U594485.pdf (22.66 MB)
Diurnal variation in excitation-contraction coupling in rat ventricular myocytes
thesis
posted on 2014-08-01, 09:31 authored by Helen Elizabeth CollinsDiumal variation has been reported in many cardiovascular haemodynamics parameters
such as heart rate and blood pressure and the cardiac action potential. This variation
may result from the diurnal variation in sympathetic activity or in cardiac gene
expression. However, it is unknown whether these time-of-day dependent changes
impact on excitation-contraction (EC) coupling. There is also a morning peak in the
onset of ventricular arrhythmias and associated sudden cardiac death in man, which
appear linked to the increase in sympathetic activity. Therefore, the aims of this
investigation were to determine whether there was a time-of-day dependent variation in
EC-coupling and its modulation by sympathetic stimulation.
Left ventricular myocytes were isolated during either the resting period (ZT3) or the
active period (ZT15) o f the adult Wistar rat. [Ca[superscript 2+]][subscript i] was determined using Fura-2 and
contraction strength was determined using cell-edge detection in response to electrical
field stimulation, and gene expression was determined using quantitative real-time RT-PCR.
To determine the effects of hypertension-induced hypertrophy, myocytes were
isolated from pre- and post-hypertensive spontaneously hypertensive rats (SHR).
The basal Ca[superscript 2+] transient, contraction strength and SR Ca[superscript 2+] content were significantly
greater in resting period (ZT3) myocytes than active period (ZT15) myocytes. Systolic
[Ca[superscript 2+]], amplitude of Ca[superscript 2+] transient and SR Ca[superscript 2+] content in response to isoproterenol (>
3nM) were significantly greater in resting period (ZT3) myocytes. The percentage of
myocytes developing arrhythmic activity in response to isoproterenol was greater in
resting period (ZT3) myocytes. Nitric oxide synthase (NOS) inhibition using L-NNA
significantly increased systolic [Ca[superscript 2+]], amplitude of Ca[superscript 2+] transient, SR Ca[superscript 2+] content and
the percentage of myocytes developing arrhythmic activity in active period (ZT15)
myocytes thereby depressing time-of-day dependent variation in these parameters. In
addition, expression of NOS1 was significantly greater in active period (ZT15)
myocytes. Diurnal variation in the Ca[superscript 2+] transient and its responsiveness to isoproterenol
were depressed in adult SHR, however, this did not reflect a depression of diurnal
cycling in NOS1 expression.
This shows for the first time a time-of-day dependent variation in the Ca[superscript 2+]-transient and
resulting contraction strength, reflecting levels of SR Ca[superscript 2+]-loading, due to a NOS-signalling
pathway. There was also a reduction in sympathetic-induced arrhythmic
activity in active period (ZT15) myocytes which was associated with increased NOS
activity. Therefore, variation in NOS may be a means of protecting against arrhythmias
during severe sympathetic stimulation. Loss of protection through disruption to the
circadian clock resulting from cardiomyopathies such as hypertension-induced
hypertrophy may result in a decreased threshold for sympathetic-induced arrhythmias,
however; this requires further work to elucidate the underlying molecular mechanisms.
History
Supervisor(s)
Rodrigo, GlennDate of award
2011-01-01Author affiliation
Department of Cardiovascular SciencesAwarding institution
University of LeicesterQualification level
- Doctoral
Qualification name
- PhD