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D Bownds

Publications and source records attributed to D Bownds.

12 recordsLinked to original sources
PubMed1977

Guanosine 3',5'-cyclic monophosphate and the in vitro physiology of frog photoreceptor membranes.

Frog rod outer segments freshly detached from dark-adapted retinas contain approximately 1-2 molecules of guanosine 3',5'-cyclic monophosphate (cyclic GMP) for every 100 molecules of visual pigment present. This cyclic GMP decays to 5'-GMP, and the conversion is accelerated upon illumination of the outer segments. Bleaching one rhodopsin molecule can lead to the hydrolysis of 1,000-2,000 molecules of cyclic GMP within 100-300 ms. The decline in cyclic GMP concentration becomes larger as illumination increases, and varies with the logarithm of light intensity at levels which bleach between 5 X 10(2) and 5 X 10(5) rhodopsin molecules per outer segment-second. Light suppression of plasma membrane permeability, assayed in vitro as light suppression of outer segment swelling in a modified Ringer's solution, occurs over this same range of light intensity. The correlation between cyclic GMP and permeability or swelling is maintained in the presence of two pharmacological perturbations: papaverine, a phosphodiesterase inhibitor, increases both cyclic GMP levels and the dark permeability of the plasma membrane; and beta,gamma-methylene ATP increases the effectiveness of light in suppressing both permeability and cyclic GMP levels.

Animals↗
PubMed1976

Biochemical correlates of adaptation processes in isolated frog photoreceptor membranes.

Frog rod outer segments isolated in suspension can maintain much of their in vivo activity. This observation provides us with a simpler system than the intact retina for correlating biochemical and physiological changes. The relevant physiological process, a decrease of sodium permeability by illumination, is assayed as light suppression of outer segment swelling in a modified Ringer's solution. We report here that this decrease is observed over approximately 4 log units of input light intensity and varies with the logarithm of intensity at light levels which bleach between 5.102 and 5.104 rhodopsin molecules/outer segment-second. In this illumination range responsiveness to light decreases as intensity increases. This sensitivity control system may be linked to light-activated rhodopsin phosphorylation, for inhibitors of this reaction increase light sensitivity. The presence of a second system, which controls the maximum amplitude of in vitro response to light, is revealed in experiments with cyclic nucleotide phosphodiesterase inhibitors. Papaverine addition raises intracellular cyclic GMP (guanosine monophosphate) levels and increases the magnitude of the dark permeability, but does not have a large influence on the amount of illumination required for suppression of this permeability. The data suggest that sensitivity and amplitude, as they are expressed in this in vitro system, are regulated by pharmacologically distinct pathways which use two different light-sensitive enzyme systems.

Adaptation, Ocular↗
PubMed1975

Light-sensitive swelling of isolated frog rod outer segments as an in vitro assay for visual transduction and dark adaptation.

Frog rod outer segments swell slowly after being shaken from an excised retina into a modified Ringer's solution. The swelling has the following characteristics: (a) It is suppressed by illumination which bleaches only 500 rhodopsin molecules per outer segment per second. This is approximately the level required to saturate the in vivo receptor potential. (b) Light suppression is seen in NaCl but not in KCl solutions. (c) Dark swelling is labile and is enhanced by calf serum, low calcium concentrations, dithiothreitol, and cyclic nucleotide phosphodiesterase inhibitors. (d) Lowering the pH to 5.5 or removing magnesium reversibly reduces dark swelling to the same extent as illumination. (e) The amount of light required for maximal suppression of dark-swelling increases approximately 10-fold if the calcium concentrations is lowered by EGTA addition. (f) The effect of illumination is irreversibly abolished by antimycin and other inhibitors of mitochondrial electron transport. (g) A process analogous to dark adaptation in vivo can be observed: If 10-50% of the rhodopsin present is bleached and the outer segments are then kept dark, rapid dark swelling returns after a period of 15-45 min. This swelling is again sensitive to light. We tentatively ascribe the light suppression of swelling to the same decrease in sodium permeability which is observed on illuminating living receptor cells. The experiments suggest that outer segments retain their competence to perform both transduction and dark adaptation after their separation from the retina.

Animals↗
PubMed1971

Characterization and analysis of frog photoreceptor membranes.

Frog photoreceptor membranes contain 54,000 g of protein per mole of visual pigment chromophore, virtually all of it insoluble membrane protein. Acrylamide gel electrophoresis indicates one major polypeptide class, most likely the visual pigment apoprotein. Suspensions of these photoreceptor membranes accumulate calcium ions when ATP is present, a characteristic that may play a part in visual excitation.

Adenosine Triphosphatases↗