onset of block and its steady state level increased with depolarization. The extent of block by diltiazem was not enhanced as the cyclic-GMP concentration was increased, suggesting that the channel is not required to be open for block to occur. Complete block was never attained even for high concentrations of diltiazem. However, the diltiazem-resistant component of the cyclic-GMP-activated current could be blocked by tetracaine.

Excised patches of plasma membrane from vertebrate rod outer segments retain a functional phototransduction enzymatic cascade

Ion channels in excised patches of plasma membrane are generally considered to be isolated from any intracellular regulation mechanisms. For example, in excised patches of vertebrate rod outer segment plasma membrane, the cGMP-activated cation channels have traditionally been studied in room light because the enzyme cascade linking photon absorption to channel closure was assumed to be inoperative. To investigate the possibility that, in fact, such excised patches retain a functional phototransduction enzymatic cascade, this same preparation was studied in darkness. Patches excised in the dark were found to retain the light sensitivity of their cGMP-induced conductance and the ability to synthesize cGMP. In the presence of guanosine 5'-[gamma-thio]triphosphate (GTP[gamma S]), a nonhydrolyzable GTP analog, light suppresses the cGMP-induced conductance irreversibly. Furthermore, inhibitors of phosphodiesterase activity reduce light sensitivity, whereas activated phosphodiesterase or activated transducin does not directly affect the channels. These results (i) establish that excised patches from rod outer segment retain functional phototransduction enzymes, (ii) support the classical view that channel opening is modulated by phosphodiesterase-mediated cGMP hydrolysis, and, most surprisingly, (iii) demonstrate that diffusion in excised patches is so restricted that local enzymes can induce variations in the concentration of small molecules. The indication that excised patches are not as simple as usually surmised opens the possibility of using them to study other intracellular transduction mechanisms.

Currents carried by monovalent cations through cyclic GMP-activated channels in excised patches from salamander rods

1. Ionic selectivity and affinity for monovalent cations of channels activated by guanosine 3',5'-cyclic monophosphate (cyclic GMP) were studied in excised inside-out patches of plasma membrane from retinal rods of the tiger salamander. Channels were activated by addition of cyclic GMP to the medium bathing the cytoplasmic side of the membrane. The ionic solution at the cytoplasmic side was rapidly changed using the method of Nunn (1987 a). 2. Permeability ratios were calculated with the Goldman-Hodgkin-Katz potential equation from reversal potential measurements for alkali monovalent cations in bi-ionic conditions. The permeability sequence was: Li+:Na+:K+:Rb+:Cs+ = 1.14:1:0.98:0.84:0.58. 3. The selectivity sequence obtained from macroscopic current measurements in bi-ionic conditions at +100 mV was: Na+:K+:Rb+:Li+:Cs+ = 1:1:0.67:0.36:0.25. 4. The organic cations tetramethylammonium (TMA+), choline and tetraethylammonium (TEA+) were not permeant through the cyclic GMP-activated channels and caused a reduction of the Na+ inward current. At -100 mV the current ratio for inward current was 1:0.75:0.58:0.2 in the presence, at the cytoplasmic side, of 110 mM-Na+, TMA+, choline or TEA+ respectively. 5. The concentration dependence of the macroscopic current and the reversal potential was studied by changing the internal concentration of Na+ or K+ or Li+ from 5 mM to 500 mM. The permeability ratios were nearly constant regardless of the permeant ion concentration. 6. The current as a function of internal ion activity could be described by a Michaelis-Menten relation with a half-saturating activity, Km, at +90 mV equal to 249, 203 and 160 mM for Na+, K+ and Li+ respectively. The ratio of the extrapolated saturating current Imax at +90 mV was 1:0.86:0.26 for Na+, K+ and Li+ respectively. 7. The outward currents and the reversal potentials measured in different mixtures of Na+ and Li+ were monotonic function of the mole fraction. 8. These results can be explained by assuming that, at least in a narrow region, the cyclic GMP-activated channel is a one-ion channel, possibly with other poorly voltage-dependent binding sites in a large inner vestibule.