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New view of heart muscle cells shows movement of ß2 adrenergic receptors

BioPhotonics
Apr 2010
Lynn Savage, lynn.savage@photonics.com

LONDON – Building better beta-blockers for various heart ailments may be on the horizon because of a novel imaging technique used by researchers at Imperial College London.

Beta-blockers are drugs that inhibit the action of β1 and β2 adrenergic receptors – G-protein-coupled receptors that reside on the surface of cardiac muscle cells, or cardiomyocytes. Both β1 and β2 mediate specific events that affect heart function; for example, β1 strongly stimulates heart contractions, while β2 has a mildly stimulating effect but a strong cardioprotective one. Although the roles played by β1 and β2 adrenergic receptors are well-known, their location inside the cardiomyocytes and the potential effects their placement might have on their functionality have remained undetermined.

Dr. Julia Gorelik and her colleagues at the college and at the University of Würzburg in Germany used a nonoptical imaging technique called scanning ion-conductance microscopy (SICM) to examine where in the heart cells β1 and β2 were sited and how they behaved when activated. They eschewed more typical imaging techniques, such as electron microscopy, because of insufficient expression levels of G-protein-coupled receptors in general.

SICM works by using an electrolyte-filled glass pipette with a 50- to 100-nm tip as a scanning probe that traverses the entire cell. Unlike the cantilever tip of an atomic force microscope, the nanopipette does not contact the cell or its components. Instead, it measures changes in ion flow through the pipette. The resolution of the device is equal to the diameter of the pipette’s tip, which means that the structural features of the cardiomyocyte, including its crests and transverse tubules (T-tubules), are readily visualizable.

To see the effect that β1 and β2 have on heart muscle, the researchers used SICM to locate the receptors and used Förster resonance energy transfer (FRET) to look for production of cyclic adenosine monophosphate (cAMP), a molecule important to cell signaling. For the FRET measurements, they used a Nikon microscope, an Osram mercury lamp and a Hamamatsu CCD camera.

They locally stimulated β1 and β2 at the T-tubules and crests of rat cardiomyocytes. They found that the β1 receptors dramatically decreased FRET signals in both locations, which showed activation of cAMP production. The β2-cAMP FRET response occurred only at the T-tubules, not at the cell crests. From this, the researchers concluded that the β2 receptors are normally located at the T-tubules but change location to match β1 receptors when the heart muscle is damaged. They believe that this shift in distribution could affect the ability of β2 to protect cardiac muscle during heart failure.

The new information regarding translocation of β2 adrenergic receptor could provide the next step toward improving beta-blocker drugs currently used to thwart the cascade of damage during heart failure and could provide insight about whether β1 or β2 should be blocked together or separately.


GLOSSARY
cantilever
A projecting beam or other structure supported only at one end.
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