Contraction in a muscle with negligible sarcoplasmic reticulum: The longitudinal retractor of the sea cucumberIsostichopus badionotus (Selenka), holothuria aspidochirota

Role of Ca(2+) in excitation-contraction coupling in echinoderm muscle: comparison with role in other tissues

The longitudinal muscle of the body wall of Isostichopus badionotus may be considered a model for excitation-contraction coupling in echinoderm muscle. Other echinoderm muscles are reviewed by comparison with the model. Echinoderm muscle is also of interest as a model for ‘mutable collagenous tissue’; however, in that tissue, Ca(2+) has been proposed to function both in living control systems and in regulation of non-living interstitial substance.

Ca(2+)transport by the sarcoplasmic reticulum Ca(2+)-ATPase in sea cucumber (Ludwigothurea grisea) muscle

In muscle cells, the excitation-contraction cycle is triggered by an increase in the concentration of free cytoplasmic Ca(2+). The Ca(2+)-ATPase present in the membrane of the sarcoplasmic reticulum (SR) pumps Ca(2+) from the cytosol into this intracellular compartment, thus promoting muscle relaxation. The microsomal fraction derived from the longitudinal smooth muscle of the body wall from the sea cucumber Ludwigothurea grisea retains a membrane-bound Ca(2+)-ATPase that is able to transport Ca(2+) mediated by ATP hydrolysis. Immunological analyses reveal that monoclonal antibodies against sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA1 and SERCA2a) cross-react with a 110 kDa band, indicating that the sea cucumber Ca(2+)-ATPase is a SERCA-type ATPase. Like the mammalian Ca(2+)-ATPase isoforms so far described, the enzyme also shows a high affinity for Ca(2+) and ATP, has an optimum pH of approximately 7.0 and is sensitive to thapsigargin and cyclopiazonic acid, specific inhibitors of the SERCA pumps. However, unlike the mammalian SERCA isoforms, concentrations of ATP above 2 mmol l(−1) inhibit Ca(2+) transport, but not ATP hydrolysis, in sea cucumber vesicles, suggesting that high ATP concentrations uncouple the Ca(2+)-ATPase. Another unique feature observed with the sea cucumber Ca(2+)-ATPase is the high dependence of maximal activity on K(+) or Na(+). Similar activation promoted by these cations was observed with various mammalian Ca(2+)-ATPase preparations when they were incubated in the presence of low concentrations of sulphated polysaccharides. In control experiments, K(+) and Na(+) have almost no effect on Ca(2+) transport, but in the presence of heparin or fucosylated chondroitin sulphate, the activity of the different mammalian Ca(2+)-ATPases is inhibited and they are activated by either K(+) or Na(+) in a manner similar to the native sea cucumber ATPase. These results led us to investigate the possible occurrence of a highly sulphated polysaccharide on vesicles from the SR of sea cucumber smooth muscle that could act as an ‘endogenous’ Ca(2+)-ATPase inhibitor. In fact, SR vesicles derived from the sea cucumber, but not from rabbit muscle, contain a highly sulphated polysaccharide. After extraction and purification of these polysaccharide molecules, their effect was tested on vesicles obtained from rabbit muscle. This compound inhibited Ca(2+) uptake in rabbit SR vesicles, at concentrations lower than heparin, and restored the dependence on monovalent cations. These results strongly suggest that the sea cucumber Ca(2+)-ATPase is activated by monovalent cations because of the presence of endogenous sulphated polysaccharides.

Catalytic activity and heat production by the Ca(2+)-ATPase from sea cucumber (Ludwigothurea grisea) longitudinal smooth muscle: modulation by monovalent cations

In muscle cells, excitation-contraction coupling involves the translocation of Ca(2+) between intracellular compartments and the cytosol. Heat derived from the hydrolysis of ATP by the sarcoplasmic reticulum Ca(2+)-ATPase of skeletal muscle plays an important role in the thermoregulation and energy balance of the cell. Although several Ca(2+)-ATPase isoforms have been described in vertebrates, little is known about Ca(2+) transport in invertebrates. In this report, a Ca(2+)-ATPase is identified in the microsomal fraction obtained from sea cucumber (Ludwigothurea grisea) smooth muscle. The activity of this enzyme is enhanced three- to fivefold by K(+) and Na(+). During Ca(2+) transport, the ATPase can synthesise ATP from ADP and inorganic phosphate (P(i)) using the energy derived from the Ca(2+) gradient formed across the microsomal membrane (ATP<->P(i) exchange). The apparent affinity of the enzyme for P(i) is increased by more than one order of magnitude by K(+). In the presence of K(+), the fraction of ATP synthesised during the exchange reaction by sea cucumber microsomes was found to be larger than that measured in microsomes derived from either rabbit or trout muscle. Like the isoforms found in skeletal muscle, the sea cucumber Ca(2+)-ATPase can convert osmotic energy into heat. The amount of heat produced after the hydrolysis of each ATP molecule increases two- to threefold when a Ca(2+) gradient is formed across the microsomal membrane.

