2. Excess carbon dioxide causes vasodilatation.
3. Increased hydrogen ion concentration causes
vasodilatation. Adenosine, lactic acid, increased potassium ions, diminished calcium ion concentration,
and increased body temperature can all cause local vasodilatation.
To inhibit contraction, other receptor mechanisms are known to activate the enzyme adenylate cyclase or guanylate cyclase in the cell membrane; the portions of the receptors that protrude to the
interior of the cells are coupled to these enzymes, causing the formation of cyclic adenosine monophosphate (cAMP) or cyclic guanosine monophosphate (cGMP), socalled second messengers. The cAMP or cGMP has many effects, one of which is to change the degree of phosphorylation of
several enzymes that indirectly inhibit contraction. The pump that moves calcium ions from the
sarcoplasm into the sarcoplasmic reticulum is activated, as well as the cell membrane pump that moves
calcium ions out of the cell itself; these effects reduce the calcium ion concentration in the sarcoplasm,
thereby inhibiting contraction. Smooth muscles have considerable diversity in how they initiate
contraction or relaxation in response to different hormones, neurotransmitters, and other substances. In
some instances, the same substance may cause either relaxation or contraction of smooth muscles in
different locations. For example, norepinephrine inhibits contraction of smooth muscle in the intestine
but stimulates contraction of smooth muscle in blood vessels.
Source of Calcium Ions That Cause Contraction Through the Cell Membrane and
