Biomedical subjects
Publications and source records attributed to H Machemer.
An account is given of approaches to gravireception, terminology mechanisms of responses to gravity as investigated and documented in the literature, and sensorimotor coupling properties in ciliates. Current theories and methods are discussed, and previously published experimental data on graviresponses are reviewed.
Motor responses of the frontal cirri of the ciliate Stylonychia were recorded at the axial view of the ciliary base with high-speed cinematography. Voltage-clamp applying sustained hyperpolarizing voltage steps was used to explore the properties of the ciliary cycle modulated by the membrane potential. Upon hyperpolarization between -1 and -13 mV, a previously inactive frontal cirrus reoriented from a neutral posture and started beating so that the axis of the beating cone of a proximal cirral segment assumed an orientation near 100 degrees (proceeding counterclockwise from posterior = 0 degrees) and inclination near 60 degrees (0 degrees = perpendicular to the cell surface). The major beating amplitude was limited to about 150 degrees. Increasing hyperpolarization increased the spatial polarity of the cycle (ratio of major over minor amplitude, from 2 to 2.4). Rates of the power stroke increased with hyperpolarizations up to -4 mV but were consistently smaller than those of the return stroke during the ciliary cycle (ratio: 0.4 to 0.6; = temporal polarity). Comparison of different hypothetical beat forms (0-shape, D-shape, and egg-shape) showed that the orientation-time data are the major determinants of the angular velocity and rate of reorientation of the cilium during the cycle. Geometric transformation of these data led to descriptions of the cycle of a proximal ciliary segment in terms of active sliding velocities and rates of unidirectional sliding translocation between identified doublets. Three voltage-sensitive functional parameters of the cilium--the inclination (which is noncyclic) and the rates of active sliding and sliding translocation (both of which are cyclic in nature)--are discussed as generating the spatial and temporal properties of the ciliary beat.
Injections of cyclic AMP (cAMP) and 8-Br-cAMP into Paramecium and external application of isobutylmethylxanthine (IBMX), an inhibitor of cAMP breakdown, to these cells increased the frequency of ciliary beating and hyperpolarized the membrane potential. When the membrane potential was held equal to the resting potential under voltage clamp, the same experimental conditions which serve to increase intracellular cAMP did not raise the ciliary frequency. We conclude that cAMP is presumably not the direct mediator of the hyperpolarization-induced ciliary activation, although it may be associated with this motor response.
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1. Paramecium caudatum was deciliated with ethanol. The ionic conductance of the membrane was investigated with constant current, voltage clamp and mechanical stimuli. 2. The resting potential was not modified by the removal of the cilia. The dependence of the resting potential on the extracellular concentrations of Ca and K was the same in deciliated and control cells. 3. The input resistance in deciliated and ciliated cells increased after the ethanol treatment. 4. The membrane capacitance decreased to 48% after deciliation, suggesting that the ciliary surface area is equal to the somatic surface area. 5. Deciliation completely removed the regenerative response (graded action potential) elicited by depolarizing current pulses or mechanical stimuli. 6. Deciliated cells retained the depolarizing and hyperpolarizing mechanoreceptor responses. 7. Voltage-clamp experiments demonstrated the loss of the early inward current in deciliated cells; it was restored during ciliary regeneration. Steady-state current-voltage relationships were unchanged by deciliation. 8. The time courses of the recovery of the membrane capacitance and of the early inward current were similar, suggesting that the number of voltage-sensitive Ca channels is proportional to the ciliary membrane area. 9. We conclude that the voltage-sensitive Ca channels reside in the ciliary membrane (in confirmation of Dunlap, 1976; Ogura & Takahashi, 1976), while mechanoreceptor channels, rectifier channels and resting conductances are localized in the somatic membrane.
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Ciliary activity in Paramecium was investigated in different external solutions using techniques of voltage clamp and high frequency cinematography. An increase in the external concentration of K, Ca or Mg ions decreased the resting potential. It had no effect on ciliary activity. When the membrane potential was fixed, an increase in external Ca or Mg and, to a lesser extent, an increase in K concentration, raised the frequency of normal beating or decreased the frequency of reversed beating of the cilia. Similar effects resulted from membrane hyperpolarization with constant ionic conditions. Increase in concentration of Ca, but not of Mg or K, enhanced hyperpolarization-induced augmentation of ciliary frequency. Increase in Ca concentration also specifically augmented the delayed increase in inward current during rapid hyperpolarizing clamp. The results support the view that [Ca]i regulates the frequency and direction of ciliary beating. It is suggested that the insensitivity of the ciliary motor system to elevations of the external concentrations of ions results from compensation of their effects on [Ca]i. Depolarization itself appears to increase [Ca]i while elevation of the external ion concentrations at a fixed membrane potential appears to decrease [Ca]i.
