Visceral muscles

Botulism was described much later than tetanus (Kemer 1817 Midura and Amon 1976 Pickett et al. 1976), and this delayed recognition is due to its much less evident symptoms, which include a generalized muscular weakness with diplopia, ptosis, dysphagia, facial paralysis, and reduced salivation and lacrimation. The paralysis then progressively descends to affect the muscles of the trunk, including respiratory and visceral muscles. All the symptoms of botulism can be ascribed to the blockade of skeletal and autonomic peripheral cholinergic nerve terminals (Tacket and Rogawski 1989). 

In the early 1980 s, dramatic Improvements in both peptide isolation technology and the instrumentation for structural characterization resulted in an avalanche of new insect neuropeptide structures. Of the 50-60 known structures, about half exhibit effects on the contractile activity of insect visceral muscle at physiological concentrations. This report describes the strategies and tactics that were utilized to successfully isolate, purify, and structurally characterize this group of insect neuropeptides. 

The remainder of this report describes how these visceral muscle bioassays were used to successfully isolate and structurally characterize more than two dozen insect neuropeptides. 

Based upon the observations of Nachman et al. (20), we predicted that the PBAN s would stimulate the isolated visceral muscles of Leucoohaea. This prediction was correct. Hez-PBAN (Nle, Nle ) stimulated the contractions of the isolated Leucoohaea hindgut above a threshold concentration of 14.5 4.2 nM (Holman, G. M. and Nachman, R. J., unpublished.). At threshold concentration, Hez-PBAN (Nle, Nleu ) was about four-fold less active than LPK on the isolated Leucoohaea oviduct (6.3 2.5 nM vs. 

Only two neuropeptides that inhibit the spontaneous contractions of visceral muscle have been structurally characterized. These two peptides, which contain a C-terminal FLRFamide tetramer are almost identical as shown ... 

Like LMS, SchistoFLRFamide is a potent inhibitor of visceral muscle contraction. When applied to the semi-isolated Schistocerca heart preparation, concentrations of SchistoFLRMamide between 1 nM and 0.1 nM produced an observable inhibitory effect. At 1000 nM, spontaneous heart contractions were abolished and remained so for several minutes after removal of the peptide-containing solution... 

LMS was isolated on the basis of its action upon visceral muscle. However, it also effects motor neurons (26). IMS attenuated the evoked transmitter release from the presynaptic membrane of excitatory motor neurons terminating on the skeletal muscle of the mealworm, Tenebrio molitor. but had no postsynaptic effects on that preparation. Although the mechanisms of IMS-induced inhibition of excitatory presynaptic potentials have not been precisely identified, one preliminary experiment suggested that metabolites of arachidonic acid may function as a second messenger for IMS at that site (26). 

The majority of insect neuropeptides discovered thus far seem to have the property of regulating the contractile activity of either skeletal and/or visceral muscles. Whether these are the principal or auxiliary modes of action for a given peptide must await a more comprehensive assessment of their individual profiles of activity in a variety of physiological systems. What follows is a summary of our current understanding of the physiological and pharmacological properties of the insect neuropeptides that regulate muscle activity. 

The presence of proctolin in the reproductive tract of a variety of insects and the action of this peptide on such tissues has been a current focus for many studies. The visceral muscles of the oviducts of Locusta migratoria. for example, are remarkably sensitive to proctolin (30)- Amounts of the peptide as low as 2 fmol induce a tonic contraction that is dose-dependent up to 200 fmol. A proctolin-like immunoreactive substance was found in the axons of nerves leading to the oviduct as well as in a number of cell bodies in the Vllth abdominal ganglion. Moreover, reverse-phase HPLC of tissue extracts of oviducts, oviducal nerves, and the Vllth abdominal ganglia indicate that the proctolin-like bioactive substance co-elutes with authentic proctolin. The peptide was present in areas of the oviduct that receive extensive innervation while it was ten-fold less in areas that receive little or no innervation. 

Two recent studies have shown that proctolin has a potent stimulating effect on the visceral muscles of the oviduct in Rhodnius prolixus (33,34) and in the stable fly Stomoxvs calcitrans (35). In both of these insects, reverse-phase HPLC and quantitative bioassay were used to demonstrate the presence of a proctolin-like substance in extracts of the reproductive tissues. Immunohistochemical preparations revealed intense proctolin-like immunoreactivity in terminal arborisations of nerves on the spermathecae, common oviduct and bursa of Rhodnius. The muscles of the ovarian sheath in stably fly responded to proctolin, and 0.5 to... 

