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P Fatt

Publications and source records attributed to P Fatt.

At least 19 recordsLinked to original sources
PubMed1982

An extended Ca2+-hypothesis of visual transduction with a role for cyclic GMP.

A model is described having the following features: Light induces Ca2+ release from vertebrate rod outer segments discs via pores composed of multimeric rhodopsin. Cytoplasmic Ca2+ reversibly blocks Na+ channels of the surface membrane, with the time course of development and amplitude of the response to light being influenced by restrictions on intradiscal Ca2+ diffusion. The falling phase of response reflects a decline in cytoplasmic [Ca2+] due to a Ca2+-binding protein controlled by cyclic GMP so that its binding capacity is increased by the reduction in cytoplasmic [cyclic GMP] which follows rhodopsin bleaching.

Animals↗
PubMed1973

An analysis of light-induced admittance changes in rod outer segments.

1. Measurements were made of the time course and amplitude of the change in real part of admittance, DeltaG, of a suspension of frog rod outer segments, following a flash of light bleaching about 1% of the rhodopsin content of the rods. The measurements, based on the use of a specially designed marginal oscillator, covered the frequency range between 500 Hz and 17 MHz.2. The components of response, previously described for rods prepared by a method involving exposure to strongly hypertonic sucrose solutions, are present in similar form when rods are isolated and maintained in isotonic solutions made up with equi-osmotic concentrations of NaCl and sucrose or with Na(2)SO(4).3. Component I, identified as a slowly developing positive DeltaG apparent at very low frequencies, is frequency-independent up to the characteristic frequency of admittance for the suspension, f(Y) (about 2 MHz for rods suspended in a solution having the conductivity of Ringer solution), but decreases at still higher frequencies.4. Component II, identified as a rapidly developing positive DeltaG which appears only above a critical frequency about 2.5 decades below f(Y), increases approximately logarithmically with frequency to reach a limiting amplitude in the region of f(Y).5. The amplitude of component II, DeltaG(II), measured in the region of f(Y), varies linearly with the conductivity of the suspending medium, G(o), under conditions in which the conductivity of the rod interior is also a linear function of the external conductivity. The relation for a flash bleaching 1% of the rhodopsin content of the dark-adapted rod is [Formula: see text]6. Measurements made on rods suspended in a low-conductivity solution, which has the effect of reducing the conductivity of the rod interior to about one ninth its value for rods suspended in Ringer solution, reveal a decline in component II for frequencies above 8 MHz.7. To explain the frequency dependence of component II and its dependence on conductivity, it is proposed that component II arises from a light-induced increase in conductance of the disk membranes which obstruct the longitudinal flow of current through the rod interior except at very high frequencies.8. The disk-membrane conductance increase for rods suspended in a solution having the conductivity of Ringer solution is calculated to be 4.3 x 10(-11) mho/rhodpsin molecule bleached, a value which is similar to what has been found for ionic channels operated by membrane potential change in the nerve membrane and by synaptic transmitter in the postjunctional membrane.9. No component of response has been observed which could be reliably attributed to a surface membrane conductance decrease of the type observed in receptor cells in the retina.

Animals↗
PubMed1973

Isolation of components of admittance change in rod outer segments.

1. Rods were separated by equilibration on a bovine serum albumin (BSA) density gradient into two major fractions, differing in their response to light.2. In one fraction the response, measured as a change in the real part of admittance DeltaG, appeared to consist exclusively of component I, while in the other, component II was prominent.3. Evidence is presented that component I arose in damaged rods. This follows from observations on rods which have been deliberately damaged by freezing followed by thawing, or by fragmentation.4. In such damaged rods, component II was absent while component I was increased in amplitude.5. The frequency dependence of component I in isolation was characterized as a positive DeltaG of constant amplitude from low frequencies up to the characteristic frequency f(Y) for the major dispersion of admittance. Above this frequency, it declined to a variable extent.6. The frequency dependence of component II observed in isolation was consistent with the previous analysis.7. A negative-going DeltaG is described which was linear with the amount of rhodopsin bleached and which was frequency independent up to the highest frequency of measurement (17 MHz).8. The origins of component I and the negative component are discussed.

