Bioelectric control of ciliary activity

Slow inactivation of the calcium current of Paramecium is dependent on voltage and not internal calcium

The isolated Ca2+ current from Paramecium caudatum was examined under voltage clamp with long conditioning depolarizations lasting for up to 5 min. The isolated transient Ca2+ current inactivates with tens of milliseconds due to Ca2+ -dependent Ca2+ -channel inactivation (Brehm & Eckert, 1978). When this fast inactivation was blocked by internally delivered EGTA, a much slower inactivation of the Ca2+ current was discovered. This slow inactivation had time constants of tens of seconds, depending on voltage. The development of this slow inactivation was further examined by following the Ca2+ transient after 1 s interruptions of the long depolarization. This development is voltage dependent; the rate of inactivation is higher with a larger depolarization. After a long depolarization, the Ca2+ current returns in two clearly separable steps. A portion of the current returns rapidly along an exponential time course with time constants of tens to hundreds of milliseconds. The remainder of the current returns slowly with time constants of tens of seconds. A longer conditioning depolarization generates a larger portion that recovers slowly. Internally delivered EGTA, sufficient to prevent most of the fast inactivation, did not change the time course or the extent of either the onset or the removal of the slow inactivation. The compound W-7, which inhibits the Ca2+ current itself, does not block the onset of this slow inactivation during depolarization. We conclude that the slow inactivation of the Ca2+ channel is a mechanistically different phenomenon from the fast Ca2+ -dependent Ca2+ -channel inactivation. The possible physiological and behavioural roles of this slow inactivation are discussed.

Ca2+ transport studied with arsenazo III in Tetrahymena microsomes. Effects of calcium ionophore A23187 and trifluoperazine

Transport of Ca2+ in microsomal membrane vesicles of the Tetrahymena has been investigated using arsenazo III as a Ca2+ indicator. The microsomes previously shown to carry a Mg2+-dependent, Ca2+-stimulated ATPase (Muto, Y. and Nozawa, Y. (1984) Biochim. Biophys. Acta 777, 67-74) accumulated calcium upon addition of ATP and Ca2+ sequestered into microsomal vesicles was rapidly discharged by the Ca2+ ionophore A23187. Kinetic studies indicated that the apparent Km for free Ca2+ and ATP are 0.4 and 59 microM, respectively. The Vmax was about 40 nmol/mg protein per min at 37 degrees C. The calcium accumulated during ATP-dependent uptake was released after depletion of ATP in the incubation medium. Furthermore, addition of trifluoperazine which inhibited both (Ca2+ + Mg2+)-ATPase and ATP-dependent Ca2+ uptake rapidly released the calcium accumulated in the microsomal vesicles. These observations suggest that Tetrahymena microsome contains both abilities to take up and to release calcium and may act as a Ca2+-regulating site in this organism.

Evidence for two voltage-dependent calcium currents in the membrane of the ciliate Stylonychia

Action potentials and voltage-dependent membrane currents have been investigated in the fresh-water hypotrich ciliate Stylonychia mytilus, using two intracellular micro-electrodes. The inward current-voltage (Iin-V) relationship has two maxima, the first around -45 mV, and the second around -17 mV (resting and holding membrane potential being -50 mV). The shape of the Iin-V relationship is virtually unaltered in the presence of the K-channel blockers tetraethylammonium, 4-aminopyridine or internal Cs. The inward currents exhibit a differential sensitivity to both external CO and Cd; the inward current activated at potentials greater than or equal to -40 mV is more sensitive to these divalent cations than the inward current activated at around -45 mV. This suggests the presence of two different types of Ca inward currents. Both types of inward currents are present when Ca is replaced by Ba (or Sr). The small inward current recorded between -48 and -40 mV relaxes similarly in Ca and in Ba solutions. The larger inward current, recorded at -30 or -20 mV, relaxes rapidly in Ca solution but only slowly and incompletely in Ba solution. A two-pulse protocol revealed that for both types of inward currents inactivation may depend partially upon the influx and/or intracellular accumulation of the charge-carrying divalent cation. There appears to be a significant difference in the degree of inactivation of the two types of inward currents, however, when Ba is the charge carrier. When the cell spontaneously released, or was induced to release its membranellar band (row of compound cilia), the second, "all-or-none' component of the action potential, and the maximum of the Iin-V relationship at -45 mV disappeared. The first, graded peak of the action potential and the larger maximum of the Iin-V relationship remained essentially unaltered. The smaller Ca current and the action potential shoulder also disappeared when the anterior half of the cell (with most of the membranellar band) was severed, but not when the posterior half was cut off. When recording from a membranellar band vesicle both types of inward currents were present. The results suggest that the two components of the action potential may correspond to the two types of Ca currents. These Ca currents are separable by their localization in the membrane. The smaller Ca current appears to be restricted to the membranellar band.(ABSTRACT TRUNCATED AT 400 WORDS)

