8. Perfusion of isolated heart and effects of drugs (Adrenaline & Acetyl choline)
- Pre-load/Free load: The load acting on the muscle before it
begins to contract.
- Afterload: The load acting on the muscle after the muscle
starts to contract.
Q1. What is the mechanism of action of Adrenaline on heart?
- Adrenaline increases the ionotropic, chronotropic,
dromotropic and bathmotropic actions of the heart. It exerts its effect on the
heart by acting on β2 adrenergic receptors that increase intracellular cyclic
AMP concentration which facilitates the opening of long standing calcium
channels which increases the depolarisation phase of the impulse, thereby
increasing the heart rate. Adrenaline also increases the intracellular calcium
in the myocardial cells by acting on β1 receptors present on the cardiac muscle
and there by increases the force of contraction.
Q2. What is the mechanism of action of Acetylcholine on
heart?
- Acetylchloine decreases the heart rate and the force of
contraction. The heart rate decreases because the membrane becomes
hyperpolarized due to the action of acetylcholine on muscarinic receptors and K+
channels. Acetylcholine increases the K+ conductance in the nodal tissue by
directly opening the K+ channels by acting on M2 receptors on the nodal cells,
acetylcholine decreases the concentration of intracellular cyclic AMP that
slows down the opening of calcium channels. This decrease the firing rate of
the nodal tissue. Therefore the heart rate decreases. Acetylcholine also
decreases cyclic AMP in the myocardial cell and therefore decreases the
magnitude of contraction.
