Appropriate chemical stimulation of cnidocytes along with mechanical stimulation is required to trigger discharge of cnidae. It has been generally assumed that such chemosensitization is mediated via specific chemoreceptors. Such chemoreceptors and their complementary ligands have never been identified. We now identify two groups of naturally occurring substances that chemosensitize cnida discharge in the feeding tentacles of the sea anemone, Aiptasia pallida. In addition, using a novel technique to quantify cnida discharge we demonstrate that these chemosensitizers act through at least two distinct classes of receptors. One class is broadly specific toward a variety of amino and imino acids and histamine (Ko 5 = 1 \-30nM), but is competitively inhibited by antihistamines (K, = 0. 1 -7.4 nM). A second class is specific for N-acetylated sugars (K0.5 = 0.1-1.5 nM), but not affected by antihistamines. Presumably, these chemoreceptors detect specific substances from potential prey. Thus, cnidocytes are sensitized to discharge their cnidae in response to mechanical stimuli originating from the prey.
"Back transfer" of metabolites from food to endosymbiotic algae in the digestive cells of Hydra viridis was demonstrated. Brine shrimp nauplii labeled with tritiated precursors of protein and nucleic acids (DNA and RNA) were fed to light and dark grown hydras. The fate of the label after a single feeding with radioactive material in hydra and algal fractions was followed by scintillation counting and autoradiographic techniques. Labeled thymidine was incorporated into DNA in both light- and dark-grown hydras. Although the symbiosis persists indefinitely in hydras in darkness (7-10 days) the number of algae per cell is reduced. Tritiated orotic acid and tritiated uridine, RNA precursors, were incorporated into peptides and proteins, and to a lesser extent into simple sugars, oligosaccharides, and oligonucleotides in hydra and algal fractions. Thus the metabolites of the brine shrimp food are available to both partners. A decrease over time in label introduced as 3H-orotic acid and 3H-uridine and incorporated into hydra RNA is compensated for by an increase in label in the algae, implying competition for constant quantities of metabolites from the single feeding. Although food availability, light, number of algae per cell, and other factors influence the quantity and rate of nutrient transfer between the partners, in both light and dark grown hydras the amount of "back transfer" of metabolites to the symbiotic algae is impressive.
Tao X, Lin MT, Thorington GU, Wilson SM, Longo LD, Hessinger DA. Long-term hypoxia increases calcium affinity of BK channels in ovine fetal and adult cerebral artery smooth muscle. Am J Physiol Heart Circ Physiol 308: H707-H722, 2015. First published January 16, 2015; doi:10.1152/ajpheart.00564.2014.-Acclimatization to high-altitude, long-term hypoxia (LTH) reportedly alters cerebral artery contraction-relaxation responses associated with changes in K ϩ channel activity. We hypothesized that to maintain oxygenation during LTH, basilar arteries (BA) in the ovine adult and near-term fetus would show increased large-conductance Ca 2ϩ activated potassium (BK) channel activity. We measured BK channel activity, expression, and cell surface distribution by use of patch-clamp electrophysiology, flow cytometry, and confocal microscopy, respectively, in myocytes from normoxic control and LTH adult and nearterm fetus BA. Electrophysiological data showed that BK channels in LTH myocytes exhibited 1) lowered Ca 2ϩ set points, 2) left-shifted activation voltages, and 3) longer dwell times. BK channels in LTH myocytes also appeared to be more dephosphorylated. These differences collectively make LTH BK channels more sensitive to activation. Studies using flow cytometry showed that the LTH fetus exhibited increased BK 1 subunit surface expression. In addition, in both fetal groups confocal microscopy revealed increased BK channel clustering and colocalization to myocyte lipid rafts. We conclude that increased BK channel activity in LTH BA occurred in association with increased channel affinity for Ca 2ϩ and left-shifted voltage activation. Increased cerebrovascular BK channel activity may be a mechanism by which LTH adult and near-term fetal sheep can acclimatize to long-term high altitude hypoxia. Our findings suggest that increasing BK