Animals↗
PubMed1968

Passive electrical properties of rod outer segments.

1. Measurements on a packed suspension of randomly oriented, dark-adapted frog rods at frequencies of 15 c/s-0.5 Mc/s indicate a behaviour similar to that of other biological materials.2. Results are analysed on the assumption that the low-frequency limiting resistance is determined by current flowing in the suspending medium and that, of the rods, two thirds are oriented perpendicular to the applied field and one third parallel to it. Those parallel to the field are treated as non-conductors.3. From the high-frequency limiting resistance the conductivity of the rod interior is calculated to vary linearly with the conductivity of the medium. The slope of the relation of internal to external conductivity is 0.50 with a limiting internal conductivity (at zero external) of 280 mumho/cm.4. On the assumption that the suspension can be represented as a single-capacitance network, the characteristic frequency of impedance is used to calculate a capacitance for the rod surface of 1.54 muF/cm(2). On the assumption of a distribution in properties of the suspension according to the theory of Bruggeman, the capacitance is calculated to have a value of about one half this.5. At frequencies below 5 kc/s the impedance locus deviates from the curve describing the behaviour at higher frequencies. It is suggested that this may involve conduction in a thin layer extending along the surface of the rod.

Animals↗
PubMed1968

Conductance changes produced by light in rod outer segments.

1. Changes in the admittance of rod outer segments produced by illumination with brief flashes were studied by two methods: one, in which maintained changes in real and imaginary parts of admittance were observed in the frequency range 15 c/s-60 kc/s; the other, in which the time course of change in absolute value of admittance (Delta|Y|) was observed at frequencies of 100 kc/s-1.0 Mc/s.2. The response to light absorbed by rhodopsin was resolved into components. One of these components was a transient increase in conductance which arose from a rapid degradation into heat of the light energy. Another component, prominent at high frequencies where the conductivity of the rod interior was accessible to measurement, was produced by the uptake of H(+) by visual pigment in its conversion from metarhodopsin I to metarhodopsin II, causing a change in ionization of buffer.3. Two other components, designated I and II, appeared as maintained changes of admittance involving the organized structure of the rod. Component I appeared as a frequency-independent increase in the real part of admittance (DeltaG), the amplitude of which varied in proportion to the conductivity of the medium, without specificity as to ion species. Component II appeared as a DeltaG which rose linearly with log frequency over the range 1-60 kc/s, while the imaginary part of admittance change (DeltaB) rose to a plateau which was maintained for more than a tenfold frequency range. This component was unaffected by variations in conductivity in the region of low conductivities.4. When rods were suspended in a solution containing 100 mM hydroxylamine, component II no longer appeared as a maintained admittance change while component I was unaffected. Examination of the time course of response showed component II to appear transiently, decaying over the course of 2 sec following a flash.5. Measurements of Delta|Y| for rods in solutions of widely different conductivities showed component II to have a more rapid time course of development than component I and to be only slightly delayed in its early part relative to the buffer component.6. The amplitude of component I varied with temperature to the extent of 4.1%/ degrees C (Q(10) of 1.5) over the range -2-25 degrees C. The amplitude of component II was nearly constant over the range 15-27 degrees C, but fell steeply at temperatures below 10 degrees C, the Q(10) at low temperatures being about 2.4. The effect of temperature on amplitude and time course of component II is consistent with its dependence on the formation and continued presence of metarhodopsin II. The failure of component I to decrease steeply at temperatures below 10 degrees C indicates a dependence on an earlier stage in the thermal conversion of rhodopsin photoproducts.7. With light flashes each bleaching less than 1% of the rhodopsin content of the rod, all components of response were proportional to the amount of rhodopsin bleached (which would be proportional to the light absorbed). For brighter flashes components I and II failed to increase in proportion to the amount of rhodopsin bleached, the deviation from proportionality being greater for component I than for component II. The failure of summation of response extended to successive responses separated by up to 5 min.8. It is suggested that component I arises from a non-selective increase in ionic permeability of the surface membrane of the rod or from a change in rod volume, while component II arises from a change in conduction along the surface membrane.

Animals↗