Mechanical Response Of Smooth Muscle In Vivo

Most smooth muscles form, along with other tissues, the walls of hollow organs such as the gastrointestinal tract, the uterus, and the blood vessels. In these organs, smooth muscle contractions serve many purposes such as the movement of lumenal contents, the regulation of lumenal volume, and the alteration of the resistance to flow through the lumen. For some of these functions, contractions must be phasic to allow the lumens to refill between contractions. For others, contractions must be tonic to resist continual distending forces. Thus, it is not surprising to learn that many factors regulate smooth muscle contractions in vivo and that the exact factors responsible for variations in the contractile activities of most individual smooth muscles are unknown.

Regulation of smooth muscle contraction is difficult to study in vivo for many reasons. Most smooth muscles are multiply innervated. Many have membrane receptors for circulating hormones and locally released paracrines and autocoids. In addition many smooth muscles respond directly to stretch of their membranes. Also, in contrast to what occurs in striated muscle, certain ligand-receptor interactions in smooth muscle lead to inhibition of contraction rather than excitation. Thus, at any one time, a given smooth muscle cell will be receiving multiple inputs, some excitatory and some inhibitory.

Response to Stretch

Quick or sustained stretch of many smooth muscles results in contraction of that muscle. In many instances, contraction occurs even in the absence of nerves; thus, it is not due to activation of a neural reflex as in skeletal muscle. Contraction most likely results from membrane depolarization or from the opening of stretch-activated calcium channels. Such responsiveness may explain why organs such as the stomach, bladder, and small arte-rioles contract to oppose distension. In some smooth muscles, especially those of some larger blood vessels, a quick stretch is followed by a temporary increase in wall tension; however, this increase is quickly followed by a relaxation toward the original wall tension referred to as stress relaxation (Fig. 11). The opposite, reverse stress relaxation, occurs when the muscle is allowed to shorten; that is, if an external stress is removed, wall tension temporarily decreases. However, the decrease is followed quickly by muscle contraction to return to the original wall tension. Such responses allow the larger blood vessels to accommodate to different blood volumes while maintaining transmural pressure nearly constant (see Chapter 14).

Response to Nerve Stimulation

There is wide diversity in the type and degree of innervation of smooth muscle tissue in the body. For

FIGURE 11 Length and isometric force changes of a strip of smooth muscle in vitro. (A) Stress relaxation; when the muscle is stretched (seen as a quick increase in length), force increases at first but then returns to almost the previous level. (B) Reverse stress relaxation; when the muscle is allowed to shorten (seen as a quick decrease in length), force decreases at first but then returns to almost the previous level.

FIGURE 11 Length and isometric force changes of a strip of smooth muscle in vitro. (A) Stress relaxation; when the muscle is stretched (seen as a quick increase in length), force increases at first but then returns to almost the previous level. (B) Reverse stress relaxation; when the muscle is allowed to shorten (seen as a quick decrease in length), force decreases at first but then returns to almost the previous level.

example, smooth muscle of the gastrointestinal tract is innervated by both the parasympathetic and sympathetic divisions of the autonomic nervous system, as well as by postganglionic nerves from the enteric nervous system. On the other hand, many blood vessels receive only sympathetic innervation. In addition to differences in the degree and type of innervation, the response of smooth muscle to a given neurotransmitter varies from one smooth muscle to another. For example, norepi-nephrine causes a marked contraction of vascular smooth muscle; however, this same neurotransmitter causes inhibition of most gastrointestinal smooth muscles. Also, many smooth muscles respond to neuro-transmitters, even if not innervated by nerves possessing those transmitters. Thus, it is not surprising to find that dysfunctions of the autonomic nervous system and pharmacologic agents that alter the function of the autonomic nervous system markedly affect many smooth muscle tissues.

Smooth muscles differ in several ways from skeletal muscle in their response to nerve stimulation. In innervated skeletal muscle, a single nerve impulse leads to release of enough neurotransmitter (acetylcholine, ACh) to cause an end-plate potential that will invariably lead to an action potential and contraction of the muscle fiber. For innervated smooth muscle, there are more types of neurotransmitters (such as ACh, norepineph-rine, and serotonin); not every nerve impulse leads to a mechanical response of the muscle, and the response of the smooth muscle membrane may be either a depolarization or a hyperpolarization (Fig. 12). Thus, integration takes place at the level of the smooth muscle cell and not just within the central nervous system, as with skeletal muscle.

FIGURE 12 Recordings of contractile (Gm, where Gm indicates that the force is measured in grams) and intracellular electrical activities (mV) from an innervated smooth muscle. Arrows indicate stimulation of excitatory (1) and inhibitory (2) nerves. Mild stimulation of an excitatory nerve leads to a small depolarization but no change in tension. Stronger stimulation leads to an action potential and contraction. Weak stimulation of an inhibitory nerve leads to a membrane hyperpolarization but no change in resting tension. Stronger stimulation leads to a larger hyperpolarization and relaxation.

FIGURE 12 Recordings of contractile (Gm, where Gm indicates that the force is measured in grams) and intracellular electrical activities (mV) from an innervated smooth muscle. Arrows indicate stimulation of excitatory (1) and inhibitory (2) nerves. Mild stimulation of an excitatory nerve leads to a small depolarization but no change in tension. Stronger stimulation leads to an action potential and contraction. Weak stimulation of an inhibitory nerve leads to a membrane hyperpolarization but no change in resting tension. Stronger stimulation leads to a larger hyperpolarization and relaxation.

Response to Circulating and Locally Released Chemicals

In addition to responding to many neurotransmitters, most smooth muscles respond to many hormones, paracrines, autocoids, and tissue metabolic products. Just a few examples can illustrate that the function of most organ systems is influenced in this manner.

1. In the cardiovascular system, circulating epinephrine and angiotensin cause marked contraction of vascular smooth muscle, whereas local tissue metabolites such as adenosine cause arteriolar dilatation.

2. Both the growth and contractile activity of uterine smooth muscle are influenced by the hormones estrogen and progesterone. Also, it is believed that the initiation of labor is brought about by hormonal changes that induce uterine contractions.

3. Airway smooth muscle contracts dramatically in response to the local release of histamine and relaxes when circulating levels of epinephrine are increased (note the diametrically opposite effects that epineph-rine has on airways as compared to vascular smooth muscle).

4. Both glomerular filtration and tubular reabsorption of fluid are influenced dramatically by the state of contraction of the smooth muscle in the afferent and efferent arterioles of the kidney. Although little understood, there are local and circulating chemicals that markedly affect the smooth muscle of these arterioles.

An important result of this responsiveness is that practically every disease state has a component that is characterized by altered smooth muscle function. Furthermore, just about every pharmacologic intervention that is attempted will, in one system or another, affect smooth muscle.

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