Question

You are relaxing in a hot tub for 15 minutes until your peripheral blood vessels are very vasodilated. As you stand up to leave the hot tub, your blood pressure rapidly changes from 120/70 to 90/60. As compensatory mechanisms begin to be activated, discuss the cellular chain of events that will occur in your vascular smooth muscle cells.

Answer 1

The following steps leads to vascular smooth muscle contraction and regulation of blood pressure.

• Vascular smooth muscle membrane depolarization or activation of hormones or neurotransmitters occurs
• Opening of the L type calcium channel
• Calcium induced calcium release in the sarcoplasmic reticulum
• Intracellular calcium level increases
• Calcium binds with and activates calmodulin
• Activation of MLCK
• Myosin light chain phosphorylation
• Myosin ATPase activity increases
• Myosin P binds to actin
• Cross bridge cycling occurs that increases the muscle tone
• Vascular smooth muscle contraction causing vasoconstriction and autoregulation of blood pressure.

Explanation of steps:

Drop in blood pressure may be due to the combined effects of vasodilation and sudden position change causing orthostatic hypotension. Vascular smooth muscle contraction may help in autoregulation of blood pressure in a normal healthy individual. Smooth muscle contraction depends on the flow of calcium into the cells. The factors that may affect the influx of calcium ions are hormonal changes, cellular depolarization and effect of neurotransmitters. These factors leads to calcium influx into the cells of the vascular smooth muscles in order to improve the muscle tone and thereby increasing the vascular resistance and a hike in the blood pressure. Calcium enter the cells through an L shaped canal which is located in the caveolae of the membrane. Then a process called calcium-induced calcium release takes place in which the intracellular calcium stimulates further release of calcium from the sarcoplasmic reticulum through the intracellular calcium channels ryanodine receptors and inositol trisphosphate receptor (IP3). Calcium inside the cell binds with calmodulin / calcium modulated protein and activates it. Activated calmodulin activates MLCK/myosin light chain kinase which then leads to its phosphorylation into a regulatory light chain on myosin. This phosphorylation leads to a conformational change on the myosin head that increases the myosin ATPase activity and promotes myosin head and actin interaction. This causes a cross bridging cycling generating tension which is directly proportional to the amount of intracellular calcium. Cross bridging cycling improves muscle tone. Smooth muscle contraction leads to vasoconstriction and regulation of blood pressure to previous stage gradually.