Background:
Transgenic rats expressing the human angiotensinogen gene [TGR (hAogen) 1623] (hAogen) exhibit sustained hypertension and systolic dysfunction associated with altered cardiac renin-angiotensin system and β- adrenergic receptor (AR) signal transduction systems. We have shown previously, in contrast to β
1
- and β
2
-ARs, β
3
-ARs are linked to G
i
proteins, and stimulation of β
3
-ARs inhibits cardiac contraction and relaxation. Hypertension is associated with increased cardiac expression of G
i
proteins. This may alter β
3
-AR stimulation and play an important role in the cardiac functional impairment of this humanized model of hypertension. However, the alteration and functional effect of β
3
-AR activation in this model are unknown. We tested the hypothesis that high cardiac tissue Ang II content in hAogen may cause an up-regulation of cardiac β
3
-AR, which exacerbates myocyte dysfunction and impairs calcium regulation, thereby directly contributing to the cardiac dysfunction.
Methods:
We compared LV myocyte β
3
-AR expression and myocyte functional and [Ca
2+
]
i
transient ([Ca
2+
]
iT
) responses to β- and β
3
-AR stimulation in myocytes obtained from 8 Sprague Dawley (SD) control and 8 hAogen male adult rats.
Results:
Versus SD, in hAGT rats, basal myocyte contraction (dL/dt
max
, 107.1 vs 138.8 μm/s), relaxation (dR/dt
max
, 94.0 vs 103.9 μm/s) and [Ca
2+
]
iT
(0.15 vs 0.21) were depressed. A non-selective β-AR agonist, isoproterenol (ISO) (10
-8
M) produced significantly smaller increases in dL/dt
max
, (42% vs. 61%), dR / dtmax (35% vs. 51%), and [Ca
2+
]
iT
(20% vs. 30%). Moreover, versus SD, in hAogen rats, LV myocyte β
3
-AR protein level increased by 32% (0.29 vs 0.22) with resulted significantly altered myocyte functional response to β
3
-AR stimulation. Compared with the changes in SD myocytes, in hAogen myocytes, BRL (10
-8
M) produced a significantly greater decreases in dL/dt
max
, (27% vs 12%), dR/dt
max
(28% vs 11%), and [Ca
2+
]
iT
(17% vs 10%).
Conclusions:
TGR (hAogen) 1623, this humanized model of hypertension is associated with an up-regulation of LV myocyte β
3
-AR, which enhances β
3
-AR-caused inhibition of myocyte contractile, relaxation and [Ca
2+
]
iT
and exacerbates β-AR desensitization, thereby directly contributing to the cardiac dysfunction.