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Light adaptation in the toad rod: its mechanism and the role of calcium ions
Part I: Abstracts
354
P. A. LCEBMAN (Pennsvlvania) and E. N. PUGH JR ~00trol of phosphodiesterase in rod disk membra~kinetics. vision
*The
possible mechanisms aud significance for
Using a continuous recording method with disk vesicle suspensions, we find that weak impulses of light initiate rapid on-off cycles of cyclic nucleotide (CN) hydrolysis. The “on” component requires GTP cofactor (but will work with y-methylene GTP) and appears to be rate limited by a property resembling rhodopsin-PDE lateral diffusional interaction in the disk membranes. The ‘-off’*component is fueled by ATP. “Off” cannot operate with y-methylene ATP, nor is ordinary ATP effective when “on‘ is activated by 7-MeGTP. Properties of “off” are reminiscent of opsin phosphorylation but rather faster than previously reported. On the assumption that we are studying the membrane-bound steps in visual transduction our findings provide a mechanistic basis for the delays in the photoreceptor response via intramembrane diffusion and for the “explosive” behaviour via enzymatic cascading.
H. KUHN (KFA, Jiilich) Light-recta binding of proteins to cattle photoreceptor membranes Three
proteins of cattle ROS are water-soluble in the dark but membrane-bound after bleaiI,ing. Their polypeptide molecular weights are approx 67,ooO. 48,000 and 37.000. The 67.000 polypeptide corresponds to the enzyme “rhodopsin kinase”. Both the kinase and the 48,000 protein bind to freshly bleached ROS membranes but are then slowly released in the dark. The time course of this release is compared with that of other slow dark reactions after bleaching; it is suggested that rhodopsin or the membrane, after bleaching. rr~ltl.sirnt/~ exhibits binding sites for the kinase and other proteins, and that the activity of some enzymatic reactions in ROS is regulated by this tight- and time-dependent binding.
THE ROLE OF CALCIUM IONS VERTEBRATE TRANSDUCTION
IN
G. B. ARDEN and J. C. LOW The role of calcium ions in pigeon cones
The extracellular phot~urrents recorded from the outer~inner limbs of pigeon cones in an isolated retina preparation can be modified by changing extracellular calcium levels. The immediate effect of decreasing [Ca’ *lUy,from IO-’ to IO-’ M IS to increase the photocurrent ( x 7) without changing the intensity/amplitude relationship. The delayed effects include changes in both sensitivity and response waveform, presumably due to loss of calcium from the cytosol. The effects are not reversed by background illumination. These observations are not consistent with the simplest form of the “‘calcium hypothesis” for cones.
D. BERTRAND (Geneva) Action of ECTA and high calcium on the cones in the turtle retina The effects of Ca”
on the activity of turtle cones were investigated by intracellular recording in perfused retinas. Lowering the external Ca” concentration with EGTA evoked a depolarization of the cones; while responses to bright light were increased, responses to dim flashes were usually reduced. T’he time course of the responses was also modified by EGTA. most prominently in the later phases. An increase of the external Cal + concentration decreased the responses to bright light but had little or no effect on responses to dim flashes; the time course of the responses was not modified. The changes caused by EGTA and high Ca’* can be explained assuming that external Ca’” concentration controls the value of the conductance ci in the model of Baylor, Hodgkin and Lamb.
B. L. BASTIAN (UCLA) Light adaptation in the toad rod: its mechanism and the role of calcium ions
The mechanism of adaptation was studied by intracellular recordings in the rods of Bufo mrinus. Any putative model of adaptation must contend with two findings of this study. First, the mechanism of adaptation is found to be so sensitive that desensitizations in the rod must spread, or be amplified, beyond the initial site of stimulation (a disc). Second, it is not possible, by raising the external Ca*+ concentration, to reproduce the sensitivity or the waveform kinetic changes characteristic of adaptation in the rod. fonophore studies suggest that internal and external Ca” concentrations are correlated. Thus adaptaiion requires an internal transmitter but this agent cannot be Ca”.
