Biomedical subjects
Publications and source records attributed to B Minke.
Many photoreceptor cells in invertebrates have a dual-peak spectral sensitivity. Evidence is presented that in fly photoreceptors the ultraviolet peak is due to a photostable pigment that absorbs light quanta and transfers the energy to the blue-absorbing visual pigment.
The trp is a conditional phototransduction mutant of Drosophila. Direct electrical measurements and shot noise analysis suggest that a prolonged intense light causes in the mutant a reduction in the quantum efficiency for quantum bump production that does not arise from bleaching of the visual pigment. This effect depends on the duration of the light and only weakly on its intensity. In the normal fly, an intense blue light that shifts the visual pigment from rhodopsin to metarhodopsin, induces an excitatory process manifested by a prolonged depolarizing after potential (PDA). In the mutant, the PDA has a small amplitude and bump noise is superimposed on the response. It can thus be shown that the excitatory process underlying the PDA is also present in those trp mutants where the PDA voltage response is small or absent. It is suggested that the absence of the PDA voltage response in the mutant is probably due to a defect in an intermediate process, which links the excitatory process to the membrane conductance change.
We have recently demonstated the dissection of the transduction process in the barnacle photoreceptor into antagonistic "excitor" and "inhibitor" processes. We now show that (a) the interaction between the two processes proceeds even when they are induced in different pigment molecules; (b) the excitor process appears to be slightly facilitated if those pigment molecular unaffected by the stimulus are in the stable metarhodopsin state or slightly inhibited if they are in the rhodopsin state; (c) there is a facilitatory interaction among the excitor processes induced in different pigment molecules. In case a, the interaction has a range of at least a few hundred angstroms, taking place in a time of less than a fraction of a second; in cases b and c, the range could be as little as "nearest neighbors" and the time as much as a few seconds. All these interactions could be intermediated by the "excitor" if it is a transmitter.
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A "complete" and quantitative kinetic model for the states and transitions of the barnacle visual pigment in situ has been constructed from intracellular recordings of the early receptor potential responses to long light pulses. The model involves two stable and four thermolabile states and 10 photochemical, thermal, and metabolic transitions among them. The existence of each state and transition is demonstrated by qualitative examination of the response resulting from a carefully chosen experimental paradigm (combination of intensity, duration, and wavelength of adaptation and stimulation). Quantitative examination of the same responses determines all of the model transition rates, but only puts constraints on the state dipole moments. The latter are determined, and the former refined, by quantitative comparison of the predictions of the complete model with the responses to a set of paradigms chosen to involve as many states and transitions as possible. The fact that good fits can be obtained to these responses without further modification of the model supports its completeness.
The late receptor potential (LRP) recorded in barnacle photoreceptor cells exhibits, at high light levels, a strong dependence on the color of the stimulus and of the preceding adaptation. Most strikingly, red illumination of a cell previously adapted to blue light results in a depolarization which may last for up to 30 min after the light goes off, while blue illumination of a cell previously adapted to red light cuts short this extended depolarization or prevents its induction by a closely following red light. Comparison of the action spectra for the stimulus-coincident LRP and for the extended depolarization and its curtailment with those previously measured for the early receptor potential (ERP) confirms that these phenomena derive from the same bi-stable pigment as the ERP. The stimulus-coincident response and the extended depolarization appear to arise from substantial activation of the stable 532 nm state of the pigment, while activation of the stable 495 state depresses or prevents the extended depolarization and probably also depresses the stimulus-coincident response. Since either process can precede the other, with mutually antagonistic effects, one is not simply the reversal of the other; they must be based on separate mechanisms. Furthermore, comparison with ERP kinetics shows that both processes involve mechanisms additional to the pigment changes, as seen in the ERP. A model is proposed and discussed for the LRP phenomena and their dependences on wavelength, intensity, and duration of illumination based on excitor-inhibitor interactions.
The recovery in the dark of the early receptor potential, as a direct manifestation of the state of the visual pigments, has been studied by intracellular recording in the ventral photoreceptors of Limulus and lateral photoreceptors of Balanus. The recovery is exponential with 1/e time constants of about 80 ms at 24 degrees C for both preparations and 1800 ms at 4 degrees C for Balanus. The 24 degrees C rate extrapolates to total recovery of the pigment within 2 s. The later part of the dark adaptation of the late receptor potential, which may take from seconds to minutes in these preparations, appears thus to be unrelated to the state of the pigment.
