Rod disk calcium movement and transduction: a poorly illuminated story

A historical perspective on the lateral diffusion model of GTPase activation and related coupling of membrane signaling proteins

Aspects of our discovery of lateral diffusion of the G protein coupled receptor (GPCR) rhodopsin and that a single activated rhodopsin can non-covalently catalyze GTP binding to thousands of GTPases per second on rod disk membranes via this diffusion are summarized herein. Rapid GTPase coupling to membrane-bound phosphodiesterase (PDE) further amplifies the signal via cGMP hydrolysis, essential to visual transduction. Important generalizations from this work are that biomembranes can uniquely concentrate, orient for reaction and provide a solvent appropriate to rapid, powerful and appropriately controlled sequential interaction of signaling proteins. Of equal importance to function is timely control and termination of such powerful amplification via receptor phosphorylation (quenching) and arrestin binding. Downstream kinetic modulation by GTPase activating proteins (GAPs) and regulators of G protein signaling (RGS) and related mechanisms as well as limitations set by membrane domain fencing, structural protein binding etc. can be essential in relevant systems.

Heat produced by the dark-adapted bullfrog retina in response to light pulses

By using a pyroelectric detector constructed with a polyvinylidene fluoride film, a rapid rise in the temperature of the dark-adapted bullfrog retina induced by light was demonstrated. In the bullfrog retina, as in the squid retina examined previously, the heat generated in response to a brief light pulse was found to be far greater than the amount produced by conversion of the entire radiant energy of the stimulus into heat. The thermal responses consist of the heat generated by the photoreceptor and the postsynaptic elements in the retina, preceded by a small signal reflecting conversion of a portion of the radiant energy of the stimulus into heat. The dependence of the thermal responses on the light intensity, on the wavelength and on a variety of physical and chemical agents was examined. The exothermic process underlying the production of heat by the photoreceptor was found to precede the electrophysiological response of the retina.

Calcium content and calcium exchange in dark-adapted toad rods

We have used laser-activated micro mass analysis (l.a.m.m.a.) and energy-dispersive X-ray analysis (e.d.x.) to measure Ca content and Ca movements in 'red' rod photoreceptors in the dark-adapted retina of the toad, Bufo marinus. Measurements with both l.a.m.m.a. and e.d.x. show that intact rod outer segments contain 4-5 mmol total Ca/l wet tissue volume, or 1-2 Ca per rhodopsin. We could detect no significant variation in the total Ca as a function of distance across or up and down the outer segment. In the inner segment, Ca could be detected only within the mitochondria-rich ellipsoid body, where the total Ca concentration was of the order of 100-400 mumol/l wet tissue volume. To measure the exchange of Ca in outer segments from intact photoreceptors, we exposed the dark-adapted retina to Ringer containing the stable isotope 44Ca. Since l.a.m.m.a. can measure separately the concentrations of each of the isotopes of the elements, and since native rods contain almost exclusively 40Ca, the increase in 44Ca and decrease in 40Ca could be used as a measure of Ca influx and efflux. Ca exchange in intact rod outer segments in darkness is very slow. The rate of accumulation of 44Ca was only 10(5) Ca/rod.s, or about 10% of the total outer segment Ca/h. This slow rate of exchange is apparently not the result of restricted movement of Ca across the plasma membrane. Ca exchange was also measured in outer segments which were either partially or entirely detached from the rest of the photoreceptor. In broken-off outer segments, Ca exchange is faster than in the intact organelles, and in 1 h, half of the 44Ca exchanges for 40Ca. When the retina was incubated in Ringer for which all of the Na was substituted with Li or choline, there was an increase in the rate of 44Ca accumulation in intact outer segments, probably due to an inhibition of Na-Ca counter transport across the plasma membrane. Our measurements indicate that the great majority of the Ca in the rod appears to be inaccessible to exchange under physiological conditions, probably because it is sequestered within the disks which in intact rods appear to be nearly impermeable to Ca in darkness.

Bound trace element content of bovine retinal disk membranes as determined by particle-induced x-ray emission

Particle-induced x-ray emission (PIXE) was used to determine the trace element content of bovine retinal disk membranes. PIXE is a multielemental analytical technique capable of the simultaneous detection and quantization of all elements from sodium and above in atomic number. The multielemental capability of PIXE allows the analysis time per element to be very low if a number of elements are detected in each sample. In addition, the multielemental capability of PIXE can be used to determine elemental content with respect to an internal reference. Here the content of detected trace element per rhodopsin was determined without recourse to an external rhodopsin assay. This was accomplished by using the sulfur content of rhodopsin as an internal reference. Detected trace element contents per rhodopsin were 1.58 +/- 0.049 Ca, 0.081 +/- 0.024 Fe, 0.393 +/- 0.200 Cu, and 0.150 +/- 0.031 Zn. Upper limits were placed on the amount of manganese, molybdenum, and nickel per rhodopsin as 0.019, 0.019, and 0.006, respectively. Two proteins known to be present in disk membranes, retinol dehydrogenase and a large protein, approximately 238,000 mol wt, are considered as potential metallo-proteins. No correlation was observed between the content of any detected element and bleaching levels.