The peritrich ciliate Vorticella sp. exhibits cellular contraction of an all-or-nothing type in response to a mechanical stimulus. Many authors have suggested that the contraction may be controlled by the cytosolic level of Ca2+, since glycerol-extracted Vorticella contracts when Ca2+ is added to the external solution. However, no direct evidence for the increase in cytosolic [Ca2+] has yet been obtained in living Vorticella.
In the present study, by injecting a fluorescent Ca2+ indicator into living Vorticella and monitoring the cytosolic [Ca2+] with a confocal microscope, we have demonstrated that a mechanical stimulus evoked an all-or-nothing rise in cytosolic [Ca2+] (Ca2+ ‘spike’). The onset of the Ca2+ spike was similar in its time course to that of cellular contraction. Since the Ca2+ spike was recorded in a Ca2+-deprived solution containing 1 mmol l−1 EGTA, we concluded that release of Ca2+ from intracellular Ca2+ storage site(s) is responsible for the Ca2+ spike.
The peritrich ciliate Vorticella sp. exhibits contraction of an all-or-nothing type involving shrinkage of the cell body and coiling of the stalk in response to mechanical stimulation (Sugi, 1960). As early as 1958, Hoffmann-Berling found that the stalk of glycerol-extracted Vorticella contracted when Ca2+ was added to the external solution and relaxed when Ca2+ was removed (see also Amos, 1971). Many authors later examined the Ca2+-dependent contraction in glycerol-extracted Vorticella in detail and reported that the degree of contraction depended on the Ca2+ concentration ([Ca2+]) in the reactivation medium (Townes and Brown, 1965; Weis-Fogh and Amos, 1972; Routledge et al. 1975; Ochiai et al. 1979). A Ca2+-binding protein, spasmin, was extracted from contractile elements of Zoothumnium geniculatum, a close relative of Vorticella (Amos et al. 1975; Ochiai et al. 1988). These findings suggest that an increase in cytosolic [Ca2+] may activate the contractile system of Vorticella (Amos et al. 1976).
Several authors assumed that the Ca2+ required to activate the contractile system is released from intracellular Ca2+ storage site(s) in Vorticella, for living Vorticella can contract even in Ca2+-deprived media containing EGTA (Allen, 1973a; Katoh and Naitoh, 1992, 1994). Electron microscopic studies of vorticellid ciliates showed that membranous tubules (tubular endoplasmic reticulum) are present in the spasmoneme, which is responsible for coiling of the stalk, and in the myoneme, which is responsible for shrinkage of the cell body. These studies also showed that endoplasmic reticulum is present around the myoneme (Carasso and Favard, 1966; Amos, 1972; Allen, 1973a,b). Moreover, Carasso and Favard (1966) demonstrated the presence of Ca2+ in the tubules and endoplasmic reticulum using cytochemical methods. These membranous systems are candidates for the Ca2+ storage sites.
Recently, cytosolic [Ca2+] has been monitored in various cells using AM esters of fluorescent Ca2+ indicators, which permeate the cell membrane of higher animals. However, in protozoan cells, it is difficult to measure cytosolic Ca2+ levels, because their cell membrane is less permeable to the AM ester. In some protozoans other than Vorticella, researchers have detected an increase in cytosolic [Ca2+]; for example, in Spirostomum ambiguum (Ettienne, 1970) and in Didinium nasutum (Pernberg and Machemer, 1995). However, no one has observed a rise in cytosolic [Ca2+] in living Vorticella, in which it has been well established, in glycerol-extracted cell models, that the contractile system is activated by Ca2+.
The primary objective of the research described in this paper was to detect a rise in the cytosolic [Ca2+] of living Vorticella. Under a confocal microscope, we observed the cytosolic [Ca2+] of living Vorticella loaded with a fluorescent Ca2+ indicator and demonstrated an all-or-nothing rise in cytosolic [Ca2+] (Ca2+ spike). Some of these results have been presented verbally elsewhere (Katoh, 1995; Katoh and Kikuyama, 1996).
