Smooth muscle types

Somljo It all depends on the fixation method. We use osmium ferricyanide to selectively infiltrate the SR. If we use intermediate high-voltage electron microscopy, we can look at thicker specimens, and this technique provides more extensive views than obtained from the usual thin sections. This is the same information we get when we infiltrate the SR with Fluo-3. These pictures are pretty reliable. Furthermore, if you want to confirm without chemical fixation, methods such as rapid freezing are available. All these techniques give the same pictures, which vary according to the smooth muscle type. 

To date, all smooth muscle types examined display characteristic Ca2+ release events known as Ca2+ sparks (Fig. 1 see Jaggar et al 2000 for review). Ca2+ sparks are localized Ca2+ release events through RyRs in the SR located very close (10—20nm) to the surface membrane. Ca2+ sparks can activate Ca2+ -dependent ion channels in the cell surface, and the physiological response of the particular cell type will reflect the type of ion channels involved (Fig. 1). 

Young In the myometrium, the Ca2+ waves are from the deep cytoplasmic Ca2+ to the plasma membrane. The intracellular Ca2+ waves should be considered separately from either phasic or tonic smooth muscle types. The myometrium is phasic smooth muscle. And intercellular Ca2+ waves are an entirely different phenomenon altogether. 

Dabrowski M, Wahl P, Holmes WE, Ashcroft FM. Effect of repaglinide on cloned beta cell, cardiac and smooth muscle types of ATP-sensitive potassium channels. Diabetologia 2001 44(6) 747-56. 

Pinacidil (6) itself exhibits no selectivity for a particular smooth muscle type and at high concentrations has a mechanism of action that is independent of channel opening [3]. Thus, caution must be exercised when studying SAR relationships as this KCA-independent mechanism can be enhanced. 

In contrast to many other smooth muscle types, ASM is regarded as being multi-unit rather than single unit, which means that each smooth muscle cell is individually innervated. This is associated with a relative lack of gap junctions which normally provide a low resistance electrical pathway for cell to cell communication. Spontaneous electrical activity is rarely observed in ASM cells and action potentials only occur in exceptional circumstances, the usual electrical response to spasmogens being graded depolarization without the overall muscle bundle acting as a syncytium. 

One should be cautious, however, to extrapolate such quantitative data to all smooth muscle types or even to similar ones of other species because of the heterogeneity of smooth muscle. For instance, Stehno-Bittel and Sturek (1992) found evidence for preferential release of Ca + from the ER toward the sarcolem-ma in bovine but not in porcine coronary artery. The ER also differs between different smooth muscles in the expression of different types of calcium-buffering proteins (Raeymaekers etal., 1993) and of the regulator protein phospholamban (see Section V). Furthermore, the data of Eggermont et al. (1988) indicate that the ratio of the number of ER and PM pump proteins also varies considerably between different smooth muscle types. 

Initiation of postjunctional activity. If an EPSP exceeds a certain threshold value, it initiates an action potential in the postsynaptic membrane by activating voltage-sensitive channels in the immediate vicinity. In certain smooth muscle types in which propagated impulses are minimal, an EPSP may increase the rate of spontaneous depolarization, cause Ca + release, and enhance muscle tone in gland ceds, the EPSP initiates secretion through Ca + mobdization. An IPSP, which occurs in neurons and smooth muscle but not in skeletal muscle, wdl tend to oppose excitatory potentials simultaneously initiated by other neuronal sources. The ultimate response depends on the summation of ad the potentials. 

Smooth muscle-type actin isoforms Six isoforms of actin are expressed in mammalian SM tissues that are generated by distinct genes [79] and show an highly conserved amino acid sequences across species. The expression of actin isoforms is developmen-tally regulated in a temporal-spatial manner [80-83]. Fully differentiated SMC contain a- (mainly in vascular tissue) and y-(mainly in the enteric structures) actins and trace amounts of (3- and y-cytoplasmic actin isoforms [79,84-87]. The SM a-isoform is expressed also in striated muscle [88], and the SM y-isoform in post-meiotic sperm [89], and the increased SM y-actin content in hypertrophied bladder SM [86,90]. The unique intracellular distribution of actin filamentous networks [91-93] and the decreased SM a-actin content in proliferating vascular SMC ([94] see Section 6) point to an isoform diversification based on specific functional requirements. 

Capriani A, Chiavegato A, Franch R, Azzarello G, Vinante 0, Sartore S (1997) Oestrogen-dependent expression of SM2 smooth muscle type myosin isoform in rabbit myometrium. J Muscle Res Cell Motil 18 413-427... 

Chiavegato A, Pauletto P, Sartore S (1996) Smooth muscle-type myosin heavy chain isoforms in bovine smooth muscle and non-muscle tissues. Biol Cell 86 27-38... 

D L Miller and J C Owicki, PM A induces a change m phenotype of TE671 cells from a smooth muscle-type towards a skeletal or cardiac muscle-type, 33d Annual Meeting of the ASCB New Orleans, 1993... 

Duodenal tumours are rare. They account for about one-third of all small bowel neoplasms, which in turn represent approximately 5%-6% of all GIT neoplasms (Kazerooni et al. 1992). Benign tumours include adenoma, adenomatous polyp, lipoma, and leiomyoma. The latter is a benign gastrointestinal stromal tumor (GIST) of smooth-muscle type. A lipoma is the only tumour that can be diagnosed by certainty on CT as a well-circumscribed, homogeneous intraluminal mass with characteristic fat density, based on negative attenuation numbers (Fig. 9.16). 

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