Types of smooth muscle

The two major types of smooth muscle (although many smooth muscles exhibit properties of each type) are  

Multiunit smooth muscle is located in the large blood vessels, eyes (iris and ciliary muscle of the lens), and piloerector muscles at the base of hair follicles. This type of muscle consists of discrete smooth muscle cells or units that function independently. Each of these units is innervated by the 

Action potentials are generated in single-unit smooth muscle. Simultaneous depolarization of 30 to 40 smooth muscle cells is required to generate a propagated action potential the presence of gap junctions allows this to occur readily. Because single-unit smooth muscle is self-excitable and capable of generating action potentials without input from the autonomic nervous system, it is referred to as myogenic. In this muscle, the function of the autonomic nervous system is to modify contractile activity only. Input is not needed to elicit contraction. 

The ability to depolarize spontaneously is related to the unstable resting membrane potentials in single-unit smooth muscle. Two types of spontaneous depolarizations may occur  

A pacemaker potential involves gradual depolarization of the cell membrane to threshold. The subsequent generation of an action potential causes smooth muscle contraction. This type of spontaneous depolarization is referred to as a pacemaker potential because it creates a regular rhythm of contraction. 

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The autonomic nervous system (ANS) modifies contractile activity of both types of smooth muscle. As discussed in Chapter 9, the ANS innervates the smooth muscle layer in a very diffuse manner, so neurotransmitter is released over a wide area of muscle. Typically, the effects of sympathetic and parasympathetic stimulation in a given tissue oppose each other one system enhances contractile activity while the other inhibits it. The specific effects (excitatory or inhibitory) that the two divisions of the ANS have on a given smooth muscle depend upon its location. 

In most types of smooth muscle cells (SMCs), stimulation of G protein-coupled receptors (GPCRs) results in a large Ca2+ transient and a sustained increase in intracellular Ca2+ concentration, which are thought to be mediated by inositol-1,4,5-trisphosphate (L1SP3)-mediated Ca2+ release and accompanying transmembrane Ca2+ entry, respectively. Various experimental techniques and protocols have revealed that although there appear to be some differences in the extent of their contribution, at least three distinct Ca2+-mobilizing mechanisms can... 

Bolton I call interstitial cells of Cajal (ICCs) a type of smooth muscle. 

FIG. 2. Confocal image of an isolated smooth muscle cell from guinea-pig ileum which has been permeabilized with staphylococcal os-toxin and incubated with 150 /iM Fluo-3 acid which stains the SR. The SR is predominantly localized to the periphery of this type of smooth muscle as seen on the left hand side where the image plane is through the centre of the cell, whereas an extensive network is seen where the image plane is adjacent to the plasma membrane as seen in the right hand portion of the cell. 

The physiological function of heparin is not completely understood. It is found only in trace amounts in normal circulating blood. It exerts an antihpemic effect by releasing lipoprotein lipase from endothehal cells heparinlike proteoglycans produced by endothelial cells have anticoagulant activity. Heparin decreases platelet and inflammatory cell adhesiveness to endothelial cells, reduces the release of platelet-derived growth factor, inhibits tumor cell metastasis, and exerts an antiproliferative effect on several types of smooth muscle. 

Nitroglycerin relaxes all types of smooth muscle regardless of the cause of the preexisting muscle tone (Figure 12-3). It has practically no direct effect on cardiac or skeletal muscle. 

Most types of smooth muscle are dependent on transmembrane calcium influx for normal resting tone and contractile responses. These cells are relaxed by the calcium channel blockers (Figure 12-3). Vascular smooth muscle appears to be the most sensitive, but similar relaxation can be shown for bronchiolar, gastrointestinal, and uterine smooth muscle. In the vascular system, arterioles appear to be more sensitive than veins orthostatic hypotension is not a common adverse effect. Blood pressure is reduced with all calcium channel blockers. Women may be more sensitive than men to the hypotensive action of diltiazem. The reduction in peripheral vascular resistance is one mechanism by which these agents may benefit the patient with angina of effort. Reduction of coronary artery tone has been demonstrated in patients with variant angina. 

The prostaglandins and thromboxanes have major effects on four types of smooth muscle airway, gastrointestinal, reproductive, and vascular. Other important targets include platelets and monocytes, kidneys, the central nervous system, autonomic presynaptic nerve terminals, sensory nerve endings, endocrine organs, adipose tissue, and the eye (the effects on the eye may involve smooth muscle). 

It is Intriguing to speculate whether this same kind of ultra-structural differentiation (different repeating units) underlies the recognized ability of a single type of smooth muscle cell, for example, to respond to stimulation by a number of different hormones and humoral substances, each in a chemically precise but different way. Hokin has already suggested the existence of "membrane patches of active llproproteln... 

Although the physiological events occurring subsequent to a- or P-recep-tor stimulation are beyond the scope of this article, a brief outline of the major events following pharmacological stimulation or blockade of these receptors may be of some use to the reader. Activation of a receptors is associated with a peripheral excitatory action on certain types of smooth muscle, such as in blood vessels supplying skin and mucous membranes a peripheral inhibitory action on other types of smooth muscle, such as in the gut and excitatory actions generalized in the central nervous system, such as... 

The distribution of the LClTa and LClTb in different types of smooth muscles is shown in Table I. In many cases, LClTa is the major isoform and it is the only isoform in most mammalian gastrointestinal muscles and chicken gizzard. In uterus, LClTa gradually increases during pregnancy, whereas in the nonpregnant state LClTb is the predominant isoform. There is some variation in the isoform distribution when the same tissue is analyzed by different laboratories or even by the same laboratory. 

It is not known if this difference reflects a real difference in the magnitude of the assembly-disassembly process in different types of smooth muscle cells. 

In intact smooth muscle, phosphorylation often declines while force is maintained, the so-called "latch" state (Dillon etal., 1981). Regulation of the latch state is still poorly understood. It has been very difficult to induce a latch state in skinned smooth muscle. The closest may be triton skinned chicken gizzard, which contracts independent of MLC phosphorylation (Wagner and Ruegg, 1986). In skinned chicken gizzard, but also in other types of smooth muscle (Bialojan et al., 1987 Siegman et al., 1989 Kenney et al., 1990 Schmidt et al, 1995), a steep and nonlinear relation between force and MLC phosphorylation was observed, which was postulated by the "latch model" of Hai and Mur-... 

In each section on the different ion channels, some unresolved issues and future directions will be addressed. In general, little is known about the precise molecular structures of the ion channels (e.g., K+ channels) in smooth muscle and our knowledge of endogenous agents as well as key signal transduction pathways that may modulate smooth muscle ion channels is far from complete. Further, as indicated previously, the modulation and expression of ion channels vary with the type of, and even within (e.g., large versus small arteries), smooth muscle. Studies on K+ channels in nonvascular types of smooth muscle will be discussed if similar material from arterial smooth muscle is limited. 

Does isosorbide mononitrate have effects on all types of smooth muscle ... 

Yes. Isosorbide nitrates relax all types of smooth muscle including bronchial, biliary, gastrointestinal (GI), ureteral, and uterine. 

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