EN.580.421
MYOCARDIAL CONTRACTILITY LAB
SUBMITTED APRIL 2023
1) Would tau (Time constant for relaxation) be increased or decreased in a
phospholamban knockout mouse. Why?
Tau would decrease. Phsopholambam (PLB)is important in the regulation of SERCA, the
sarcoplasmic endoplasmic reticulum Ca2+ ATPase, which is critical in pumping Ca2+
out of the cytoplasm and initiating myocyte relaxation. In its unphosphorylated state it
constrains the activity of SERCA. When phosphorylated (as by protein kinase A during
adrenergic receptor activation), the brake on SERCA is removed and the pump activity
is enhanced. When phospholamban is absent (KO mouse) the SERCA is unrestrained
and rate of relaxation would increase, tau would decrease and lucitropy (diastolic
function) would be enhanced
2) Phosphodiesterases (PDE) breakdown cAMP to AMP. What would you predict that
PDE inhibitors do with respect to myocardial contractility and why?
They would increase inotropy. If cAMP breakdown is inhibited, increased accumulation
of cAMP would increase activation of PKA and all of the downstream effectors that
enhance inotropy.
, 3) a) List 2 classes of drugs that:
1) Increase myocardial contractility
Beta adrenergic receptor agonist (epinephrine, isoproterenol etc )
Phosphodiesterase inhibitors (milrinone etc)
2) Decrease myocardial contractility
Beta-adrenergic receptor antagonists
Ca2+ channel Blockers
b) Explain their mechanism of action.
Self explanatory
4) Invent a drug that increases myocardial contractility by a mechanism independent of
the above classes. (Use your knowledge of signal transduction mechanisms in the heart).
Anything where they mention either increasing Ca2+ or increasing the myofilament
sensitivity to Ca2+
5) Describe briefly how beta-adrenergic receptor agonists improve diastolic function.
Similar to answer in question 3. Adrenergic receptor activation results in increase in
cAMP. PKA activation and phosphorylation of phospholamban. PLB phosphorylation
results in the removal of the constraint on SERCA with an increase in the rate of Ca2+
being pumped back into the sarcoplasmic reticulum. This enhances the rate of relaxation
and improves diastolic function.
6) Extra Credit! Although counterintuitive, low dose beta adrenergic receptor
antagonists (beta-blockers ) are used to treat patients with heart failure. Using your
knowledge of how G- protein coupled receptors are regulated, explain why beta
blockers which are negative inotropes, may be important in the treatment of heart
failure in which the inotropic state of the heart is already low.
Heart failure leads top a secondary chronic increase in sympathetic tone. This leads to
an agonist dependent uncoupling of the adrenergic receptor from its signal transduction
cascade. This is primarily mediated by beta-adrenergic receptor kinase mediated
uncoupling and other mechanisms which lead to a decrease in the abundance and
internalization of receptor. Beta –antagonists prevent desensitization and improve
coupling and thereby myocardial contractile reserve.
(Anything that sounds vaguely like this is OK )
MYOCARDIAL CONTRACTILITY LAB
SUBMITTED APRIL 2023
1) Would tau (Time constant for relaxation) be increased or decreased in a
phospholamban knockout mouse. Why?
Tau would decrease. Phsopholambam (PLB)is important in the regulation of SERCA, the
sarcoplasmic endoplasmic reticulum Ca2+ ATPase, which is critical in pumping Ca2+
out of the cytoplasm and initiating myocyte relaxation. In its unphosphorylated state it
constrains the activity of SERCA. When phosphorylated (as by protein kinase A during
adrenergic receptor activation), the brake on SERCA is removed and the pump activity
is enhanced. When phospholamban is absent (KO mouse) the SERCA is unrestrained
and rate of relaxation would increase, tau would decrease and lucitropy (diastolic
function) would be enhanced
2) Phosphodiesterases (PDE) breakdown cAMP to AMP. What would you predict that
PDE inhibitors do with respect to myocardial contractility and why?
They would increase inotropy. If cAMP breakdown is inhibited, increased accumulation
of cAMP would increase activation of PKA and all of the downstream effectors that
enhance inotropy.
, 3) a) List 2 classes of drugs that:
1) Increase myocardial contractility
Beta adrenergic receptor agonist (epinephrine, isoproterenol etc )
Phosphodiesterase inhibitors (milrinone etc)
2) Decrease myocardial contractility
Beta-adrenergic receptor antagonists
Ca2+ channel Blockers
b) Explain their mechanism of action.
Self explanatory
4) Invent a drug that increases myocardial contractility by a mechanism independent of
the above classes. (Use your knowledge of signal transduction mechanisms in the heart).
Anything where they mention either increasing Ca2+ or increasing the myofilament
sensitivity to Ca2+
5) Describe briefly how beta-adrenergic receptor agonists improve diastolic function.
Similar to answer in question 3. Adrenergic receptor activation results in increase in
cAMP. PKA activation and phosphorylation of phospholamban. PLB phosphorylation
results in the removal of the constraint on SERCA with an increase in the rate of Ca2+
being pumped back into the sarcoplasmic reticulum. This enhances the rate of relaxation
and improves diastolic function.
6) Extra Credit! Although counterintuitive, low dose beta adrenergic receptor
antagonists (beta-blockers ) are used to treat patients with heart failure. Using your
knowledge of how G- protein coupled receptors are regulated, explain why beta
blockers which are negative inotropes, may be important in the treatment of heart
failure in which the inotropic state of the heart is already low.
Heart failure leads top a secondary chronic increase in sympathetic tone. This leads to
an agonist dependent uncoupling of the adrenergic receptor from its signal transduction
cascade. This is primarily mediated by beta-adrenergic receptor kinase mediated
uncoupling and other mechanisms which lead to a decrease in the abundance and
internalization of receptor. Beta –antagonists prevent desensitization and improve
coupling and thereby myocardial contractile reserve.
(Anything that sounds vaguely like this is OK )