Physiological and mutational protein variations in the ciliary membrane of Paramecium

The proteins in the ciliary membrane of wild-type and mutant Paramecium tetraurelia are examined with SDS and IEF gels. Over 80% of the proteins in the ciliary membrane belong to two groups: the immobilization antigen (I-Ag), which is a 220-280 kD surface protein, and a set of at least four integral proteins slightly over 40 kD (the 40 k), most of which focus near pH 4.0 (the acidic 40 k). Variations of the I-Ag in its apparent molecular weight appear spontaneously in different clones of the same strain and can be triggered by changing the culture temperatures. We discovered that the members of the acidic 40 k family also vary in their relative proportion. Furthermore, the variations in I-Ag and those in acidic 40 k are tightly coupled. The concerted changes suggest a co-regulation in the synthesis of these proteins. The ciliary membranes of 20 mutants of 11 complementation groups known for their behavioral and electrophysiological defects are examined. Coupled variations of I-Ag and acidic 40 k among clones, similar to those of the wild type, are seen. Besides the I-Ag and the acidic 40 k, this membrane has over 60 other species of proteins, most of which are invariant. Shifts in the isoelectric points of two of these minor proteins have been correlated with two different mutations, 'fast-2' and 'paranoiac A'. No electrophoretic shifts can be correlated with the 'pawn B' mutation as found by Merkel et al. [35].

Diffusion of Ca2+ and the quiescent response of sea urchin sperm flagella

The starting and stopping transients observed in sea urchin sperm flagella in the presence of high Ca2+ are believed to begin with an influx of Ca2+ into the axoneme and to end, as indicated by resumption of normal beating, when the Ca2+ has been reduced to very low levels by an active extrusion process. If the influx and efflux processes were uniformly distributed along the length of the flagellum, it is not likely that the starting and stopping transients would occur as a well defined sequence of events that always proceeds from the proximal to the distal end. Theoretical analysis of the concentration profiles of Ca2+ expected if Ca2+ influx occurred along the length of the flagellum but efflux was restricted to the proximal end shows that the time required to reduce [Ca2+] in the distal portion of the flagellum would generally be longer than the observed recovery times. Localization of both the influx and efflux processes near the proximal end, however, yields concentration profiles consistent with observations on the duration of starting and stopping transients.

Changes in morphology and physiology of olfactory receptor cilia during development

Ciliated olfactory receptor neurons in vertebrates turn over throughout life. We show that these neurons bear different types of cilia at different developmental stages; cilia on newly differentiating cells are short and motile; cilia on mature cells are longer and immotile; Mg2+ and adenosine 5'-triphosphate are requisite for ciliary motion; stimulation with odorants can induce synchronous motion and that this process is mediated by Ca2+. We propose that receptor neurons have two distinguishable developmental states. In the first, before the growing axon establishes synaptic connection to the brain, the cells bear motile cilia and are generally irritable. In the second, the cilia are long and immotile and the cells can distinguish between odorants.

A Ca2+-activated ATPase specifically released by Ca2+ shock from Paramecium tetraurelia

Deciliation of Paramecium tetraurelia by a Ca2+ shock procedure releases a discrete set of proteins which represent about 1% of the total cell protein. Marker enzymes for cytoplasm (hexokinase), endoplasmic reticulum (glucose-6-phosphatase), peroxisomes (catalase), and lysosomes (acid phosphatase) were not released by this treatment. Among the proteins selectively released is a Ca2+-dependent ATPase. This enzyme has a broad substrate specificity which includes GTP, ATP, and UTP, and it can be activated by Ca2+, Sr2+, or Ba2+, but not by Mg2+ or by monovalent cations. The crude enzyme has a specific activity of 2-3 mumol/min per mg; the optimal pH for activity is 7.5. ATPase, GTPase, and UTPase all reside in the same protein, which is inhibited by ruthenium red, is irreversibly denatured at 50 degrees C, and which has a sedimentation coefficient of 8-10 S. This enzyme is compared with other surface-derived ATPases of ciliated protozoans, and its possible roles are discussed.

