Background:
Angiotensin-(1-12) [Ang-(1-12)] is a chymase-dependent source for Angiotensin II (Ang II) inotropic activity that may be impaired in a model of sustained hypertension with high cardiac Ang II content due to insertion of the human angiotensinogen (AGT) gene in the rat genome. Accordingly, we evaluated the effects of Ang-(1-12) and Ang II on myocyte contractility and [Ca
2+
]
i
regulation in 9 adult male transgenic rats expressing the human AGT gene [TGR(hAGT)L1623)] and 9 SD controls.
Methods:
We compared LV myocyte contraction, relaxation and [Ca
2+
]
i
transient ([Ca
2+
]
iT
) responses to Ang-(1-12) (4x10
-6
M) and Ang II (10
-6
M) in freshly isolated LV myocytes.
Results:
Myocyte contraction (dL/dt
max
, 109.6 vs 127.9 μm/s), relaxation (dR/dt
max
, 95.3 vs 107.5 μm/s) and [Ca
2+
]
iT
(0.15 vs 0.24) were depressed in TGR(hAGT)L1623 rats compared to SD controls. Moreover, cell contractile and [Ca
2+
]
iT
responses following exposure to Ang-(1-12) or Ang II were markedly blunted. In SD myocytes, versus baseline, Ang II or Ang-(1-12) superfusion produced significant increases in dL/dt
max
[Ang II: 44% vs Ang-(1-12): 34%], dR/dt
max
(33% vs 26%) and [Ca
2+
]
iT
(31% vs 25%). Importantly, the magnitude of the responses to the two agents in TGR(hAGT)L1623 myocytes was significantly reduced. Versus the changes in SD myocytes, Ang-(1-12) caused significantly less increases in dL/dt
max
(22%), dR/dt
max
(16%) and [Ca
2+
]
iT
(15%) in TGR(hAGT)L1623 myocytes . Ang II also caused similar significantly attenuated increases in dL/dt
max
(27%), dR/dt
max
(25%) and [Ca
2+
]
iT
(23%). The Ang-(1-12)-induced inotropic effects were completely prevented in the presence of the inhibitory cAMP analog, Rp-cAMPS (10
–4
M, 2 hours) in both SD and TGR(hAGT)L1623 myocytes, but were further augmented only intransgenic rats after incubation of myocytes with the G
i
inhibitor, pertussis toxin (PTX, 2 μg/ml, 36°C, 5 hours).
Conclusions:
Ang-(1-12) stimulates LV myocyte contractile function and [Ca
2+
]
iT
in both SD and TGR(hAGT)L1623 rats. Furthermore, we now show that the blunted inotropic responses to Ang-(1-12) and Ang II in rats expressing the human AGT gene is mediated through a cAMP-dependent mechanism that is coupled to both stimulatory G and inhibitory PTX-sensitive G proteins.
Dissociation of Chronotropic and Inotropic Responses in the Rat Heart During Sympathetic Stimulation