Cardiac Muscle

 Cardiac Muscle

actin - thin   regulated by calcium in cardiac muscle
Myosin - thick - regulated by calcium in smooth muscle

Autonomic Control of Heart Rate

NB: factors other than autonomic tone affect heart rate

• adenosine and high K lower heart rate by moving resting membrane potential toward K equilibrium potential.  from Katzung:potassium concentration has more to do with the permeability rather than resting membrane potential!
• Hyperkalemia: reduces eq. potential for K+ but on the other hand increases permeability therefore potassium current will be higher and RMP will come closer to K+ eq. potential!  significance: in cardiac pacemaker cells during phase IV ,due to high K+ permeability it will effectively counteract hyperpolarization induced depolarizing current via rectifier channels therefore late approach towards threshold!
• Hypokalemia: increases eq. potential for K+ but on the other hand reduces permeability ;therefore K+ current will be low and RMP will be farther away from the eq. potential (K+ eq. pot. has the major influence on RMP) in other words membrane will be destabilized!  significance: in cardiac pacemaker cells low K+ current during phase IV will not be able to effectively counteract the depolarizing current and therefore early approach towards threshold [enhanced automaticity]

• Hypoxia - interferes with If - slows inward Na current and slows heart rate
• Thyroid hormone (T3)  - upregulated beta receptors (1 and 2), increase chronotropy, inotropy, vasodilation.  (results in increased stroke volume SBP and decreased DBP  = increased pulse pressur ("bounding pulse")

does calcium channel blocker slow heart rate?  yes

In normal subject with resting HR of 60, the intrinsic heart rate is about 100 (blockade of sympathetic and parasympathetic tone).  e.g., resting HR is determined mainly by vagal tone.

Cardiac Signal Transduction Mechanisms

Beta agonists produce increased inotropy (contractility) AND lusitropy (relaxation) due to inhibition of phospholamden and increased activity of SERCA

Frank-Starling Law of the Heart - 

increased preload causes increased stroke volume

Myocardial Response to Increased Afterload

will reduce stroke volume initially.  However, smaller SV means larger ESV.  Then, when normal venous return enters ventrical, the EDV will now be greater, and SV will return to normal.  e.g., people with hypertension do not have low SV and high heart rate.