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

Microanatomy and ultrastructure of the foot of the infaunal bivalve Tegillarca granosa (Bivalvia: Arcidae)

In this study, the morphology and ultrastructure of the foot of Tegillarca granosa was compared with the bivalves from different habitats. The sediment of habitat of T. granosa is mostly a mixture of sand (68.93%) and mud (24.12%). The foot is wedge-shaped with multiple projections on the surface and covered with ciliary tufts. The epithelial layer is simple and composed of ciliated columnar epithelia and mucous cells. Although the mucous cells are distributed mostly in the epithelial layer, they are developed even in the connective tissues and muscle layers, and the mucous cells mostly contain acidic carboxylated mucosubstances. From the TEM observation, secretory cells are classified into three types. Type A secretory cell has a goblet form and is most widely distributed among the three types. Type B secretory cell has an oval form and the secretory granule has fibrous substance. Type C secretory cell has an elongated elliptic form and membrane-bounded secretory granules. The muscle fiber bundles are composed mainly of smooth muscle fibers. The smooth muscle fibers can be divided into two types. Type A muscle fibers have evenly distributed thick microfilaments between the thin microfilaments of cytoplasm. Type B muscle fiber has cluster of condensed microfilaments in the medulla cytoplasm while the cortical cytoplasm has loose distribution of thin microfilaments.

RFamide neuropeptide actions on the molluscan heart

FMRFamide and the related tetrapeptide FLRFamide are highly excitatory in molluscan non-cardiac smooth muscle. They are also exceptionally excitatory in the atrium and internally perfused ventricle of Busycon canaliculatum. These two peptides, usually thought of as classic molluscan cardio-acceleratory agents are in fact simply two members of a large and ever growing superfamily, the RFamide family, whose phylogenetic distribution has been so elegantly mapped by Walker. Members of this family, often with extended peptide chains (e.g. penta, hepta and decapeptides), stretch in their known distribution from the cnidaria to the chordates. The effects of some of the members of this superfamily (FMRFamide. FLRFamide, YMRFamide, TNRNFLRFamide, SDPFLRFamide, LMS) were examined. The neuropeptides were found to be very potent at very low concentrations (10(-9) M) in the ventricle of both Buccinium and Busycon. Other neuropeptides (HFMRdFamide, SCPb, NLERFamide and pEGRFamide) were found to be without any effect. The Ca2+ dependency of these neuropeptides was also tested. The peptides appear to induce contraction of the ventricles by release of Ca2+ from internal pools. The neuropeptides appear to stimulate contraction in these cardiac muscles through a completely different pathway to Serotonin (the main excitatory neurotransmitter for the cardiac muscle). When the peptides were applied together with Serotonin an additive effect was observed clearly indicating the release of Ca2+ through different pathways. The nature of the RFamide receptor was also tested. It appears that the RFamide neuropeptides mobilize the 2nd messenger IP3 (Inositol trisphosphate), since the IP3 blocker Neomycin Sulphate inhibited the response of the neuropeptides.

Ordered distribution of membrane-associated dense plaques in intact quail gizzard smooth muscle cells revealed by freeze-fracture following treatment with cholesterol probes

The surface distribution of membrane-associated dense plaques in intact quail gizzard smooth muscle cells was investigated by freeze-fracture. Replicas of fractured smooth muscle cell plasma membrane showed caveola-free regions with few intramembrane particles, interspersed with caveola-populated areas with a higher intramembrane particle density. Electron microscopy of thin sections of quail gizzard smooth muscle revealed the regions free of caveolae to be occupied by membrane-associated dense plaques; anchoring sites for the contractile filaments of the cell. Demarcation between the caveola-populated and caveola-free regions on the relicated intramembrane surface was not clear and thus provided little information concerning the distribution of dense plaque sites. However, treatment of the smooth muscle tissue with the cholesterol-binding agents filipin or tomatin prior to freeze-fracture allowed the dense plaque sites to be easily observed as the sites remained free of the membrane deformations characteristic of these agents. The dense plaque sites consist of caveola-free oval areas juxtaposed in regular bands that traverse the long axis of the cell. The dense plaque sites on the freeze-fracture replica were confirmed by electron microscopy of thin sections of filipin-treated quail gizzard smooth muscle cells, which showed the plasma membrane associated with the dense plaques to be unaffected by the actions of filipin, whereas that of the caveola-populated region was severely deformed. The observations presented in this study provide evidence for a highly ordered distribution of dense plaques at the cell surface of intact quail gizzard smooth muscle cells and thus corroborate existing evidence for an organized substructure of smooth muscle cells.

Dependence upon external calcium for contractile activity in two molluscan proboscis muscles

1. Both the radular sac and odontophore retractor muscles of Buccinum undatum depend upon [Ca]0 to raise the [Ca]i concentration of the contractile system to activation level. 2. The K-induced responses of the muscles depend mainly upon [Ca]0 for activator Ca while the ACh responses depend upon [Ca]0 to raise stored intracellular Ca to activation levels. 3. In the radular sac muscle, it is probable that the inward current is carried by Na+ or is Na(+)-dependent and this current may release [Ca]i for contraction since the muscle became spontaneously active during ACh- and K-contractures in Ca-free seawater containing 2 mM EGTA as a calcium chelator. 4. It is proposed that since calcium antagonists are more inhibitory on ACh responses than on K-contractures, ACh releases the activator calcium for the contractile system through a slow-type Ca channel while high K releases Ca through a fast-type calcium channel in these muscles.

Electromechanical uncoupling in a molluscan muscle examined by the sucrose gap technique. The effect of calcium antagonist and agonist agents

Membrane potential and tension of Busycon radular protractor muscles were studied by sucrose gap methods. Excitation-contraction (EC) coupling was examined in response to acetylcholine (ACh) and high K which depolarized the fibres and induced tension, but without action potential firing. Potassium depolarization did not follow predictions expected from the Nernst equation at low and very high K levels, and maximum tension was found at about 100 mM K. EC coupling was very sensitive to [Ca]0. Ca-free media eliminated K- and ACh-induced tension but with normal depolarization, showing full electromechanical uncoupling. Ionophore A23187 enhanced K- and ACh-induced responses and X-537A enhanced ACh responses, demonstrating acute dependence of activation on [Ca]0 in this muscle. The calcium antagonists nifedipine and nisoldipine reduced tension in the muscle only at very high concentrations, and both agents slightly reduced K- and ACh-induced depolarization. Verapamil reduced K- and ACh-induced tension but paradoxically it enhanced the depolarizing actions of these agents leading to electromechanical uncoupling. Abscisic acid (ABA) enhanced ACh- and K-induced tension and simultaneously enhanced their depolarizing actions. Ionophores and ABA appear to enhance calcium influx which may secondarily influence sodium influx. Calcium antagonists have no consistent actions on this muscle, suggesting that calcium channel activity of the radular protractor may be different from that seen in mammalian visceral muscles.