Figure 36.7 Neurotransmitters and Stretch Alter the Membrane Potential of Smooth Muscle Cells
Contraction of smooth muscle can be induced by both stretch and parasympathetic stimulation. This effect was demonstrated using a strip of smooth muscle incubated in a saline bath and measurement of action potentials and the force of contraction in response to both types of stimuli. Two electrodes, one attached to the muscle tissue and one as reference in the saline bath, were used to measure muscle membrane action potentials; the force of contraction was measured with a force transducer. In the first experiment, the muscle was attached to a device that applied force in order to stretch the muscle. Muscle membrane potentials and action potentials were recorded and analyzed. The results showed that stretching depolarizes the smooth muscle membrane, increasing the probability of propagation of an action potential and, as a result, an increase in muscle contraction. In the second set of experiments, the response of the muscle to the neurotransmitters acetylcholine and norepinephrine was assayed. The results showed that the muscle cell membrane depolarized in response to acetylcholine, leading to an increased firing rate of action potentials and, therefore, an increase in the force of muscle contraction. By comparison, the application of norepinephrine resulted in the hyperpolarization of the muscle cell membrane, leading to decreased firing rate of action potentials and, thus, a decrease in the force of muscle contraction. These findings indicated that both of the neurotransmitters can alter the resting potential of smooth muscle cell membrane, thereby influencing the rate at which smooth muscle cells fire action potentials. Together, the results of these experiments indicate that contraction of smooth muscle can be induced by both stretch and acetylcholine. The relationship between the electrical event of membrane depolarization and the mechanical event of muscle contraction is referred to as electromechanical coupling. These events are linked via the influx of Ca2+ from the extracellular space into the sarcoplasm, where it binds to calmodulin and stimulates contraction. Importantly, smooth muscle contraction can also be induced through a mechanism independent of a change in membrane potential. This mechanism, termed pharmacomechanical coupling, occurs through the activation of receptors by hormones or drugs that induce muscle contraction. Ca2+ also links these two events, but in this instance Ca2+ is released from the sarcoplasmic reticulum.
Original Papers
Bolton, T. B. 1972. The depolarizing action of acetylcholine or carbachol in intestinal smooth muscle. Journal of Physiology 220: 647-671.
http://jp.physoc.org/content/220/3/647.full.pdf+html
Bolton, T. B., R. J. Lang, and T. Takewaki. 1984. Mechanisms of action of noradrenaline and carbachol on smooth muscle of guinea-pig anterior mesenteric artery. Journal of Physiology 351: 549–572.
http://jp.physoc.org/content/351/1/549.full.pdf+html
Benham, C. D., T. B. Bolton, and R. J. Lang. 1985. Acetylcholine activates an inward current in single mammalian smooth muscle cells. Nature 316: 345–347.
http://www.nature.com/nature/journal/v316/n6026/abs/316345a0.html
Bolton, T. B., S. A. Prestwich, A. V. Zholos, and D. V. Gordienko. 1999. Excitation-contraction coupling in gastrointestinal and other smooth muscles. Annual Review of Physiology 61: 85–115.
http://dx.doi.org/10.1146/annurev.physiol.61.1.85
Links
Webb, R. C. 2003. Smooth muscle contraction and relaxation. Advances in Physiology Education 27: 201–206.
http://advan.physiology.org/cgi/content/full/27/4/201
Kimball’s Biology Pages: Muscles
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/Muscles.html
University of Illinois at Chicago: Biochemistry of Muscle Contraction: Smooth Muscle
http://www.uic.edu/classes/phyb/phyb516/smoothmuscleu3.htm
University of Texas Southwestern Medical Center: UT Southwestern researchers reveal mechanisms of smooth-muscle contraction
http://www.utsouthwestern.edu/utsw/cda/dept37389/files/161304.html