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Ca
+
Tropomyosin
alpha-1 chain
Tropomyosin
beta chain
Beta-1
adrenergic
receptor
Voltage-
depenent L-type
calcium channel
Voltage-
depenent L-type
calcium channel
Voltage-
depenent L-type
calcium channel
Ryanodine
receptor 2
cAMP-dependent
protein kinase
type I-alpha
regulatory
subunit
Sodium/calcium
exchanger 1
Voltage-gated
T-type calcium
channel (ICaT)
Voltage-gated
T-type calcium
channel (ICaT)
Sarcoplasmic/endoplasmic
reticulum
calcium ATPase
2
cAMP-dependent
protein kinase
catalytic
subunit alpha
Laβlol
Ca
+
Ca
+
Ca
+
Na
+
Na
+
Ca
+
Ca
+
Ca
+
Laβlol
Laβlol
ATP
H
2
O
Ca
+
ADP
P
i
Ca
+
Ca
+
Troponin
Troponin
G-Protein
Signaling
Cascade
Muscle
Relaxation
Cardiac Myocyte
Sarcoplasmic Reticulum
Normally, calcium binding to
troponin displaces
tropomyosin from the myosin
binding sites on the actin
filaments. Low
concentrations of calcium
disallow this from
occurring.
Myosin and actin filaments
slide further away from each
other leading to muscle
relaxation.
Actin Filament
Myosin Filament
The beta-1 adrenergic
receptor is coupled to and
antagonizing this receptor
results in the G protein
signalling cascade not being
activated.
The G-protein signalling
cascade activates protein
kinase which activates
calcium channels on the
membrane, resulting in
calcium influx.
Low calcium cannot activate
the ryanodine receptor on
the sarcoplasmic reticulum.
Time
Membrane potential (mV)
-40mV (threshold)
Pacemaker Action Potential
Phase 0- depolarization
Phase 3- repolarization
Phase 4-spontaneous
depolarization
With labetalol
Without labetalol
Labetalol, by antagonizing
beta-1 adrenergic receptors,
decreases intracellular
Ca2+. Phase 4 spontaneous
depolarization is Ca2+
dependent, therefore
propranolol decreases the
slope of phase 4, increasing
the time it takes for
threshold to be reached and
thus slows heart rate
Cytosol
Since calcium is linked to
activation of muscle
contraction. Reducing
cytosolic calcium levels
reduces muscle contraction
Labetalol enters the
bloodstream and once it
reaches the heart, it
antagonizes the beta-1
adrenergic receptor on the
cell membrane of myocytes
such that epinephrine
stimulation of the heart is
reduced.
T-tubule
Labetalol is administered as
an oral tablet or via
intravenous injection
Mitochondria
Calcium
TPM1
TPM2
ADRB1
CACNA1C
CACNA2D2
CACNB1
RYR2
PRKAR1A
SLC8A1
CACNA1H
CACNA1G
ATP2A2
PRKACA
Laβlol
Calcium
Calcium
Calcium
Sodium
Sodium
Calcium
Calcium
Calcium
Laβlol
Laβlol
Adenosine
triphosphate
Water
Calcium
Adenosine
diphosphate
Phosphate
Calcium
Calcium
Troponin
Troponin
G-Protein
Signaling
Cascade
Muscle
Relaxation