A non-invasive vibrating calcium-selective electrode measures acetylcholine-induced calcium flux across the sarcolemma of a smooth muscle

To determine possible sources of Ca2+ during excitation-contraction coupling in smooth muscle, a vibrating Ca2(+)-selective electrode was used to measure Ca2+ flux during the process of contraction. The smooth muscle model was the longitudinal muscle of the body wall of a sea cucumber Sclerodactyla briareus. Because acetylcholine caused slow contractions of the muscle that were inhibited by Ca2+ channel blockers diltiazem and verapamil in earlier mechanical studies, we chose a vibrating Ca2(+)-selective electrode as our method to test the hypothesis that acetylcholine may be stimulating Ca2+ influx across the sarcolemma, providing a Ca2+ source during excitation-contraction coupling. Acetylcholine treatment stimulated a net Ca2+ efflux that was both dose and time dependent. We then tested two L-type Ca2+ channel blockers, diltiazem and verapamil, and two non-specific Ca2+ blockers, cobalt (Co2+) and lanthanum (La3+) on acetylcholine-induced Ca2+ flux. All four Ca2+ blockers tested potently inhibited Ca2+ efflux induced by physiological doses of acetylcholine. We propose that the acetylcholine-induced Ca2+ efflux was the result of, first, Ca2+ influx through voltage-sensitive L-type Ca2+ channels, then the rapid extrusion of Ca2+ by an outwardly directed carrier such as the Na-Ca exchanger as suggested by Li+ substitution experiments. The vibrating Ca2+ electrode has provided new insights on the active and complex role the sarcolemma plays in Ca2+ homeostasis and regulating Ca2+ redistribution during excitation-contraction coupling.

Morphological and physiological properties of non-striated muscle from the tunicate, Ciona intestinalis: parallels with vertebrate skeletal muscle

Non-striated muscle from the longitudinal body wall muscle in an adult ascidian (tunicate) is characterized morphologically and physiologically. These muscles are unique among chordate non-striated (i.e. 'smooth') muscles in that they are composed of discrete bundles of several small diameter (4-10 microns) muscle cells (fibers) arranged in parallel functional units. Each bundle is wrapped by a basal lamina, and at least some appear to be directly innervated at a neuromuscular junction similar to an end plate. In these regards, a bundle of Ciona smooth muscle cells is analogous to a skeletal muscle fiber of a vertebrate. This analogy also extends to physiological properties. Ciona muscle generates a rapid all-or-none Ca action potential which gives rise to a brisk twitch with brief latency. These anatomical and physiological adaptations are discussed in terms of the evolution of vertebrate skeletal muscle.

Effects of calcium antagonists on contraction of a holothurian muscle

Contractions evoked by acetylcholine or by tetanic stimulation differed from caffeine contractures, in muscle strips isolated from the longitudinal muscle bands of the body wall of Isostichopus badionotus (Selenka), an aspidochirote holothurian. Tetanic contractions and responses to acetylcholine remained reproducible for hours in sea water or artificial sea water. Caffeine contractures declined rapidly during a series of repetitions, carried out in a bath medium which had a calcium content equivalent to that of sea water. Manganese, lathanum, and dantrolene have been used as calcium antagonists, with the objective of differentiating the calcium stores used in reproducible contractions from the calcium stores used in rapidly extinguishing contractures. Muscle strips were pretreated with an ionophore (X-537A) to confer reproducibility in a series of caffeine contractures, before use of calcium antagonists. For reproducible caffeine contractures, the order of effectiveness of calcium antagonists was lanthanum greater than manganese greater than dantrolene. The order of reversibility was manganese greater than dantrolene greater than lanthanum. For acetylcholine contractions, or tetanic contractions, the order of effectiveness of calcium antagonists was lanthanum greater than manganese and the order of reversibility was manganese greater than lanthanum. Dantrolene reversibility enhanced contractions.

Evidence that myosin light chain phosphorylation regulates contraction in the body wall muscles of the sea cucumber

The Ca2+ activation mechanism of the longitudinal body wall muscles of Parastichopus californicus (sea cucumber) was studied using skinned muscle fiber bundles. Reversible phosphorylation of the myosin light chains correlated with Ca2+-activated tension and relaxation. Pretreatment of the skinned fibers with AT-P gamma S and high Ca2+ (10(-5)M) resulted in irreversible thiophosphorylation of the myosin light chains and activation of a Ca2+ insensitive tension. In contrast, pretreatment with low Ca2+ (10(-8)M) and ATP gamma S results in no thiophosphorylation of the myosin light chains or irreversible activation of tension. These results are consistent with a Ca2+-sensitive myosin light chain kinase/phosphatase system being responsible for the activation of the muscle. Other agents known to have an effect upon the Ca2+-activated tension in skinned vertebrate smooth muscle fibers (trifluoperazine, catalytic subunit of the cyclic AMP-dependent protein kinase, and calmodulin) did not have an effect on myosin light chain phosphorylation or Ca2+-activated tension. These results suggest a different type of myosin light chain kinase than is found in vertebrate smooth muscle is responsible for the activation of parastichopus longitudinal body wall muscle.

Molluscan visceral muscle fine structure. General structure and sarcolemmal organization in the smooth muscle of the intestinal wall of Buccinum undatum L

Fine structure of intestinal muscle in the gastropod Buccinum undatum is described. Myofibrillar organization is typical of non-pseudostriated molluscan muscles. The dense body system is poorly developed but there are extensive attachments plaques. The sarcolemma is elaborately modified. Deep infoldings of the membrane give the cells an irregular outline. Such infoldings enclose extracellular matrix and are associated with attachment plaques. Arising from these and from the general sarcolemma are numerous tubular membranous invaginations ending blindly at varying depth in the sarcoplasm. These structures have a helical coat of particles on the cytoplasmic face. Associated with both types of invagination are subsarcolemmal vesicles. The possibility that the tubular invaginations are analogues of vertebrate smooth muscle caveolae or striated muscle T-tubules and that the vesicles are the corresponding sarcoplasmic reticulum is discussed. The occurrence of such structures in molluscan muscle and elsewhere is reviewed.