Smooth muscle cells line all of the hollow organs of the body; the GI tract, all of the blood vessels, the bronchi and all areas of the lung, the bladder, the urogenital tract, the uterus, they control the diameter of the iris of the eye and even make your hair stand on end. This tissue has such a wide distribution that it is difficult to underestimate its importance in health and disease. A few examples of conditions which involve alterations in the function of smooth muscle cells include hypertension, atherosclerosis, and asthma. These muscles have dynamic and complex mechanisms to regulate their intracellular calcium concentration and their force of contraction. This section of the gallery is devoted to experiments which have helped to discover some of these mechanisms.

a and b subunits of the Na+/K+
ATPase Distribution of the a (red) and b (green) subunits of the Na+/K+ ATPase in a smooth muscle cell. The superimposed image demonstrates that the pump is a heterodimer that is distributed in strands that run roughly parallel to the long axis of the cell, although individual strands merge and bifurcate.

Coupling of Sarcoplasmic Reticulum Calcium Storage to Sarcolemma Ca2+Transport
The distribution of the a subunit of the Na+/K+ ATPase was compared to other proteins involved in ion regulation or involved in maintenance of the cytoskeleton. The Na+/K+ ATPase was located within 150 nm of the Na+/Ca2+ exchanger on the sarcolemma and within 150 nm of calsequestrin in the lumen of the sarcoplasmic reticulum, but was in a different domain of the sarcolemma than the cytoskeletal protein vinculin.

Ca2+ Induced Ca 2+ Release in Smooth Muscle Cells
Voltage-gated Ca2+ channels (green) on the sarcolemma are physically juxtaposed to ryanodine receptors (red) in the SR membrane in mammalian smooth muscle cells. This provides the structural verification that calcium-induced calcium release can operate in mammalian smooth muscle cells.

Distribution of Active Protein Kinase C in Single Smooth Muscle Cells
In a smooth muscle cell at rest there is a network of active protein kinase C (green) roughly 250 nm beneath the sarcolemma whose distribution mirrors that of the cytoskeletal protein vinculin (red).

After stimulation with carbachol, active protein kinase C (green) is seen throughout the myoplasm, but not in the nucleus. Vinculin remains at the sarcolemma (red). (SMMUSC4.JPG). Protein Kinase C has been implicated in regulating contractile force in smooth muscle cells and its distribution to what appears to be contractile filaments following cell activation with a muscarinic agonist is consistent with this hypothesis.

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