The early receptor potential (ERP) in the barnacle photoreceptor is shown by intracellular recording to exhibit a strong dependence on the color of the stimulus and of the preceding adaptation. The adaptation effects appear to be stable for at least 3 h in the dark. Most strikingly, the ERP is positive after red adaptation and mainly negative after blue adaptation. The simplest hypothesis which accounts for these observations is that two thermally stable pigment states with different absorption spectra contribute to the ERP. All ERP responses appear to be consistent with the sums of different ratios of the ERP's of the two pure states. The relative populations of the two states are shown to vary reciprocally, suggesting that the two are states of the same closed pigment cycle. Both states have approximately Dartnall nomogram-shaped absorption spectra, one peaked near 495 nm, and the other near 532 nm.
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Red illumination of a Balanus amphitrite photoreceptor that has been adapted to blue light leads to prolonged depolarization in the late receptor potential. This depolarization can be switched off by further exposure to a blue stimulus. The early receptor potential in this cell is purely depolarizing or largely hyperpolarizing; the former is true if the cell has been adapted to red light, and the latter, if blue light has been used. The color-adaptation "memories" for both early and late receptor potentials appear to be permanent. The existence of two stable states for the early receptor potential directly implies a pigment with two stable states, and these apparently contribute antagonistically to the late receptor potential.
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The hypothesis that a large, possibly toxic, increase in cellular calcium accompanies photoreceptor cell degeneration in several different Drosophila mutants was tested. The calcium content of wild type and mutant photoreceptors of Drosophila was measured using rapid freezing of the eyes and energy-dispersive x-ray analysis (e.d.x.) of cryosections and semithin sections of cryosubstituted material. Light- and dark-raised mutants of the following strains were studied: retinal degeneration B (rdgB); retinal degeneration C (rdgC); neither inactivation nor afterpotential C (ninaC), and no receptor potential A (norpA). These are light-dependent retinal degeneration mutants in which the affected gene products had been previously shown as myosin-kinase (ninaC), calcium-dependent phosphoprotein phosphatase (rdgC), phosphoinositide transfer protein (rdgB), and phospholipase C (norpA). In light-raised mutants, ommatidia of variable degrees of degeneration were observed. Mass-dense globular bodies of 200-500 nm diameter in relatively large quantities were found in the degenerating photoreceptor of all the mutants tested. These subcellular globules were found to have a very high calcium content, which was not found in wild type or in nondegenerating photoreceptors of the mutants. Nondegenerating photoreceptors were found not only in dark-raised mutants, but in smaller quantities also in light-raised mutants. Usually these globular structures contained high levels of phosphorus, indicating that at least part of the calcium in the mutant photoreceptors is precipitated as calcium phosphate. The results indicate that a large increase in cellular calcium accompanies light-induced photoreceptor degeneration in degenerating Drosophila mutants even when induced by very different mutations, suggesting that the calcium accumulation is a secondary rather than a primary effect in the degeneration process.
The formation of metarhodopsin in fly photoreceptor no. 1--6 occurs at room temperature with a time constant of 125 microseconds (Q10 approximately 2.5). The formation of rhodopsin is faster by factor of 1/10 to 1/100 at least.
Bleaching adaptation is the reduction in sensitivity of photoreceptors to light, following large photoconversion of rhodopsin molecules. It is accompanied by a prolonged dark excitation, and the two processes may be causally linked. There are remarkable similarities in the prolonged dark excitation of vertebrate rods and invertebrate photoreceptors. There is much evidence to indicate that prolonged dark excitation is tightly linked to processes that occur at the photo-pigment level. A molecular mechanism is suggested to explain the prolonged dark excitation. According to this model, quenching of photoreceptor excitation operates by phosphorylation of metarhodopsin (M) molecules. When unphosphorylated M molecules exist in the dark, they give rise to prolonged dark excitation. This prolonged dark excitation seems to be coupled to photoreceptors' desensitization by an unknown mechanism.