During compensatory hypertrophy (26 weeks of age) in Spontaneously Hypertensive Rat (SHR) hearts, the inotropic response to b-adrenergic stimulation is decreased, despite normal baseline contractile function, thus indicating a specific defect in the b-adrenergic pathways. These investigators have shown that this decreased responsiveness is not due to decreased availability of Ca2+. for activation of contraction (Moravec et al, 1995) and have proposed the alternative hypothesis that during b-adrenergic stimulation, the sensitivity of the myofilaments to Ca2+ is decreased in the SHR heart. This hypothesis is supported by (I) their observations of a greater increase in troponin I (TN-I) phosphorylation, in SHR versus WKY myocytes following b-adrenergic receptor stimulation and downstream activation of the b-adrenergic pathway (McConnell et al, in review); (ii) their evidence of decreased Ca2+ sensitivity of actomyosin ATPase activity in the SHR versus the WKY after b-adrenergic stimulation: (iii) their demonstration that the greater impairment of the inotropic response to b-adrenergic stimulation in 76 weeks old SHR is accompanied by a further increase in TN-I phosphorylation. The investigators therefore propose that this greater increase in TN-I phosphorylation in the SHR contributes to the impaired inotropic response to sympathetic stimulation. In the studies proposed in this application, they will investigate the mechanism responsible for the greater increase in PKA phosphorylation of TN-I in the SHR, they will address the functional significance of these observations and will extend their studies to the failing human heart which also shows a depressed response to the b-adrenergic stimulation. In human heart failure, a greater increase in PKA-dependent TN-I may represent a novel mechanism, downstream of b-receptor activation of adenylyl cyclase, to protect the failing heart against over-stimulation.
Their Specific Aims are therefore (1) to compare the stoichiometry of PKA phosphorylation of TN-1 in the SHR and WKY using 2-D gel electrophoresis; (2) to determine whether myofilament-associated PKA activity is increased in the SHR; (3) to determine if increased PKA phosphorylation of TN-1 contributes to the impaired response of the failing human heart to sympathetic stimulation; (4) to determine the functional significance of increased PKA phosphorylation by comparing the effect of b-adrenergic stimulation on Ca2+ sensitivity of force development in skinned trabeculae from SHR and WKY and from failing and non-failing human hearts. Overall, these studies may help to define a novel mechanism by which the response to b-adrenergic stimulation is down-regulated in hypertrophied and failing hearts.
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