Effect of ovariectomy and estrogen replacement on the -Adrenergic receptor signaling pathway and intracellular Ca2+ homeostasis in the rat heart
Abstract of the thesis entitled ?ffect of ovariectomy and estrogen replacement on the ?adrenergic receptor signaling pathway and intracellular Ca2+ homeostasis in the rat heart?submitted by Mr. Kam Wan Lung for the Degree of Doctor of Philosophy at the University of Hong Kong in January, 2005
Estrogen protects the heart against ischaemic insults and decreases the cardiac contractility. There is evidence that estrogen down-regulates ?-adrenergic receptor (?-AR) and affects intracellular Ca2+ ([Ca2+]i) homeostasis. It was hypothesized that estrogen might confer cardioprotection and decrease contractility by suppressing the expression of ?-AR, altering its signaling mechanism, and affecting the [Ca2+]i homeostasis. Three series of experiments were conducted to address the chronic effects of estrogen on 1) the expression of ?-AR, 2) the signaling intermediates mediating the action of ?-AR activation and 3) [Ca2+]i homeostasis. These were correlated with the effects of estrogen on injury in response to ischaemic insult and/or contractile functions. In order to determine the action of estrogen on the myocardium, two in vitro preparations, the isolated perfused heart and isolated ventricular myocytes, were used. Ovariectomy (Ovx) was performed to remove the source of estrogen and the cardiac responses were compared with those from sham control and Ovx rats with estrogen replacement (Ovx+E2).
The first important observation of the present study is that the cardioprotective effect of estrogen
is due to its direct suppressive effect on the enhanced ?-AR expression in the heart from Ovx rats,
which in turn reduces the injury upon ?AR stimulation during ischaemia. This is based on two
observations. Myocardial infarct size was significantly greater in Ovx than in the sham and Ovx+E2 rats
upon ?AR activation during ischaemia. There was a positive correlation between the infarct size and
the expression level of ?-AR. Furthermore, incubation of ventricular myocytes from Ovx rats with
10-9M estrogen for 24 h, which significantly reduced the expression of ?-AR, was accompanied by
cardioprotection. On the other hand, incubation of the myocytes with 10-9M estrogen for 12h neither
affected the ?-AR expression nor reduced cardiac injury in response to ischaemic insults. The
observation is evidence of a causal relationship between ?-AR and cardiac injury.
The second main finding is that both the basal and isoproterenol (Iso)-induced protein
kinase A (PKA) activity were significantly higher in the heart from Ovx than those from the sham
and Ovx+E2 rats. This was accompanied by a significantly higher basal and Iso-stimulated increase
in L-type Ca2+ channel (CaCh) activity. Blockade of PKA with a selective inhibitor, KT5720, at a
concentration that reduced the effect of PKA on basal CaCh activity to that of the sham control,
significantly inhibited the Iso-stimulated increase in CaCh activity in all three groups of rats and the
inhibition was significantly greater in the Ovx rat than in the other two groups of rats, indicating a
greater contribution of PKA on ?AR stimulated increase in CaCh activity in the heart from the Ovx
rats. The result indicated that estrogen suppresses PKA activity and its effect on CaCh, thus
reducing cardiac responses to ?AR stimulation.
The third important observation is that in the heart from Ovx rats, Ca2+ release from
sarcoplasmic reticulum (SR) during excitation-contraction (E-C) coupling was greater and faster,
due to increases in expression and activity of both CaCh and ryanodine receptor, and SR Ca2+
content. The removal of Ca2+ from cytoplasm was faster due to a marked increase in Na+-Ca2+ exchanger activity, but not to any change in SR Ca2+-ATPase. These were accompanied by increased contractility and speedier relaxation. The effects were reversed to normal by estrogen replacement. The observation is evidence that estrogen may affect the contractile functions by altering [Ca2+]i homeostasis in the heart.
In conclusion, the result from the present study demonstrates that estrogen protects the heart and decreases contractility by suppressing the ?-AR expression and the basal activity of PKA, and by altering [Ca2+]i homeostasis.
School:The University of Hong Kong
School Location:China - Hong Kong SAR
Source Type:Master's Thesis
Keywords:estrogen ovariectomy beta adrenoceptors cellular signal transduction calcium ions homeostasis heart physiology rats
Date of Publication:01/01/2005