Positive inotropic action of angiotensin II in the pithed rat

The cardiovascular effects of angiotensin II were examined in aortic blood pressure-controlled and -uncontrolled pithed rats. Angiotensin II induced a dose-dependent increase in diastolic blood pressure, left ventricular pressure (LVP), dP/dt (the first derivative of LVP) and heart rate in pithed rats. The maximal responses for these parameters were similar to those to noradrenaline, except for the rise in diastolic blood pressure, where noradrenaline caused a greater increase than angiotensin II. After treatment with propranolol, the positive chronotropic effect of angiotensin II was abolished. Angiotensin II produced a dose-dependent increase in diastolic blood pressure, which was similar to that of vasopressin, and an increase in dP/dtmax, which proved much greater than that of vasopressin. When aortic blood pressure was controlled and the beta-receptors were blocked by propranolol, angiotensin II caused a dose-dependent increase in dP/dtmax without affecting the left ventricular enddiastolic pressure. The same results were obtained after both beta- and alpha-adrenoceptors were blocked by propranolol and phentolamine. Losartan but not PD123177 caused parallel rightward shifts of the dose-response curve of angiotensin II for dP/dtmax in the aortic blood pressure controlled pithed rat without altering the maximal response. It is concluded that in the pithed rat angiotensin II produced an increase in myocardial contractile force which is not mediated by beta- or alpha-adrenoceptors. The inotropic effect appears to be mediated by angiotensin receptors, of the AT1-subtype.