Glyne U. Thorington

Control of Cnida Discharge: II. Microbasic p-Mastigophore Nematocysts are Regulated by Two Classes of Chemoreceptors

Using tentacles of the sea anemone, Aiptasia pallida, Thorington and Hessinger ( 1984, 1988a, b) re cently identified two classes of Chemoreceptors involved in sensitizing cnidocytes to discharge cnidae in response to mechanical stimuli. Discharge of cnidae was quanti fied by measuring adhesive force between the tentacles and a test object. This measurement is assumed to reflect the contribution of the three types of cnidae present in the tentacles of A. pallida: the adherent spirocysts and two types of penetrant nematocysts, the predominant microbasic p-mastigophores and the basotrichous isorhi- zas. In the present paper we directly measure the dis charge of the microbasic p-mastigophores and show that mastigophore-containing cnidocytes are sensitized by representative agonists for these two classes of Chemore ceptors. We also show that under certain conditions the number of discharged microbasic p-mastigophores cor relates linearly to a major component of the measured adhesive force. This enables us to calculate the contribu tion to adhesive force made by individual mastigo-

Control of Cnida Discharge: III. Spirocysts are Regulated by Three Classes of Chemoreceptors

Spirocysts are two to three times more abundant than nematocysts in the feeding tentacles of acontiate sea anemones. Despite their prevalence, little experimental work has been done on the discharge of spirocysts because of the difficulty in detecting and counting them after they have discharged. To circumvent this problem, we have developed a simple, reliable, enzyme-linked lectin sorbent assay (ELLSA) for quantifying discharged spirocysts. With this method, we have shown that the discharge of spirocysts, like that of mastigophore nematocysts, is chemosensitized in a dose-dependent manner by three classes of low molecular weight substances, typified by N-acetylneuraminic acid (NANA), glycine, and certain heterocyclic amino compounds, such as proline and histamine. We also show that spirocysts exhibit considerable agonist-specific variation in the dose-responses of discharge, suggesting the existence of multiple populations of spirocyst-bearing cnidocyte/supporting cell complexes (CSCCs). Our findings call into question commonly held views regarding the respective roles of spirocysts and mastigophore nematocysts in the retention of captured prey.

Efferent Mechanisms of Discharging Cnidae: II. A Nematocyst Release Response in the Sea Anemone Tentacle

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.

Effects of Satiation and Starvation on Nematocyst Discharge, Prey Killing, and Ingestion in Two Species of Sea Anemone

Studies spanning 60 years with several cnidarian species show that satiation inhibits prey capture and ingestion and that starvation increases prey capture and ingestion. Most have attributed the effects of satiation to inhibition of nematocyst discharge. We hypothesized that satiation inhibits prey capture and ingestion in sea anemones (Haliplanella luciae and Aiptasia pallida) primarily by inhibiting the intrinsic adherence (i.e., holding power) of discharging nematocysts. Using a quantitative feeding assay for H. luciae, we found that satiation completely uncoupled prey killing from prey ingestion, while nematocyst-mediated prey killing was only partially inhibited. Using A. pallida to measure nematocyst discharge and nematocyst-mediated adhesive force, we showed that satiation completely inhibited the intrinsic adherence of discharging nematocysts from Type B and Type C cnidocyte/supporting cell complexes (CSCCs), while only partially inhibiting nematocyst discharge from Type Bs. These inhibitory effects of satiation were gradually restored by starvation, reaching a maximum at 72 h after feeding. Thus, the effects of satiation and starvation on prey killing and ingestion in two species of acontiate sea anemones are primarily due to changes in the intrinsic adherence of nematocysts from both Type B and Type C CSCCs.

Long-term hypoxia increases calcium affinity of BK channels in ovine fetal and adult cerebral artery smooth muscle

Acclimatization to high-altitude, long-term hypoxia (LTH) reportedly alters cerebral artery contraction-relaxation responses associated with changes in K(+) 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(2+) 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 near-term fetus BA. Electrophysiological data showed that BK channels in LTH myocytes exhibited 1) lowered Ca(2+) 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 β1 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(2+) 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.