Habituation and sensitization in an aneural cell: some comparative and theoretical considerations

Several shorter- and longer-term non-associative modifications in behavior are known to occur in neural as well as aneural systems. Thus neural investment is not essential for these phenomena to occur. Cellular studies of these behaviors in Protozoa, where a single cell is also a whole organism, may be useful in investigating the evolution of mechanisms underlying these plastic behavioral changes.

Kinetic analysis of chemokinesis of Paramecium

Paramecia detect and accumulate in or disperse from some chemicals. Cells do this by changing frequency of turning and speed of swimming. There are at least two mechanisms by which cells respond: one dependent on ability to turn, one dependent on speed modulation. There are also two classes of chemicals: those that require the cells' ability to turn in order to cause accumulation and dispersal (type I), and those that apparently require only speed modulation (type II). Attractants of type I cause qualitatively similar changes in behavior to repellents of type II and the converse; therefore, assays are needed to distinguish between these two classes of chemicals, despite qualitatively similar behavior of some attractants and repellents. We examined two assays of paramecium chemoresponse, T-maze assay and well test, to understand how the T-maze distinguishes between attractants of type I and repellents of type II and why the well test does not.

Calcium and potassium currents in muscle fibres of an insect (Carausius morosus)

1. A three electrode voltage-clamp was used to investigate membrane currents in the skeletal muscle fibres of the stick insect, Carausius morosus. Contraction was blocked by hypertonic solutions. 2. Membrane currents elicited by step depolarizations consisted of an inward current, an early outward current and a delayed outward current. 3. The reversal potential of the delayed outward current did not change when SO4(2-) was substituted for Cl-, but shifted by 14.1 mV when [K]0 was increased from 20 mM to 40 mM in SO4(2-) solution, suggesting that the delayed current is carried by K+. Both early and delayed outward currents were substantially reduced by 120 mM-tetraethylammonium (TEA) ions. 4. The small size of the shift in the reversal potential of the delayed outward current with increased pulse duration suggests that the delayed current measured flows mainly across the surface membrane. 5. Increasing [Ca]o made the apparent reversal potential for the inward current (120 mM-TEA Ringer) more positive and increased the size of the maximum inward current. However, Ca-currents showed saturation with increasing [Ca]o, indicating that there is a site to which Ca ions bind during their passage through the membrane. The dissociation constant of this site was 7.3 mM at 0 mV and was voltage-dependent. 6. Inward currents were blocked by 1 mM-La3+ or Cd2+, or by substitution of Co2+ or Ni2+ for Mg2+. Strontium and barium were able to permeate the channel but Na+ and Mg2+ appear impermeant. 7. As expected from the low intracellular Ca concentration, the instantaneous current-voltage relation of the Ca current rectified strongly in the inward direction. 8. Both constant field theory and the simplest, single site, Eyring rate theory model predict the rectification of the instantaneous current-voltage relation. The rate theory model also predicts saturation of the Ca current with [Ca]o.

Electrically induced Ca2+ transport across the membrane of Paramecium caudatum measured by means of flow-through technique

Transmembrane calcium fluxes related to excitation were studied in Paramecium caudatum. Radioactive (45Ca2+) or inactive solution was flowed through a dense suspension of unlabelled or labelled cells, and radioactivity was monitored in the solution. The organisms were electrically stimulated by means of extracellular electrodes. As a result of stimulation Ca2+ uptake and release was measured. The uptake response dropped with increasing number of successive stimulation periods and increased with growing stimulus amplitude and duration. Maximum uptake was obtained at 20 V/cm and at least 60 s duration and for temperatures between 10 and 15 degrees C. A Ca2+ influx of 700 pmol/1000 cells upon 1 min stimulation was measured at 15 degrees C. This corresponds to an increment of the intraciliary [Ca2+] of about 5 x 10(-4) M. Ca2+ release was dependent on the stimulus amplitude in a similar manner as was Ca2+ uptake. Photographic recordings of the swimming behaviour of the organisms were used to interpret the flux data. At temperatures up to 15 degrees C the cells swam backward perpendicular to the applied electric field of 10 to 20 V/cm. At 25 degrees C they showed forward spiralling movement. For the first time evidence was brought for stimulated Ca2+ influx in Paramecium at physiological temperatures. It is concluded from the results that a strong active Ca2+ extrusion from the intraciliary space counteracts the influx. The Ca2+ pump rate must be at least 8 x 10(12) calcium ions per s per cm2 ciliary surface.