Materials and methods
Specimens of Vorticella sp. were grown at 20 °C on a glass slide in a bacterized saline solution (final concentrations
0.1 mmol l−1 KCl, 0.09 mmol l−1 CaCl2 and 0.1 mmol l−1 MgSO4) containing 0.1 % (w/w) dehydrated cereal leaves (Sigma). Specimens were washed with a standard saline solution containing 1 mmol l−1 KCl, 1 mmol l−1 CaCl2 and 10 mmol l−1 Tris-maleate buffer (pH 7.0) and then immersed in a Ca2+-deprived solution for approximately 10 min prior to each experiment. The Ca2+-deprived solution was a mixture of 1 mmol l−1 KCl, 1 mmol l−1 EGTA and 10 mmol l−1 Tris-maleate buffer (pH 7.0).
In order to monitor the cytosolic [Ca2+] of Vorticella, a Calcium Green solution was injected into the cell body according to the method of Hiramoto (1974). The distal end of the stalk of a specimen was first detached from its substratum (a glass slide) and the cell body was fixed at the tip of a suction pipette (approximately 5 μm in inner diameter) by lowering the hydrostatic pressure inside the pipette. The tip of a microneedle (about 1 μm in inner diameter) for injection was then inserted into the cell body. The Calcium Green solution contained 1 μmol l−1 Calcium Green-1 dextran (Mr 10 000, Molecular Probes, Inc., Eugene, USA), 150 mmol l−1 KCl and 1 mmol l−1 Pipes (pH 7.0, adjusted using KOH). The amount of solution injected into the cell body was 0.7–1 pl, which corresponded to 2–3 % of the volume of the cell body. Thus, the final concentration of Calcium Green-1 dextran contained in the cytoplasm was approximately 20–30 nmol l−1.
A mechanical stimulus was applied to the cell body with a microneedle. The microneedle was driven perpendicularly by a piezoelectric transducer according to the method of Katoh and Naitoh (1992) (see also Naitoh and Eckert, 1969). The amplitude of the motion was represented as the amplitude of a rectangular voltage pulse (2–10 V) applied to the piezoelectric transducer attached to the microneedle (Fig. 1). The tip of the microneedle moved approximately 0.15 μm in response to a rectangular electrical pulse of 5 V. The duration of the electrical pulse was constant (15 ms).
An inverted microscope (TMD, Nikon, Tokyo) equipped with a laser confocal scanner unit (Insight Plus-IQ, Meridian Instruments Far East, Inc., Tokyo) was used with excitation at a wavelength of 488 nm (Fig. 1). The magnified image of the specimen was recorded with a video recorder (BR-S811, Victor, Tokyo) through a CCD camera (HCC-600M, Flovel, Tokyo). The recorded images were transferred to a computer (Apple Macintosh 7100) and analyzed. Relative cytosolic [Ca2+] was expressed as F/F0, where F is the intensity of fluorescence and F0 is the mean value of F of the cell body immediately before stimulation. All experiments were performed at room temperature (20–23 °C).
Transient rise in cytosolic [Ca2+]
Before the experiments, the fluorescent Ca2+ indicator (Calcium Green-1 dextran) was injected into a specimen bathed in a Ca2+-deprived solution. Although the injection of the Ca2+ indicator often caused a contraction of Vorticella, the specimen soon relaxed and resumed ciliary beating within 1 min. All experiments were carried out on specimens in this state.
We observed relative cytosolic [Ca2+] (F/F0) in a cell body of Vorticella before, during and after applying a mechanical stimulus to the cell body. The mechanical stimulus evoked a transient rise in cytosolic [Ca2+]. Fig. 2 shows representative images of this transient rise. The cytosolic [Ca2+] increased rapidly and then slowly returned to the resting level.
To determine the precise time course of the Ca2+ transient, [Ca2+] in the cell body was plotted against time (Fig. 3). When a mechanical stimulus was applied to the cell body, cytosolic [Ca2+] increased rapidly and then gradually returned to the initial level. The latency between the stimulus and the onset of the Ca2+ transient was 43±10 ms (mean ± S.E.M., N=12) and the rise time of the Ca2+ transient was 92±10 ms (mean ± S.E.M., N=12).