channel activity in cerebral myocytes may be a therapeutic target to ameliorate the adverse effects of high altitude in adults or of intrauterine hypoxia in the fetus. ) channels, which are well known to modulate vascular tone and promote vasorelaxation (3,7,13,28).In contrast with the role of BK channels in mediating cerebrovascular response to short-term hypoxia, their role in acclimatization to long-term hypoxia (LTH) is less well known. During high-altitude LTH, the CBF in adult humans (23, 60) and sheep (34, 63) returns to normal following a period of transitional increased blood flow, as compiled by Brugniaux et al. (8). The ovine near-term fetus diverts an increased fraction of total cardiac output to the brain, when subjected to hypoxia during gestation. This implies that the cerebral vasculature is more dilated than the systemic vasculature (37, 38). Increased vessel dilation was also the major factor contributing to increased CBF and maintenance of oxygenation in human volunteers taken to high altitude or subjected to acute hypoxia at sea level (66). Because increased blood flow correlates directly with increased vessel diameter in mid-cerebral arteries in humans, as reviewed by Ains...
Two kinds of cnida predominate in the tentacles of the acontiate sea anemones: spirocysts and microbasic mastigophore nematocysts. These cnidae discharge in response to appropriate mechanical and chemical stimulation. In this paper, we calculate the strengths of attachment between the tentacle and the capsules (= "tentacle adherence") of discharged spirocysts and mastigophores by measuring adhesive force and by determining the numbers of spirocysts and mastigophores discharged onto targets under conditions where the attachment of everted cnida tubules to the target (= "cnida adhesion") exceeds tentacle adherence. Under these conditions, the average contribution of individual cnidae to adhesive force is called the intrinsic adherence. The intrinsic adherence is a measure of the average frictional force required to dislodge the capsule of individual discharged cnidae from the tentacle. The intrinsic adherence of discharged mastigophores varies inversely, from 0.45 to 0.15 mgf (4.41 to 1.47 μN), with the number of discharged mastigophores. The larger values characterize mastigophores discharged by mechanically triggering nonchemosensitized tentacles, whereas the lower values characterize the intrinsic adherence of mastigophores discharged from chemosensitized tentacles. In contrast, the intrinsic adherence of discharged spirocysts is very low to insignificant. Thus, by comparison to mastigophores, spirocysts contribute little, if any, to adhesive force, and, by inference, do not directly secure captured prey to the tentacle. Our measurements indicate that penetrable prey are primarily secured to the tentacle by discharged mastigophores and by the inherent stickiness of the tentacle surface.
Feeding behavior in cnidarians is a sequence of coordinated responses beginning with nematocyst discharge. The nematocyst response produces prey capture by envenomating prey and attaching prey to the tentacle. The strength of attachment of discharged nematocysts to the tentacle is termed intrinsic adherence and is calculated from measurements of adhesive force. Following prey capture, the feeding response involves movement of the tentacles toward the mouth and mouth opening. For ingestion to occur, nematocysts attaching the prey to the tentacles must be released from the tentacle. A nematocyst release response has been proposed, but never documented nor measured. Our criterion for a nematocyst release response is that the intrinsic adherence of discharged nematocysts must decrease to zero. The unit of nematocyst discharge in sea anemone tentacles is the cnidocyte/ supporting cell complex (CSCC). The nematocyst response includes nematocysts discharged from Type C CSCCs by physical contact alone and nematocysts discharged from the more numerous Type B CSCCs that require both chemosensitization and physical contact. We identify two prey-derived substances, N-acetylneuraminic acid (NANA) and glycine, both of which chemosensitize nematocyst discharge from Type B CSCCs at low concentrations. At higher concentrations NANA stimulates the release response of Type Cs, and glycine stimulates the release response of Type Bs.
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