[Study of the intra- and extracellular electrolyte content in Paramecia cultures carried out during a space flight (author's transl)]

In the results of previous investigations we have already reported that cultures of Paramecium tetraurelia submitted to a space flight present a stimulation of their proliferative ability, an increase in cell volume and a decrease in dry weight and in total protein content. These results suggest changes of cell metabolism induced by the space environment. In order to confirm this hypothesis we have studied the concentration of extracellular electrolytes in the control and the in-flight culture media with respect to the intracellular content of the same electrolytes. These measures concern Na, Cl, K, P, Mg, Ca. In this paper we report the results of these analyses and note that if no differences are noted for Na and Cl between control and in-flight cultures, modifications in P, K, Ca and Mg levels are observed. Generally there is a higher concentration of these elements in the in-flight medium but, in contrast, a lower intracellular content is noted for in-flight Paramecia. We have established a double comparison: on the one hand between control and in-flight media and between control and in flight cells, on the other hand between media and cells. All these data suggest possible changes in the membrane permeability, or of the binding proteins in Paramecia cultivated in hypogravity.

Purification and characterization of methyl-accepting chemotaxis protein methyltransferase I in Bacillus subtilis

A methyltransferase that methylates one of the proteins involved in chemotactic adaptation to sensory stimuli in Bacillus subtilis was purified to homogeneity. The enzyme utilizes S-adenosylmethionine as donor for a methyl group that is transferred to a glutamate residue in a 69 000-mol.wt. membrane protein and also to a protein of 19 000 mol.wt. The molecular weights of the denatured enzyme by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and of the native enzyme by gel-filtration chromatography both show the protein to be a 44 000-mol.wt. monomer. Isoelectric focusing of the purified methyltransferase showed the protein to be a single species with isoelectric point pI 5.4. On the basis of a molecular weight of 44 000, the molar absorption coefficient at 262 nm of the enzyme is 10.9 x 10(4) M-1 . cm-1. The Km of the enzyme for S-adenosylmethionine is about 2 microM. The Ki for S-adenosylhomocysteine is about 0.2 microM. Ca2+ is a competitive inhibitor of methylation, with a Ki of 0.065 microM. The enzyme methylates membranes from the wild-type more efficiently than membranes isolated from a mutant strain defective in chemotaxis. The enzyme is unable to methylate Escherichia coli membranes.

Chemotaxis of Spirochaeta aurantia: involvement of membrane potential in chemosensory signal transduction

The effects of valinomycin and nigericin on sugar chemotaxis in Spirochaeta aurantia were investigated by using a quantitative capillary assay, and the fluorescent cation, 3,3'-dipropyl-2,2'-thiodicarbocyanine iodide was used as a probe to study effects of chemoattractants on membrane potential. Addition of a chemoattractant, D-xylose, to cells in either potassium or sodium phosphate buffer resulted in a transient membrane depolarization. In the presence of valinomycin, the membrane potential of cells in potassium phosphate buffer was reduced, and the transient membrane depolarization that resulted from the addition of D-xylose was eliminated. Although there was no detectable effect of valinomycin on motility, D-xylose taxis of cells in potassium phosphate buffer was completely inhibited by valinomycin. In sodium phosphate buffer, valinomycin had little effect on membrane potential or D-xylose taxis. Nigericin is known to dissipate the transmembrane pH gradient of S. aurantia in potassium phosphate buffer. This compound did not dissipate the membrane potential or the transient membrane depolarization observed upon addition of D-xylose to cells in either potassium or sodium phosphate buffer. Nigericin did not inhibit D-xylose taxis in either potassium or sodium phosphate buffer. This study indicates that the membrane potential but not the transmembrane pH gradient of S. aurantia is somehow involved in chemosensory signal transduction.

