Question

The stimulation of either adenylate or guanylate cyclase induces smooth muscle relaxation. The cyclic nucleotides produced by these enzymes stimulate cAMP and cGMP-dependent kinases, respectively. These kinases phosphorylate, among other things, enzymes that remove Ca++ from the cytosol, and in doing so they inhibit contraction. In contrast, either a decrease in K⁺ permeability or an increase in Na⁺ permeability results in membrane depolarization and contraction. Likewise, inhibition of the sarcoplasmic reticulum Ca++-ATPase, one of the enzymes activated by cyclic nucleotide-dependent kinases, would also favor muscle contraction. Smooth muscle does not express troponin. What is the effect of the stimulation of adenylate or guanylate cyclase on smooth muscle?

1) Smooth muscle relaxation
2) Smooth muscle contraction
3) Increase in K⁺ permeability
4) Increase in Na⁺ permeability

Answer 1

The stimulation of adenylate or guanylate cyclase leads to the production of cyclic nucleotides, which activate kinases that remove Ca++ from the cytosol, resulting in the relaxation of smooth muscle.

Step-by-step explanation:

The effect of the stimulation of adenylate or guanylate cyclase on smooth muscle is smooth muscle relaxation. This occurs because stimulation of these enzymes increases the levels of cyclic nucleotides, namely cAMP and cGMP, which activate cAMP and cGMP-dependent kinases. These kinases then phosphorylate enzymes that are responsible for removing Ca++ from the cytosol. Since Ca++ is necessary for muscle contraction, its removal leads to relaxation of the smooth muscle. In smooth muscle, contraction is regulated by the Ca++-calmodulin complex that activates myosin light chain kinase, promoting cross-bridge cycling and contraction. However, cyclic nucleotide-dependent kinases act to reverse this process and induce relaxation.