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Quantitative relations between ciliary reversal and membrane responses were examined in electrically stimulated paramecia. Specimens bathed in 1 mM CaCl(2), 1 mM KCl, and 1 mM Tris-HCl, pH 7.2, were filmed at 250 frames per second while depolarizing current pulses were injected. At current intensities producing only electrotonic shifts the cilia failed to respond. Stimuli which elicited a regenerative response were followed by a period of reversed ciliary beating. With increasing stimulus intensities the latency of ciliary reversal dropped from 30 to 4 ms or less, and the duration of reversal increased from 50 ms to 2.4 s or more; the corresponding regenerative responses increased in amplitude and rate of rise. With progressively larger intracellular positive pulses, electric stimulation became less effective, producing responses with a progressive increase in latency and decrease in duration of reversed beating of the cilia. When 100-ms pulses shifted the membrane potential to +70 mV or more, ciliary reversal was suppressed until the end of the pulse. "Off" responses then occurred with a latency of 2-4 ms independent of further increases in positive potential displacement. These results suggest that ciliary reversal is coupled to membrane depolarization by the influx of ions which produces the regenerative depolarization of the surface membrane. According to this view suppression of the ciliary response during stimulation occurs when the membrane potential approaches the equilibrium potential of the coupling ion, thereby retarding its influx. Previous data together with the present findings suggest that this ion is Ca(2+).
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Electrophysiological and cell reactivation studies in Paramecium and other ciliates have established that depolarizing stimulation opens voltage-sensitive ciliary Ca2+ channels leading to an elevation in intraciliary Ca2+, a rapid 'reversal' in sliding-microtubule based ciliary activity and backward swimming. Regulation of cilia by hyperpolarization modulates the pitch and rate of forward locomotion. The control of this predominant behaviour has been a matter of controversy because ciliary conductances do not change with negative shifts from the resting potential. Recordings of ciliary responses during electrophysiological manipulation of the Ca driving force in the ciliates Stylonychia and Didinium now suggests that a crucial step in hyperpolarization-induced ciliary activation (HCA) is a reduction in intraciliary Ca2+ from a resting steady-state level. The data are discussed with respect to previous hypotheses for the regulation of HCA.
OBJECTIVE: To establish whether the bending and torsional stiffness of an implanted nail are influenced by nail design and nail-bolt interface, this study compared two implanted retrograde nail systems: the AO/ASIF unreamed humeral nail (UHN) and the Russell-Taylor (RT) nail. DESIGN: Pair randomization. SETTING: Mechanical laboratory testing. SPECIMENS: Twelve pairs of freshly harvested cadaveric humeri. METHODS: Transverse fractures were simulated with a standardized midshaft osteotomy and a three-millimeter gap. Both nails were proximally and distally interlocked. The RT nail has a single interlock at its base and tip. The UHN has double interlocking both proximally and distally. The screw hole design of the RT nail features slots, whereas the UHN has round screw holes. MAIN OUTCOME MEASURES: Anteroposterior and mediolateral bending stiffness and torsional stiffness. RESULTS: The RT nail showed higher bending stiffness in anteroposterior and mediolateral bending. Large differences were seen in the torsional characteristics: for the first 30 degrees, the RT nail showed a much lower resistance against torsion than the UHN. Analysis of variance of stiffness at four, six, and eight newton-meters showed statistical significance (p < 0.0001). Torsional stiffness, defined as the slope of a straight line approximated to between 75 and 100 percent of the maximum torque, was very similar in both nails. CONCLUSION: The torsional differences between the two nail systems are attributable to the nail-bolt interface of the RT nail. This dynamic system allows a clinically relevant degree of movement. The greater resistance to rotatory forces of the UHN is explained by the fact that the interlocking at its tip and base creates a static rather than a dynamic system.
Cells of Loxodes striatus were adjusted to defined culturing, experimental solution O2-supply, temperature, and state of equilibration to be subjected to step type transition of acceleration from normal gravity, (1 g) to the weightless condition (microgravity) during free fall in a 500 m drop shaft. Cellular locomotion inside a vertical experimental chamber was recorded preceding transition and during 10 s of microgravity. Cell tracks from video records were used to separate cells gliding along a solid surface from free swimmers, and to determine gravitaxis and gravikenesis of gliding and swimming cells. With O2 concentrations > or = 40% air saturation gliders and swimmers showed a positive gravitaxis. In microgravity gravitaxis of gliders relaxed within 5 s whereas gravitaxis relaxation of swimmers was not completed even after 10 s. Rates of horizontal gliders (319 micrometers/s) exceeded those, of horizontal swimmers (275 micrometers/s). Relaxation of gravikinesis was incomplete after 10 s of microgravity. Analysis of the locomotion rates during the g-step transition revealed that gliders sediment more slowly, than swimmers (14 versus 45 micrometers/s). The gravikinesis of gliders cancelled sedimentation effects during upward and downward locomotion tending to maintain cells at a predetermined level inside sediments of a freshwater habitat. At > or = 40% air saturation, gravikinesis of swimmers augmented the speed of the majority of cells during gravitaxis, which favours fast vertical migrations of Loxodes.