Although the hindgut showed high sensitivity to the LK s, other t3rpes of visceral muscle were less responsive. The foregut and oviduct, for example, were 100-1000 fold less sensitive than the hindgut. A concentration of more than 10" M of each of the peptides was required to elicit a detectable response from these... 

Visceral muscle function Visual disturbances (pupillary autonomic responses) Bladder instability (bladder capacity, detrustor contractions) Intestinal motility decreased Anticholinergic medications... 

Skeletal muscle is a major component of body tissue and accounts for 40-50% of the body weight. Skeletal tissue is composed of specialized striated cells, which function to convert chemical energy to mechanical work. Skeletal muscle plays a central role in body metabolism and serves as a source of body heat and a storage depot for energy-rich compounds, protein, and intracellular ions (e.g., potassium). It also contains up to 80% of the body water content. In contrast to cardiac and smooth visceral muscle tissue, skeletal muscle is under voluntary control. 

Ectopic expression experiments have also been performed to study the functional potential of the tinman gene. Perhaps surprisingly, the consequences of prolonged ubiquitous tinman expression in the mesoderm or in the whole embryo were relatively mild. The main effect was a transient increase in the number of heart precursors during stage 11, but the formation of the heart, visceral muscles, and body wall muscles appeared not to be disturbed (Yin and Frasch, 1998). These results indicate that coregulators, whose expression or activity is spatially restricted, are obligatory for all known aspects of tinman function. 

B. bagpipe, a Homeobox Gene Required for Visceral Muscle Formation... 

In the past few years, comparisons between invertebrate and vertebrate systems have led to the conclusion that an astonishing number of regulatory mechanisms in mesoderm development have been evolutionarily conserved. In particular, genes that are homologous to tinman and bagpipe appear to have related roles in heart and visceral muscle development, respectively, in vertebrate embryos (Lyons et al., 1995 Tribioli et al., 1997). Furthermore, the Dpp-related bone morphogenetic proteins... 

Bodmer, R. (1993). The gene tinman is required for specification of the heart and visceral muscles in Drosophila. Development 118 1X9-129. 

Georgias, C., Wasser, M., and Hinz, U. (1997). A basic-helix-loop-helix protein expressed in precursors of Drosophila longitudinal visceral muscles. Mech. Dev. 69 115-124. 

Control of Hemolymph Lipid and Visceral Muscle in Locusts... 

Two examples of the multifunctional properties of octopamine in locusts are highlighted namely the control of haemolymph lipid and oviduct visceral muscle. Adenosine 3, 5 -monophosphate (cylcic AMP) is believed to act as the cellular mediator for many of the physiological actions of octopamine, although at least one other mechanism of action exists. 

Locusta Oviduct visceral muscle Inhibits myogenic contractions Modulates neuromuscular transmission DUMOV 2... 

A similar series of experiments has now been performed in insect visceral muscle (81). Octopamine elevates cyclic AMP content of the lateral oviduct of locusts. This effect is dose-dependent in the presence of IBMX, with a threshold dose at about 10"% and half-maximal stimulation at 7 x 10"%. Forskolin also elevates cyclic AMP content and IBMX potentiates the action of forskolin. 

The evidence strongly suggests that, as with skeletal muscle, the octopaminergic control of this insect visceral muscle is mediated at least in part by cyclic AMP. 

A correlation between smooth muscle stimulation and increased metabolism of inositol phosphates has been known for some time, for example, in carbachol-or NE-stimulated rabbit iris (Abdel-Latif et al, 1977), in carbachol- or ACh-stimulated trachea (Baron et al., 1984 Hashimoto et al., 1985 Takuwa et al., 1986 Duncan et al., 1987), in angiotensin- or vasopressin-stimulated vascular muscle (Smith et al., 1984 Doyle and Ruegg, 1985), in ACh- or histamine-stimulated visceral muscle (Best et al, 1985), in oxytocin-stimulated human myometrium (Amiot etal., 1993), or in bombesin-stimulated rat myometrium (Schrey etal.,... 

---