Biochemical studies of the excitable membrane of Paramecium tetraurelia VI. Endogenous protein substrates for in vitro and in vivo phosphorylation in cilia and ciliary membranes

The endogenous protein kinases of isolated Paramecium tetraurelia cilia phosphorylated approximately 30 ciliary polypeptides in vitro. Labeling with [gamma-32P]ATP was not proportional to the amount of each protein in cilia; some minor polypeptides (e.g., 67,000 and 180,000 mol wt) were more heavily labeled than some major polypeptides. Certain of the endogenous substrates for protein kinase were localized in the ciliary membrane (130,000, 86,000, 67,000, and 45,000 mol wt); others were found in axonemes or in both fractions. With cilia from bacterized cultures in the undefined Cerophyl medium, the labeling of specific endogenous phosphate acceptors was altered by pH, cyclic AMP, and cyclic GMP, but the labeling pattern was not affected by the presence of Na+ or K+ (15 mM), Ba++ (5 mM), Ca++ (10(-5) or 10(-4) M), or EGTA. Very similar results were obtained with cilia from cells grown axenically in a semidefined medium; the molecular weights and the extent of phosphorylation of the phosphopolypeptides were comparable to those of cilia from bacterized Cerophyl cultures, although no significant cyclic nucleotide effects were observed in the axenic cilia. Most of the phosphopolypeptides labeled in vitro also turned over rapidly in vitro. The phosphoprotein phosphatase responsible for turnover was partially inhibited by 5 mM NaF. The pattern of ciliary polypeptides labeled in vivo was similar to that observed in the in vitro experiments, although the relative intensities of labeling differed. Six behavioral mutants of Paramecium, known to have defects in the excitable membrane that regulates the ciliary beat, showed normal patterns of ciliary protein phosphorylation in vitro, with and without added cyclic nucleotides, at both pH 6.0 and pH 8.0. The mutants also had apparently normal phosphoprotein phosphatase. The Paranoiac A mutant, however, showed a reduction in cyclic GMP-stimulated protein kinase activity.

Comparative psychopharmacology

Studies of the effects of psychoactive drugs and neurotransmitters on the behavior of invertebrates and poikilothermic vertebrates are reviewed. Dangers of reductive explanations are pointed out. Results and suggestions are given concerning the use of poikilothermic animals (1) in the development of screening tests, (2) in experiments on the action mechanisms of psychopharmaca, and (3) in the use of psychoactive drugs in the study of the mechanisms of animal behavior.

Distribution of calcium in the suctorian Discophrya collini: an x-ray microanalytical study

Discophrya collini is a free-living suctorian with tentacles which can be induced to contract by means of a range of experimental stimuli, including the application of CaCl2 and MgCl2 but not BaCl2. X-ray microanalysis of glutaraldehyde-only fixed cells shows Ca to be present in the cytoplasmic ground substance and elongate dense bodies (EDB). In 10(-1) M CaCl2-treated cells, calcium levels remain unchanged except for a three-fold increase in the EDB. Treatment of cells with 10(-1) M MgCl2 and 10(-1) M BaCl2 does not result in their detection in the cell. It is suggested that EDB may act as reservoirs controlling levels of calcium.

Calcium-mediated inactivation of calcium current in Paramecium

1. The Ca current seen in response to depolarization was investigated in Paramecium caudatum under voltage clamp. Inactivation of the current was measured with the double pulse method; a fixed test pulse of an amplitude sufficient to evoke maximal inward current was preceded by a conditioning pulse of variable amplitude (0-120 mV).2. The amplitude of the current recorded during the test pulse was related to the potential of the conditioning pulse. Reduction of test pulse current was taken as an index of Ca current inactivation. The current recorded during a test pulse showed a progressive decrease to a minimum as the potential of the conditioning pulse approached +10 to +30 mV. Further increase in conditioning pulse amplitude was accompanied by a progressive restoration of the test pulse current. Conditioning pulses near the calcium equilibrium potential had only a slight inactivating effect on the test pulse current.3. Injection of a mixture of Cs and TEA which blocked late outward current had essentially no effect on the inward current or its inactivation.4. Elevation of external Ca from 0.5 to 5 mM was accompanied by increased inactivation of the test pulse current. The enhanced inactivation of the test pulse current was approximately proportional to the increase in current recorded during the conditioning pulse.5. Following injection of the Ca chelating agent, EGTA, the inactivation of the test pulse current was diminished; in addition, the transient inward current relaxed slightly more slowly, and the transient was followed by a steady net inward current.6. The time course of recovery from inactivation in the double pulse experiment approximated a single exponential having a time constant of 80-110 msec. Injection of EGTA shortened the time constant by as much as 50%.7. It is concluded that interference with the entry of Ca or enhanced removal of intracellular free Ca(2+) interferes with the process of Ca current inactivation, while enhanced entry of Ca promotes the process of inactivation. While the mechanism of inactivation is unknown, arguments are presented that the accumulation of intracellular Ca influences the Ca channel conductance.

