Membranes

Question 1

Membrane architecture

Answer 1

alterations in cardiomyocyte membrane architecture contributes to reduced contractility

model: salt-resistant rats vs spontaneously hypertensive salt-intolerant rats
- hearts develop hypertrophy and CHF
- defective ECC in experimental cardiac hypertrophy and HF

Question 2

Defects in calcium handling

Answer 2

• current density through L-type is reduced
• reduced sensitivity of RyRs
• myocyte structure is disrupted so that calcium entering the cell through L-type channels does not couple to stimulate calcium release through RyRs
• defect in CICR

Question 3

ECC in spontaneously hypertensive rats

Answer 3

• ECC is compromised in SHRs
• myocyte contraction/AP reduced
• current density unaffected
• AP-induced calcium transient reduced
• calcium released through RyRs unaffected
• same in hypertrophy and HF
• hypertrophic hearts are rescued by beta stimulation
• t-tubule density reduced in disease

Question 4

SERCA in heart failure

Answer 4

SERCA in heart failure
- calcium clearance rate reduced
- diastolic calcium increases
- SR calcium reduced
- calcium transient reduced
- SERCA activity reduced

Question 5

Adenoviral SERCA

Answer 5

• adenoviral gene transfer of SERCA2a improves left ventricular function in aortic-banded rats in transition to heart failure
• contractile function in isolated cardiomyocytes of failing human HF also restored by gene transfer of SERCA2a (early studies)
• failed to show real benefit when tested in patients

Question 6

ECC - types of calcium

Answer 6

ECC-calcium is not the only type of calcium in cardiomyocytes

IP3-generated by Gq via PLC
- promotes SR calcium release
- promoted by exercise, pregnancy, genetics, hormones, stress, stretch etc.

SHRs:
- hearts hypertrophy
- SHR show higher propensity for extra systolic calcium transients
- IP3RII expression increased in SHRs
- RyR expression unaffected
- IP3Rs preferentially increased in the dyad of ventricular myocytes from hypertrophic hearts

• calcium sparks influenced by IP3-dependent calcium release
• how do more IP3Rs cause increased spontaneous calcium release?
• enhanced sensitivity of RyR-mediated calcium release?

Question 7

Endothelin

Answer 7

• ET-1 stimulates IP3-regulated calcium release in myocytes
• stimulates cardiac ionotropy and arrhythmia
• induces hypertrophy via MAPK pathway
• potent vasoconstrictor
• levels increased in patients with HF
• endothelin stored and released in the heart, where it has para/autocrine effects
• ET-1-induced phenotypic changes are IP3-dependent
• IP3-induced calcium signals stimulate the transcription of hypertrophy-associated genes without ECC
• IP3 stimulates increased calcium in the nucleus (IP3Rs located close to the nucleus)
• nuclear calcium increase is required for ET-1-stimulated ANF expression

Question 8

Specificity

Answer 8

specificity provided by coincident signals
- stimulation of kinase cascades

specificity provided by alternative calcium
- calcium signals generated from distinct calcium pools control contraction and transcription
- CICR = contraction
- IP3-induced CR = transcription
- HOW?

Question 9

Gene transcription

Answer 9

• calcium regulates hypertrophic gene expression
• more work = increased calcium = increased heart
• yet phospholamban KO = increased calcium cycling, does not induce hypertrophy
• overexpression of CaM = hypertrophy by a calcinuerin-dependent pathway
• prevented by calcineurin inhibitor
• overexpression of constitutively active nuclear factor of activated T cells (NF-AT3) = hypertrophy
• calcineurin inhibition prevents hypertrophy in mice
• CaMKII selectively signals to histone deacetylase increase during hypertrophy