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M C Cornwall

Publications and source records attributed to M C Cornwall.

47 records · Page 3Linked to original sources
PubMed1983

Colour dependence of the early receptor potential and late receptor potential in scallop distal photoreceptor.

1. Intracellular voltage and current responses to short (blue) and long (red) wave-length lights were measured in the distal hyperpolarizing photoreceptor (;off receptor') of the isolated and perfused scallop (Pecten irradians) retina.2. The early receptor potential (e.r.p.) was isolated by holding membrane potential at the reversal potential for the late receptor potential (l.r.p.) or by working at temperatures (< 5.0 degrees C) that abolished the l.r.p.3. The e.r.p., measured using intense flashes of white light, consisted of a positive phase followed by a negative phase, but was converted to a monophasic, negative-going wave following pre-adaptation with red light and to a monophasic, positive-going wave following pre-adaptation with blue light.4. The spectral sensitivity curve for the negative e.r.p. was maximum at 500 nm, whereas the spectral sensitivity curve for the positive e.r.p. was maximum at 575 nm.5. The positive or negative e.r.p.s approached their maximum amplitude exponentially when tested with red or blue flashes of increasing intensity. The results suggest that the positive (or negative) e.r.p. is proportional to the number of photopigment molecules photo-isomerized.6. The photosensitivity maximum of rhodopsin calculated at 500 nm, using the exponential constant and the spectral sensitivity data, was estimated to be 2.1 x 10(-16) cm(2) photon(-1), whereas the photosensitivity maximum of metarhodopsin calculated at 575 nm was estimated to be 2.6 x 10(-16) cm(2) photon(-1).7. In cells pre-adapted with white light, stimulation with blue light caused a hyperpolarizing l.r.p. which was followed by a prolonged hyperpolarizing after-potential (p.h.a.). Stimulation with red light under similar conditions caused an initial hyperpolarization which was followed by a small depolarization during the stimulus, but no after-potential.8. The duration of the p.h.a. was increased by pre-adaptation with a red light, which caused the maximum net transfer of metarhodopsin to rhodopsin; however, its decay was always complete in 5 min or less.9. The photo-isomerization of metarhodopsin by red light suppressed the p.h.a. and caused an after-depolarizing response that decayed in less than 1 min.10. The spectral sensitivity curve for the induction of the p.h.a. was maximum at 500 nm and corresponded to the spectral sensitivity for the negative e.r.p. and for the l.r.p. studied in the dark-adapted retina, whereas the spectral sensitivity curve for the suppression of the p.h.a. and for the induction of the after-depolarization was maximum at 575 nm and corresponded to the spectral sensitivity for the positive e.r.p.11. In photoreceptors clamped to the resting potential in normal ASW, the photo-isomerization of rhodopsin, in the absence of light absorption by metarhodopsin, activated a persistent outward current that had the same time course of decay as the p.h.a. The photo-isomerization of metarhodopsin suppressed the persistent outward current and activated an inward current whose decay took longer than the decay of the after-depolarizing response.12. In the absence of external Ca(2+) and Na(+) ions, the persistent outward current produced by light absorption by rhodopsin, and the inward current produced by light absorption by metarhodopsin, both reversed at the K(+) equilibrium potential. The results show that the induction of the prolonged hyperpolarizing after-potential and the after-depolarizing response involve only the movement of K(+) ions through the same light-dependent K(+) channels that determine the hyperpolarizing l.r.p. of the distal cells.

Action Potentials↗
PubMed1982

The prolonged hyperpolarizing afterpotential in an invertebrate photoreceptor: wavelength and ionic dependence.

A single electrode voltage clamp was used to examine the prolonged hyperpolarizing afterpotential (PHA) which accompanies photoconversion of a substantial fraction of rhodopsin (lambda max = 500 nM) to metarhodopsin (lambda max = 575 nM) in distal photoreceptor cells in the retina of the bay scallop, Pecten irradians. The PHA appears to result from a persistent light-activated outward K+ current passing through the same channels responsible for the normal receptor potential in these cells.

Animals↗
PubMed1982

Selectivity of the Ca2+-activated and light-dependent K+ channels for monovalent cations.

The ionic selectivity of the Ca(2+)-activated K(+) channel of Aplysia neurons and of the light-dependent K(+) channel of Pecten photoreceptors to metal and organic cations was studied. The selectivity sequence determined from reversal potential measurements is T1(+) K(+) > Rb(+) > NH(+) (4) > Cs(+) > Na(+), Li(+) and is identical to the sequence determined previously for voltage-dependent K(+) channels in a variety of tissues. Our results suggest that some physical aspect of the K(+) channel is conserved in phyllogenetically different tissues and cells.

Animals↗
PubMed1981

Glass microelectrode tip capacitance: its measurement and a method for its reduction.

