G1BH 2012-2013 - HC-15

Excitation-contraction coupling

Paul Steendijk, PhD

Associate Professor Cardiovascular PhysiologyDepartment of Cardiology, LUMC

p.steendijk@lumc.nl

2013.02.13

Contraction mechanisms

Excitation-contractioncoupling

Pump function

G1BH 2012-2013 HC-15

Excitation-contraction coupling

Cross-bridge cycling & calcium

Calcium & cross-bridge cycling

Calcium concentrationsExtracellular Ca concentration: [Ca]o ~ 1.2 mM (1200 μM)Intracellular Ca concentration: [Ca]i ~ 100 nM (0.1 μM)- 104-fold concentration gradient

During action potential:- Ca enters via L-type Ca channels- Free [Ca]i increases ~10-fold

[Ca]o , [Ca]i and Force

- [Ca]i depends on [Ca]o- Developed force is dependent on free [Ca]i

Calcium-induced calcium release from SR- Ca influx is not sufficient to raise [Ca]i and initiate contraction- Sarcoplasmic reticulum stores large amounts of Ca- Ca entry (L-type Ca channels) triggers Ca release from SR via

Ryanodine receptors

Calcium re-uptake into the SR- Ca is sequestered into SR by SERCA pump (SR Ca-ATPase)- Phospholamban regulates SERCA; phosphorylation removesits inhibitory effect

- Calsequestrin is a SR Ca-binding protein

Calcium removal initiates relaxation

Species differences: Humans ≈ Rabbit

- Ca must be removed from the cytosol to lower [Ca]i and allow relaxation- Routes: SERCA (SR-ATPase), Na/Ca exchange (NCX), SL-ATPase

Calcium handling

Beta-receptor stimulation & ECC- Sympathetic activation >> release norepinephrine >> stimulate beta-

receptors on SL >> stimulates cAMP production >> protein kinase A- Phosphorylation of L-type Ca-channels: positive inotropic effect

(improves contraction)- Phosphorylation of phospholamban (PLB) and troponin-I (TnI): positive

lusitropic effect (improves relaxation)

- Myofilaments (Actine-Myosine)- Myofibrils- Mitochondria (ATP)

-Sarcolemma, T-tubule- Sarcoplasmic reticulum- Cisterna

Anatomy & E-C coupling

Sarcomere length and intracellular calcium control forceIsometric force is increased by:

Increased intracellular calcium- binding to Tn-C >> actine-myosin interaction

Increased sarcomere length(“Starling’s Law of the Heart”)- Optimal actine-myosine overlap- Reduced lattice spacing- Altered myosin head orientation

Interaction of two mechanisms- increased calcium sensitivity