19.9c Inotropic Agents

What do inotropic agents affect?

Stroke volume

How do inotropic agents change stroke volume?

By altering contractility (force of contraction)

What is contractility?

The force with which the heart muscle contracts

What is contractility dependent on?

The amount of Ca2+ available in the sarcoplasm

Why is Ca2+ important for contraction?

Ca2+ determines the number of cross bridges formed

What is the relationship between Ca2+ and cross bridge formation?

More Ca2+ leads to more cross bridges

What is the relationship between cross bridges and contraction force?

More cross bridges result in stronger contraction

What is the overall relationship between Ca2+ and stroke volume?

Increased Ca2+ increases stroke volume

What do positive inotropic agents do to Ca2+ levels?

They increase available Ca2+

What is the effect of increased Ca2+ on contractility?

It increases contractility

What is the effect of increased contractility on stroke volume?

Stroke volume increases

Which hormones act as positive inotropic agents?

Epinephrine (EPI) and norepinephrine (NE)

How do EPI and NE increase contractility?

They act through β1-adrenergic receptors to increase Ca2+

What is the role of β1-adrenergic receptors in inotropic effects?

They increase Ca2+ availability inside cardiac cells

How does thyroid hormone affect contractility?

It increases the number of β1-adrenergic receptors

What is the effect of more β1 receptors?

Greater response to EPI and NE and increased Ca2+

What is an example of a drug that acts as a positive inotropic agent?

Digitalis

How does digitalis affect the heart?

It increases contractility

What is the effect of digitalis on cardiac output?

It increases cardiac output

What is the key concept of positive inotropic agents?

Increase Ca2+ → increase contractility → increase SV

What do negative inotropic agents do to Ca2+ levels?

They decrease available Ca2+

What is the effect of decreased Ca2+ on contractility?

It decreases contractility

What is the effect of decreased contractility on stroke volume?

Stroke volume decreases

What types of electrolyte imbalances act as negative inotropic agents?

Increased K+ or increased H+

How do electrolyte imbalances affect contractility?

They reduce Ca2+ availability and weaken contraction

What type of drugs act as negative inotropic agents?

Ca2+ channel-blocking blood pressure drugs

How do Ca2+ channel blockers affect the heart?

They reduce Ca2+ entry into cells

What is the result of reduced Ca2+ entry?

Decreased contractility

What is the key concept of negative inotropic agents?

Decrease Ca2+ → decrease contractility → decrease SV

What is the main mechanism by which inotropic agents affect stroke volume?

By changing Ca2+ availability in cardiac muscle cells

What is the full inotropic relationship chain?

Ca2+ levels → cross bridge formation → contractility → stroke volume

What happens when Ca2+ increases?

More cross bridges, stronger contraction, higher stroke volume

What happens when Ca2+ decreases?

Fewer cross bridges, weaker contraction, lower stroke volume