354
P. A. LCEBMAN (Pennsvlvania) and E. N. PUGH JR ~00trol of phosphodiesterase in rod disk membra~kinetics. vision
*The
possible mechanisms aud significance for
Using a continuous recording method with disk vesicle suspensions, we find that weak impulses of light initiate rapid on-off cycles of cyclic nucleotide (CN) hydrolysis. The “on” component requires GTP cofactor (but will work with y-methylene GTP) and appears to be rate limited by a property resembling rhodopsin-PDE lateral diffusional interaction in the disk membranes. The ‘-off’*component is fueled by ATP. “Off” cannot operate with y-methylene ATP, nor is ordinary ATP effective when “on‘ is activated by 7-MeGTP. Properties of “off” are reminiscent of opsin phosphorylation but rather faster than previously reported. On the assumption that we are studying the membrane-bound steps in visual transduction our findings provide a mechanistic basis for the delays in the photoreceptor response via intramembrane diffusion and for the “explosive” behaviour via enzymatic cascading.
H. KUHN (KFA, Jiilich) Light-recta binding of proteins to cattle photoreceptor membranes Three
proteins of cattle ROS are water-soluble in the dark but membrane-bound after bleaiI,ing. Their polypeptide molecular weights are approx 67,ooO. 48,000 and 37.000. The 67.000 polypeptide corresponds to the enzyme “rhodopsin kinase”. Both the kinase and the 48,000 protein bind to freshly bleached ROS membranes but are then slowly released in the dark. The time course of this release is compared with that of other slow dark reactions after bleaching; it is suggested that rhodopsin or the membrane, after bleaching. rr~ltl.sirnt/~ exhibits binding sites for the kinase and other proteins, and that the activity of some enzymatic reactions in ROS is regulated by this tight- and time-dependent binding.
THE ROLE OF CALCIUM IONS VERTEBRATE TRANSDUCTION
IN
G. B. ARDEN and J. C. LOW The role of calcium ions in pigeon cones
The extracellular phot~urrents recorded from the outer~inner limbs of pigeon cones in an isolated retina preparation can be modified by changing extracellular calcium levels. The immediate effect of decreasing [Ca’ *lUy,from IO-’ to IO-’ M IS to increase the photocurrent ( x 7) without changing the intensity/amplitude relationship. The delayed effects include changes in both sensitivity and response waveform, presumably due to loss of calcium from the cytosol. The effects are not reversed by background illumination. These observations are not consistent with the simplest form of the “‘calcium hypothesis” for cones.
D. BERTRAND (Geneva) Action of ECTA and high calcium on the cones in the turtle retina The effects of Ca”
on the activity of turtle cones were investigated by intracellular recording in perfused retinas. Lowering the external Ca” concentration with EGTA evoked a depolarization of the cones; while responses to bright light were increased, responses to dim flashes were usually reduced. T’he time course of the responses was also modified by EGTA. most prominently in the later phases. An increase of the external Cal + concentration decreased the responses to bright light but had little or no effect on responses to dim flashes; the time course of the responses was not modified. The changes caused by EGTA and high Ca’* can be explained assuming that external Ca’” concentration controls the value of the conductance ci in the model of Baylor, Hodgkin and Lamb.
B. L. BASTIAN (UCLA) Light adaptation in the toad rod: its mechanism and the role of calcium ions
The mechanism of adaptation was studied by intracellular recordings in the rods of Bufo mrinus. Any putative model of adaptation must contend with two findings of this study. First, the mechanism of adaptation is found to be so sensitive that desensitizations in the rod must spread, or be amplified, beyond the initial site of stimulation (a disc). Second, it is not possible, by raising the external Ca*+ concentration, to reproduce the sensitivity or the waveform kinetic changes characteristic of adaptation in the rod. fonophore studies suggest that internal and external Ca” concentrations are correlated. Thus adaptaiion requires an internal transmitter but this agent cannot be Ca”.