The frequency response of input amplifiers used for measurement of bioelectric signals from small cells is severely limited by the resistance and capacitance of the fine glass microelectrodes which are required for these measurements. A significant improvement in the frequency response can be realized by employing the technique of input capacitance neutralization. This capacitance neutralization, however, is incomplete since: (1) the bandwidth of the input amplifier is finite; and (2) a fraction of the electrode capacitance is isolated from the amplifier input by a part of the electrode resistance and cannot be compensated. It is therefore desirable to ensure that the electrode capacitance is as low as possible before neutralization. A method is discussed for measuring and predicting the distributed capacitance of the microelectrode and a technique is described for coating the outside of the electrode near the tip with a substance which can lower the electrode capacitance per unit length by as much as 7-fold. The significance of the improved frequency response of the input amplifier that this technique provides is discussed in light of recent advances in intracellular single electrode voltage clamp technique.

Animals↗
PubMed1979

Contribution of calcium and potassium permeability changes to the off response of scallop hyperpolarizing photoreceptors.

1. The membrane response of the distal photoreceptors in the retina of the scallop Pectin irradians to the termination of a bright white light (off response) is shown to be composed of the decay of the hyperpolarizing receptor potential and an action potential with slow kinetics. 2. The action potential can be produced in darkness in the absence of external Na+ ions by membrane depolarization. 3. The action potential is maintained by replacement of external Ca2+ with Sr2+ or Ba2+, but not by Mg2+. In normal external Ca2+ (9mM), the action potential is abolished by the addition of the Ca2+ inhibitors, La3+, Co2+, and Mn2+ or the organic Ca2+ antagonist D-600. 4. Elevated external Ca2+ concentrations increase the rate of rise and peak amplitude of the action potential as well as the rate of repolarization and after hyperpolarization, but decrease the duration. 5. The rate of rise and peak amplitude of the action potential are increased by the K+ antagonists tetraethylammonium (TEA) 4-amino-phyridine (4-AP), Ba2+ and procaine. The antagonists have different effects on subsequent phases of the response, however. External TEA and Ba2+ increase the duration, but decrease the rate of repolarization and abolish the after hyperpolarization, whereas external 4-AP and procaine increase the rate of repolarization, decrease the duration and increase the after hyperpolarization. 6. The ratio of the Ca2+ to K+ permeability (P Ca/P K) estimated from the constant field equation at the peak of the action potential in different external Ca2+ concentrations is close to 1. 7. The maximum rate of rise and the peak amplitude of the action potential are increased by membrane hyperpolarization and decreased by membrane depolarization. They are decreased by background light intensity relative to their value in the dark. 8. In normal ASW the action potential can be identified during the off response as a small overshoot of membrane potential relative to its value in the dark. 9. The rate of repolarization of the off response in normal ASW is reduced by agents or conditions which inhibit or reduce Ca2+ permeability changes, e.g. external Co2+ and La2+ or zero external Ca2+. 10. Our results suggest that a voltage-dependent increase in membrane permeability to Ca2+ and to K+ ions modifies the repolarizing phase of the receptor potential.

Action Potentials↗
PubMed1975

Spectral correlates of a quasi-stable depolarization in barnacle photoreceptor following red light.

1. Illumination of B. eburneus photoreceptors with intense red light produces a membrane depolarization that persists in darkness. This quasistable depolarization (latch-up) can be terminated with green light. The phenomenon was investigated with electrophysiological, spectrochemical, and microspectrophotometric techniques. 2. Latch-up was associated with a stable inward current in cells with the membrane potential voltage-clamped at the resting potential in darkness. The stable current could only be elicited at wave-lengths greater than 580 nm. 3. Light-induced current (LIC) was measured at various wave-lengths in dark-adapted photoreceptors with the membrane voltage-clamped to the resting potential. The minimum number of photons required to elicit a fixed amount of LIC occurred at 540 nm, indicating that the photoreceptor is maximally sensitive to this wave-length of light. The photoreceptor was also sensitive to wave-lengths in the near-U.V. region of the spectrum (380-420 nm). 4. Steady red adapting light reduced the magnitude of the LIC uniformly at all wave-lengths except in the near-U.V. region of the spectrum; sensitivity was reduced less in this region. 5. The spectrum for termination of the stable inward current following or during red light was shifted to the blue (peak about 510 nm) compared to the peak for LIC (peak about 540 nm). 6. Absorbance of single cells prepared under bright, red light decreased maximally at 480 nm following exposure to wave-lengths of light longer than 540 nm. 7. A pigment extract of 1000 barnacle ocelli prepared under dim, red light had a maximum absorbance change at 480 nm when bleached with blue-gree light. 8. There was no evidence in the latter two experiments of photointerconversion of pigments with absorbance maxima at 480 and 540 nm. Rather, the maximum absorption of the bleaching products seemed to occur at wave-lengths shorter than 420 nm. 9. Since latch-up induction occurs at wave-lengths longer than 580 nm, it may depend on the 540 pigment or on an undetected red absorbing pigment. 10. A photolabile pigment at 480 nm correlated most closely with termination of the stable inward current associated with latch-up.

