As is true for skeletal muscle, the initiating stimulus for most smooth muscle contraction is an increase in intracellular calcium ions. This increase can be caused in different types of smooth muscle by nerve stimula-tion of the smooth muscle fiber, hormonal stimulation, stretch of the fiber, or even change in the chemical environment of the fiber.
Yet smooth muscle does not contain troponin, the regulatory protein that is activated by calcium ions to cause skeletal muscle contraction. Instead, smooth muscle contraction is activated by an entirely different mechanism, as follows.
Combination of Calcium Ions with Calmodulin—Activation of Myosin Kinase and Phosphorylation of the Myosin Head. Inplace of troponin, smooth muscle cells contain a large amount of another regulatory protein called calmod-ulin. Although this protein is similar to troponin, it isdifferent in the manner in which it initiates contrac-tion. Calmodulin does this by activating the myosin cross-bridges. This activation and subsequent contrac-tion occur in the following sequence:
1. The calcium ions bind with calmodulin.
2. The calmodulin-calcium combination joins with and activates myosin kinase, a phosphorylating enzyme.
3. One of the light chains of each myosin head, called the regulatory chain, becomes phosphorylated in response to this myosin kinase. When this chain is not phosphorylated, the attachment-detachment cycling of the myosin head with the actin filament does not occur. But when the regulatory chain is phosphorylated, the head has the capability of binding repetitively with the actin filament and proceeding through the entire cycling process of intermittent “pulls,” the same as occurs for skeletal muscle, thus causing muscle contraction.
When the calcium ion concentration falls below a crit-ical level, the aforementioned processes automatically reverse, except for the phosphorylation of the myosin head. Reversal of this requires another enzyme, myosin phosphatase, located in the fluids of thesmooth muscle cell, which splits the phosphate from the regulatory light chain. Then the cycling stops and contraction ceases. The time required for relaxation of muscle contraction, therefore, is determined to a great extent by the amount of active myosin phosphatase in the cell.
Possible Mechanism for Regulation of the Latch Phenomenon
Because of the importance of the latch phenomenon in smooth muscle, and because this phenomenon allows long-term maintenance of tone in many smooth muscle organs without much expenditure of energy, many attempts have been made to explain it. Among the many mechanisms that have been postulated, one of the simplest is the following.
When the myosin kinase and myosin phosphatase enzymes are both strongly activated, the cycling fre-quency of the myosin heads and the velocity of contraction are great. Then, as the activation of the enzymes decreases, the cycling frequency decreases, but at the same time, the deactivation of these enzymes allows the myosin heads to remain attached to the actin filament for a longer and longer proportion of the cycling period. Therefore, the number of heads attached to the actin filament at any given time remains large. Because the number of heads attached to the actin determines the static force of contraction, tension is maintained, or “latched”; yet little energy is used by the muscle, because ATP is not degraded to ADP except on the rare occasion when a head detaches.