Cardiac EC Coupling I

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Cardiac E-C Coupling I

• W. Gil Wier
• gwier001_at_umaryland.edu
• http//medschool.umaryland.edu/biochemistry/course
  s.asp

Feb. 28 (Tuesday) 1000 AM to 1130 AM
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Access Web site
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B
A
Cell border
50 mm
C
Ventricular Muscle Isolated Ventricular Myocyte
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Mammalian Skeletal Muscle Sarcomeric Structure
Ventricular Myocyte Sarcomeric Structure
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Junctional SR (interior)
Foot structures at Junctional SR are SR Ca2
release channels (RyR)
Junctional SR (peripheral)
Transverse tubules at Z line
Corbular SR (peripheral)
From Ultrastructure of Cardiac Muscle J.R.
Sommer R.B. Jennings. In The Heart and
Cardiovascular System, 2nd Ed. H.A. Fozzzard et
al. Raven Press, Ltd.,, NY, 1992
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Surface Membrane Staining in ventricular and
atrial cells
Kirk, MM, Izu, LT, Izu, YC, McCulle, SL, Wier,
WG, Balke, CW, Shorofsky, S. Role of the
transverse-axial tubule system in generating
calcium sparks and calcium transients in rat
atrial myocytes J. Physiol 547.2441-451, 2003
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Transverse Axial Tubular System in Atrial Cells
Kirk, MM, Izu, LT, Izu, YC, McCulle, SL, Wier,
WG, Balke, CW, Shorofsky, S. Role of the
transverse-axial tubule system in generating
calcium sparks and calcium transients in rat
atrial myocytes J. Physiol 547.2441-451, 2003
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Electrical Activity of the Heart
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Ca2i transienttriggered by patch clamp
depolarization
(From Sobie, Ziman Lederer, work in progress)
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Beuckelmann, DJ, Wier, WG Mechanism of release
of calcium from sarcoplasmic reticulum of
guinea-pig cardiac cells. Journal of Physiology
405233-255, 1988
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Dissection of Ca2 Signaling in Cardiac Cells

• Whole cell voltage-clamp
• observe ICa, whole cell Ca2 transients, in total
  absence of Na (no Ca2 fluxes due to Na/Ca
  exchange)
• Led to quantitative observations of ICa and SR
  Ca2 flux
• Not all Ca2 currents are created equal
• in terms of ability to trigger SR Ca2 release
• local control theory of cardiac E-C coupling

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Insert summary page from J.Physiol. Paper
Balke, CW, Egan, TM, Wier, WG Processes that
remove calcium from the cytoplasm during
excitation-contraction coupling in rat heart
cells. The Journal of Physiology 474447-462,
1994
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In absence of Na/Ca exchange and SR Function the
Ca2 transient can be quantitatively
accounted for by the Ca2 current
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In absence of Na/Ca exchange and SR Function the
Ca2 transient can be quantitatively
accounted for by the Ca2 current
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Calculated SR Ca2 fluxes removal model The
ratio of Ca2 fluxes through the SR to that
through the L-type Ca2 channels is about
101 CICR has a high gain
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Direct, experimental observation of SR Ca2 flux
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Gain Function of E-C Coupling
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NCX may also trigger SR Ca release or contribute
activator Ca
Biophys J, July 1998, p. 359-371, Vol. 75, No. 1
, Litwin, Liu Bridge
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Cardiac E-C Coupling Explained in terms of Ca2
Sparks

• Ca2 sparklets
• Ca2 transients due to Ca2 entry through L-type
  Ca2 channels

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Ca2 sparks at rest
Cell does not move at the whole cell level or at
the level of the sarcomere!
(From Sobie, Ziman Lederer, work in progress)
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López-López, JR, Shacklock, PS, Balke, CW, Wier,
WG Local stochastic release of Ca2 in
voltage-clamped rat heart cells visualization
with confocal microscopy. The Journal of
Physiology 480.121-29, 1994
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Ca2 sparks confocalline-scan images
Di-8 ANEPPS
Fluo-3 Di-8 ANEPPS
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Time (1 s)
Ca2
Shacklock, PS, Wier, WG, Balke, CW Local Ca2
transients (Ca2-sparks) originate at
transverse-tubules in rat heart cells. Journal
of Physiology 487.3601-608, 1995
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Ca2 sparks evoked by voltage-clamp pulsesIca
nearly blocked
A
B
Adapted from López-López, JR, Shacklock, PS,
Balke, CW, Wier, WG Local calcium transients
triggered by single L-type Ca2-channel currents
in cardiac cells. Science 2681042-1045, 1995
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SR Ca2 Release observed in voltage-clamped
myocytes. Cytoplasm contains a high
concentration of Ca2 indicator (a Ca2 buffer)
Local SR Ca2 Release has same spatio-temporal
pattern and voltage-dependence as Ca2 sparks
Cleeman, Wang Morad, PNAS, Vol. 95, Issue 18,
10984-10989, September 1, 1998
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Ca2 sparks triggered by patch depolarization
.Shorofsky, SR, Izu, L, Wier, WG, Balke, CW
Ca2 sparks triggered by patch depolarization in
rat heart cells. Circulation Research
82424-429, 1998
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Ca2 sparklets represent Ca2 entry through
single L-type Ca2 channels
Cheng et al., Nature 410, 592 - 596 (2001)
doi10.1038/35069083
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Ca2 sparklets can trigger Ca2 sparks
Cheng et al., Nature 410, 592 - 596 (2001)
doi10.1038/35069083
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Ca2 Sparks in Cardiac E-C coupling

• Sparks occur spontaneously or can be evoked by
  depolarization of cell patches, single isolated
  cells. They occur in intact cardiac muscle.
• Evoked at Z lines (SR junctional contact with
  transverse tubules, RyR clusters)
• Evoked by single L-type Ca2 channel current
• Voltage and time-dependence of Ca2 spark
  probability is similar to that of L-type Ca2
  channels
• Appear to sum in space and time to produce the
  whole-cell Ca2 transient.

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NCX, forward reverse modes
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Ca2 Efflux mechanisms NCX