Adaptation, Ocular↗
PubMed1975

Ionic mechanism of a quasi-stable depolarization in barnacle photoreceptor following red light.

1. The membrane mechanism of a quasi-stable membrane depolarization (latch-up) that persists in darkness following red light was examined in barnacle photoreceptor with micro-electrode techniques including voltage-clamp and Na+-sensitive micro-electrodes. 2. Current-voltage (I-V) relations of the membrane in darkness following red light (latch-up) and in darkness following termination of latch-up with green light, indicate that latch-up is associated with an increase of membrane conductance. 3. The latch-current (membrane current in darkness following red light minus membrane current in darkness following a gree flash that terminates latch-up) was inward at the resting potential, reversed sign at about +26mV (mean of six cells), and became outward at more positive membrance potentials. 4. Current-voltage relations of the membrane during green light (no latch-up) closely resembled those during latch-up. The light-induced current (LIC) elicited by green ligh (membrane current during the light flash minus membrane current in darkness following the light flash) was inward from the resting potential to +26mV (mean of six cells), then reversed sign and became outward. 5. The latch-current and LIC were both augmented in reduced Ca2+ solutions and decreased as Na-+ was reduced at a fixed Ca2+ concentration. 6. Both LIC and latch-current reversed sign at a more negative membrane potential (increment V equals 14mV) in solutions containing one quarter the normal amount of Na+. 7. The internal Na-+ activity (a-iNa) of a photoreceptor increased from about 10-18 mM upon illumination with long steps of intense red or white illumination. Five minutes in darkness after white light, a-iNa had recovered significantly, whereas a-iNa remained elecated following red illumination. 8. Latch-up seems to be a persistence in darkness of the same membrane mechanism that normally occurs during illumination; i.e. a conductance increase to Na+ ions. Ca2+ ions act primarily to suppress this current. There is evidence for a net Na+ influx during illumination that is sustained in darkness during latch-up.

Animals↗
PubMedDate not supplied

Movement of retinal along cone and rod photoreceptors.

Single isolated photoreceptors can be taken through a visual cycle of light adaptation by bleaching visual pigment, followed by dark adaptation when supplied with 11-cis retinal. Light adaptation after bleaching is manifested by faster response kinetics and a permanent reduction in sensitivity to light flashes, presumed to be due to the presence of bleached visual pigment. The recovery of flash sensitivity during dark adaptation is assumed to be due to regeneration of visual pigment to pre-bleach levels. In previous work, the outer segments of bleached, light-adapted cells were exposed to 11-cis retinal. In the present work, the cell bodies of bleached photoreceptors were exposed. We report a marked difference between rods and cones. Bleached cones recover sensitivity when their cell bodies are exposed to 11-cis retinal. Bleached rods do not. These results imply that retinal can move freely along the cone photoreceptor, but retinal either is not taken up by the rod cell body or retinal cannot move from the rod cell body to the rod outer segment. The free transfer of retinal along cone but not along rod photoreceptors could explain why, during dark adaptation in the retina, cones have access to a store of 11-cis retinal which is not available to rods. Additional experiments investigated the movement of retinal along bleached rod outer segments. The results indicate that retinal can move along the rod outer segment, but that this movement is slow, occurring at about the same rate as the regeneration of visual pigment.

Ambystoma↗
PubMedDate not supplied

Evidence for the prolonged photoactivated lifetime of an analogue visual pigment containing 11-cis 9-desmethylretinal.

Following bright flashes, rod photoreceptors exhibit a period of photocurrent saturation that increases linearly with the logarithm of flash intensity. In a recent report, Pepperberg et al. (1992) presented evidence that the slope of the function relating the saturation period (T) to the natural logarithm of flash intensity (ln If) represents the exponential lifetime (tau) of photoactivated visual pigment: tau = delta T/delta [ln If]. In salamander rods, 11-cis 9-desmethylretinal combines with opsin to form 9-desmethyl rhodopsin. Dim flash responses mediated by this analogue visual pigment exhibited slow recovery kinetics relative to those of native pigment (Corson et al., 1991). This observation raises the hypothesis that the physiological lifetime of photoactivated 9-desmethyl rhodopsin is substantially longer than that of native visual pigment. To test this hypothesis, we have examined the relation between the period of photocurrent saturation and flash intensity in salamander rods containing a mixture of the two pigments. Brief stimuli at two widely separated wavelengths (440 and 640 nm) elicited saturating photocurrent responses that were preferentially mediated by 9-desmethyl rhodopsin or residual native pigment, respectively. Plots of T vs. ln If revealed a linear increase in the period of response saturation over a large range of saturating intensities at both wavelengths. However, the slope of the relation between T and ln If with 440-nm flashes was more than twice as large (4.1 +/- 0.5 s, n = 5) as that measured with 640-nm flashes (1.7 +/- 0.4 s). For rods subjected only to bleaching of the native pigment, or to bleaching and resensitization with 11-cis retinal, the slope of the relation between T and ln If remained independent of wavelength and indistinguishable from that of native pigment in unbleached cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