The effects of high concentrations of sodium or calcium ions on the lipid composition and properties of Tetrahymena membranes

Tetrahymena pyriformis cells have been grown in media varying in NaCl concentration from 3.7 mM (normal medium) to 0.3 M and varying in CaCl2 from 0.2 mM (normal medium) to 0.1 M. Tetrahymena grown in 0.3 M NaCl showed relatively few alterations in phospholipid composition, with significant changes being found only in the cell surface membranes (pellicle), which incrased in phosphatidylethanolamine content from 39% (low Na+) to 48% (high Na+) of the total phospholipids. The small decrease in fatty acid unsaturation and increase in shorter chain fatty acids in pellicle phospholipids were not statistically significant. No significant changes in phospholipid head group composition or fatty acid distribution were observed in high Ca2+-grown cells. Complementary studies of membrane fluidity, as inferred from freeze-fracture electron microscopy analysis, indicated that membranes of high Na+-acclimated cells were similar to those of control cells, when each was measured in its respective medium. However, the outer alveolar membrane of the pellicle and the food vacuolar membrane were considerably less fluid in high-Ca2+ cells. The lower fluidity in vacuolar membranes may have been responsible for alterations in the cells' capacity to form food vacuoles.

Biochemical studies of the excitable membrane of Paramecium tetraurelia. III. Proteins of cilia and ciliary membranes

As a first step in the biochemical analysis of membrane excitation in wild-type Paramecium and its behavioral mutants we have defined the protein composition of the ciliary membrane of wild-type cells. The techniques for the isolation of cilia and ciliary membrane vesicles were refined. Membranes of high purity and integrity were obtained without the use of detergents. The fractions were characterized by electron microscopy, and the proteins of whole cilia, axonemes, and ciliary membrane vesicles were resolved by SDS polyacrylamide gel electrophoresis and isoelectric focusing in one and two dimensions. Protein patterns and EM appearance of the fractions were highly reproducible. Over 200 polypeptides were present in isolated cilia, most of which were recovered in the axonemal fraction. Trichocysts, which were sometimes present as a minor contaminant in ciliary preparations, were composed of a very distinct set of over 30 polypeptides of mol wt 11,000--19,000. Membrane vesicles contained up to 70 polypeptides of mol wt 15,000--250,000. The major vesicle species were a high molecular weight protein (the "immobilization antigen") and a group of acidic proteins with mol wt similar to or approximately 40,000. These and several other membrane proteins were specifically decreased or totally absent in the axoneme fraction. Tubulin, the major axonemal species, occurred only in trace amounts in isolated vesicles; the same was true for Tetrahymena ciliary membranes prepared by the methods described in this paper. A protein of mol wt 31,000, pI 6.8, was virtually absent in vesicles prepared from cells in exponential growth phase, but became prominent early in stationary phase in good correlation with cellular mating reactivity. This detailed characterization will provide the basis for comparison of the ciliary proteins of wild-type and behavioral mutants and for analysis of topography and function of membrane proteins. It will also be useful in future studies of trichocysts and mating reactions.

Restoration of membrane excitability in a behavioral mutant of Paramecium caudatum during conjugation and by microinjection of wild-type cytoplasm

When cells of the behavioral mutant cnrC of Paramecium caudatum were mated with the wild type, phenotype change from CNR (no backward swinning) to wild type in the cnrC mate occurred immediately after the formation of tight pairs. No change of phenotype occurred when cells of cnrA or cnrB were mated with wild type. Phenotypic change from CNR to wild type in cells of cnrC was also induced by microinjection of wild-type cytoplasm. Microinjection of wild-type cytoplasm induced no change in cells of cnrA or cnrB. Phenotypic change in the cnrC mate during conjugation can be explained by cytoplasmic exchange during conjugation, though transfer of membrane sites for excitability through membrane fluidity cannot be ruled out.