Key result
Alpha 1-adrenergic stimulation with phenylephrine and increased flow synergistically increased left ventricular systolic pressure and cardiac performance (P<0.01).
p-value: p=<0.01
Direct stimulation of myocardial alpha 1-adrenoceptors and increased flow act simultaneously and synergistically to increase cardiac performance in rat hearts.
Should not yet influence clinical practice; leaves open translation of alpha1-flow synergy to human cardiac performance.
Myocardial contractility depends on several mechanisms such as coronary perfusion pressure (CPP) and flow as well as on alpha 1-adrenoceptor stimulation. Both effects occur during the sympathetic stimulation mediated by norepinephrine. Norepinephrine increases force development in the heart and produces vasoconstriction increasing arterial pressure and, in turn, CPP. The contribution of each of these factors to the increase in myocardial performance needs to be clarified. Thus, in the present study we used two protocols: in the first we measured mean arterial pressure, left ventricular pressure and rate of rise of left ventricular pressure development in anesthetized rats (N = 10) submitted to phenylephrine (PE) stimulation before and after propranolol plus atropine treatment. These observations showed that in vivo alpha 1-adrenergic stimulation increases left ventricular developed pressure (P < 0.05) together with arterial blood pressure (P < 0.05). In the second protocol, we measured left ventricular isovolumic systolic pressure (ISP) and CPP in Langendorff constant flow-perfused hearts. The hearts (N = 7) were perfused with increasing flow rates under control conditions and PE or PE + nitroprusside (NP). Both CPP and ISP increased (P < 0.01) as a function of flow. CPP changes were not affected by drug treatment but ISP increased (P < 0.01). The largest ISP increase was obtained with PE + NP treatment (P < 0.01). The results suggest that both mechanisms, i.e., direct stimulation of myocardial alpha 1-adrenoceptors and increased flow, increased cardiac performance acting simultaneously and synergistically.
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Vassallo et al. (1998) studied this question. Phenylephrine and flow changes vs. Control conditions was evaluated on Left ventricular developed pressure and isovolumic systolic pressure (p=<0.01). Alpha 1-adrenergic stimulation with phenylephrine and increased flow synergistically increased left ventricular systolic pressure and cardiac performance (P<0.01).
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