Relationship between the Ca2+ spike and contraction of the cell body
To determine the temporal relationship between the Ca2+ transient and contraction of the cell body, we compared the onset of the Ca2+ transient with that of contraction in the same specimen injected with Calcium Green. First, under a light microscope, we recorded the contraction evoked by a mechanical stimulus. We then switched the optical pathway to a laser scanning confocal microscope and recorded the Ca2+ transient evoked by mechanical stimulus in the same cell. Since the cell body of Vorticella was fixed with a suction pipette and a microneedle, the degree of contraction was smaller than that in normal specimens attached to a substratum by the stalk (see Fig. 4B). We compared the time course of the small contraction with that of the Ca2+ transient.
Fig. 4 shows representative examples of contraction and of the Ca2+ transient in Calcium-Green-injected Vorticella. Both were evoked by mechanical stimuli with the same intensity (3 V). Contraction occurred one frame after the stimulation in Fig. 4B, and the Ca2+ transient also occurred one frame after the stimulation in Fig. 4A. The onset of contraction was similar in its time course to that of the Ca2+ transient. Moreover, the threshold intensity of the mechanical stimulus required to evoke contraction (2.9±0.2 V, N=5) was similar to that required to evoke the Ca2+ transient (3.2±0.2 V, N=5).
Relationship between stimulus intensity and cytosolic [Ca2+]
To examine the relationship between the intensities of mechanical stimuli and cytosolic [Ca2+], we applied mechanical stimuli with different intensities to a Vorticella cell and measured cytosolic [Ca2+]. Fig. 5 shows representative examples of responses of cytosolic [Ca2+] to mechanical stimuli of different intensities. The Ca2+ images shown in Fig. 5 were recorded from the same specimen. No rise in cytosolic [Ca2+] was evoked when the intensity of the mechanical stimulus was lower than 2 V (Fig. 5B). When the intensity was 3 V or more, a transient rise in cytosolic [Ca2+] was evoked (Fig. 5C,D). The spatial pattern of the rise in cytosolic [Ca2+] was similar irrespective of the intensity of the mechanical stimulus provided that the intensity was over the threshold level (3 V, in Fig. 5) for evoking the Ca2+ transient (compare Fig. 5C with Fig. 5D).
To determine the effect of the intensity of the mechanical stimulus on the time course of the Ca2+ transients, cytosolic [Ca2+] was plotted against time (Fig. 6). A rise in cytosolic [Ca2+] was observed when the intensity of the mechanical stimulus was 3 V or more, as was expected from the Ca2+ images in Fig. 5. The form of the Ca2+ transient, a fast rising phase followed by a slow falling phase, was consistent irrespective of the intensity of the mechanical stimulus (compare Fig. 6B with Fig. 6C).
In the next series of experiments, the relationship between the peak level of the Ca2+ transient and the intensity of the mechanical stimulus was examined more precisely (Fig. 7). The peak level was constant irrespective of the intensity of the mechanical stimulus provided that the intensity was above a certain threshold level (3 V). This clearly shows that the Ca2+ transient is of an all-or-nothing type.
Many authors have reported that glycerol-extracted Vorticella contracts when it is bathed in a solution containing Ca2+ (Hoffmann-Berling, 1958; Townes and Brown, 1965; Weis-Fogh and Amos, 1972; Ochiai et al. 1979). They have assumed that an increase in cytosolic [Ca2+] is required to activate the contractile system of living Vorticella. However, no one has yet succeeded in recording an actual rise in cytosolic [Ca2+] in living Vorticella, probably because of the difficulty of loading a Ca2+ indicator into a protozoan cell. In the present experiments, we succeeded in injecting a Ca2+ indicator (Calcium Green-1 dextran) directly into living Vorticella and observing a transient rise in cytosolic [Ca2+] in response to mechanical stimulation. The rise in cytosolic [Ca2+] was expressed as a series of Ca2+ images (see Fig. 2). A plot of cytosolic [Ca2+] against time clearly shows the time course of the Ca2+ transient in living Vorticella (Fig. 3). The Ca2+ transient will be called a Ca2+ spike hereafter.
The relationship between the intensity of the mechanical stimulus and the peak value of the Ca2+ spike reveals that the Ca2+ spike of Vorticella is of an all-or-nothing type (Figs 5, 6, 7). This is the first report of an all-or-nothing rise in cytosolic [Ca2+] (Ca2+ spike) in contractile protozoa. Using a giant contractile ciliate Spirostomum ambiguum, Ettienne (1970) observed a small increase in light emission from a Ca2+-sensitive photoprotein, aequorin, which had been injected into the cell. However, it is not known whether the Ca2+ transient of Spirostomum is all-or-nothing in type. Moreover, the time course of the Ca2+ transient of Spirostomum was very different from that of Vorticella. The Ca2+ transient of Spirostomum never resembles a Ca2+ spike.
To examine the causal relationship between the Ca2+ spike and contraction of the cell body in Vorticella, we compared the time course of the onset of the Ca2+ spike with that of the contraction in the same specimen. As shown in Fig. 4, the time course of the onset of the Ca2+ spike was similar to that of contraction. Moreover, the threshold intensity of the mechanical stimulus required to evoke a Ca2+ spike was similar to that required to evoke cellular contraction. These results support the idea that the Ca2+ ‘spike’ is responsible for the contraction of Vorticella.
Baylor and Hollingworth (1988) reported that the time courses of Ca2+ transients were prolonged by the fluorescent Ca2+ indicator loaded into the cell because the fluorescent Ca2+ indicator acts, more or less, like a Ca2+ buffer. This may be the reason why the time courses of both the Ca2+ spike and the contraction in the Calcium-Green-injected Vorticella were more prolonged than in normal Vorticella.
It is well known that contraction of Vorticella occurs even in the absence of extracellular Ca2+ (Allen 1973a,b; Katoh and Naitoh, 1992, 1994). In the present experiment, the Ca2+ spike was recorded in the absence of extracellular Ca2+. These results show that Ca2+ required to evoke the contraction and the Ca2+ spike is released from some intracellular Ca2+ storage site(s). Katoh and Naitoh (1992, 1994) suggested that regenerative liberation of Ca2+ from intracellular storage sites is involved in the increase in cytosolic [Ca2+] that evokes all-or-nothing-type contraction. The regenerative liberation of Ca2+ may be responsible for the all-or-nothing-type Ca2+ spike in Vorticella. Moreton and Amos (1979) proposed that an influx of Ca2+ from an external solution into the cell, accompanied by a Ca2+ action potential, might be responsible for the rise in the cytosolic [Ca2+] in Zoothumnium geniculatum, a close relative of Vorticella. However, the mechanism mobilizing Ca2+ for evoking contraction in Z. geniculatum may be different from that in Vorticella.
Several authors have reported that endoplasmic reticulum and/or membranous tubules (tubular ER) are present near the myoneme, which is responsible for the contraction of the cell body (Carasso and Favard, 1966; Amos, 1972; Allen, 1973a,b). Ca2+ was detected in the endoplasmic reticulum and membranous tubules (tubular ER) (Carasso and Favard, 1966). Moreover, Katoh and Naitoh (1994) reported that drugs that affect the liberation of Ca2+ from the sarcoplasmic reticulum of skeletal muscle fibres (Endo, 1977) also induce or inhibit contraction in living Vorticella. Thus, these membranous systems are strong candidates for Ca2+ storage site(s).
In the present paper, we describe a Ca2+ spike of all-or-nothing type in living Vorticella. We show that release of Ca2+ from intracellular storage site(s) is responsible for generating the Ca2+ spike. Vorticella may utilize intracellular Ca2+ storage sites to control cytosolic [Ca2+] in a way similar to sarcoplasmic reticulum in muscle cells, although the contractile element of Vorticella is very different from that of muscle cells. Our findings are important for understanding the evolution of the Ca2+ mobilization mechanisms used to evoke cellular contraction in the animal kingdom.
We thank Professor Yukio Hiramoto of University of the Air for invaluable discussions. K.K. is indebted to Dr Mihoko Takahashi of the University of Tsukuba for her invaluable suggestions and expresses sincere thanks to Professor Shinya Inoue of the Marine Biological Laboratory. This work was supported by grants from the Japanese Ministry of Education, Science and Culture (no. 06304004) and from the University of the Air to M.K.