Induction by endogenous noradrenaline of an alpha 1-adrenoceptor-mediated positive inotropic effect in rabbit papillary muscles

Ventriculo-Arterial Coupling Analysis Predicts the Hemodynamic Response to Norepinephrine in Hypotensive Postoperative Patients: A Prospective Observational Study

OBJECTIVES

The objectives of the present study were to evaluate, in patients with persistent arterial hypotension in the immediate postcardiac surgery period, the effects of norepinephrine infusion on ventriculo-arterial coupling, its determinants: arterial elastance and end-systolic ventricular elastance, and to test the ability of arterial elastance to end-systolic ventricular elastance ratio to predict stroke volume increases.

DESIGN

Prospective observational study.

SETTING

Cardiac-vascular surgical ICU.

PATIENTS

Twenty-eight postoperative cardiac surgery patients, in whom physicians decided to administer norepinephrine infusion, were included.

MEASUREMENTS AND MAIN RESULTS

Arterial pressure, stroke volume index, cardiac index, indexed total peripheral resistance, arterial compliance, arterial elastance, and end-systolic ventricular elastance, were measured before and after norepinephrine infusion. We estimated ventriculo-arterial coupling by the arterial elastance to end-systolic ventricular elastance ratio and defined stroke volume responders by a stroke volume increase greater than or equal to 15%. Twenty-two of the 28 subjects had altered ventriculo-arterial coupling (1.87 [1.57-2.51] vs 1.1 [1-1.18]). Fifteen of the 28 subjects (54%) were stroke volume responders. At baseline, stroke volume responders had similar arterial pressure, higher indexed total peripheral resistance, arterial elastance, arterial elastance to end-systolic ventricular elastance ratio (2.21 [1.69-2.89] vs 1.33 [1.1-1.56]; p < 0.05), and lower arterial compliance, indexed total peripheral resistance and cardiac index. Norepinephrine significantly increased arterial pressure in all subjects. In stroke volume responders, norepinephrine increased arterial elastance, end-systolic ventricular elastance, cardiac index, and improved arterial elastance/end-systolic ventricular elastance coupling. The baseline arterial elastance to end-systolic ventricular elastance ratio predicted stroke volume responsiveness (area under the curve [95% CI], 0.87 [0.71-1]; p < 0.0001).

CONCLUSIONS

In patients with arterial hypotension norepinephrine increased end-systolic ventricular elastance and arterial elastance. The effects of norepinephrine on stroke volume depend on baseline ventriculo-arterial coupling. Although norepinephrine infusion corrects arterial hypotension in all subjects, increase of stroke volume occurred only in subjects with altered ventriculo-arterial coupling.

Role of alpha1-adrenoceptor subtypes in production of the positive inotropic effects in mammalian myocardium: implications for the alpha1-adrenoceptor subtype distribution

Both alpha1A- and alpha1B-adrenoceptor subtypes are present in mammalian myocardium. Alpha1-adrenoceptor activation can enhance myocardial contractility, and two possible inotropic mechanisms are proposed: an increase in myofilament Ca2+ sensitivity and an increase in transsarcolemmal Ca2+ influx during action potential prolongation that accompanies the transient outward current inhibition. We suggest that the former is mediated by the alpha1B-adrenoceptor subtype and the latter is by the alpha1A-adrenoceptor subtype. The alpha1B-adrenoceptor subtype may be located on a space more proximal to the sympathetic nerve endings than the alpha1A-adrenoceptor subtype, because the positive inotropic effect of endogenous norepinephrine was mediated entirely by the alpha1B-adrenoceptor subtype. In some species, the sustained positive inotropic effect develops following the transient negative inotropic effect, which is mediated by the alpha1A-adrenoceptor subtype.

Involvement of alpha1B-adrenoceptors in the positive inotropic effect of endogenous noradrenaline in rabbit myocardium

We studied the alpha1-adrenoceptor subtypes mediating the positive inotropic effects of phenylephrine and noradrenaline as well as endogenous noradrenaline released by tyramine in rabbit papillary muscle. In the presence of propranolol, both phenylephrine and tyramine produced a positive inotropic effect in a concentration-dependent manner. WB4101 (N-[2-(2,6-dimethoxyphenoxy]ethyl]-2,3-dihydro-1,4-benzodioxin+ ++-2-methanamine) and chlorethylclonidine each antagonized the positive inotropic effect of phenylephrine. On the other hand, only chlorethylclonidine significantly blocked the positive inotropic effect of tyramine. However, the presence of both antagonists was needed to block the positive inotropic effect elicited by the exogenous addition of the low concentration of noradrenaline. These data suggest that after extensive blockade of beta-adrenoceptors the positive inotropic effects of phenylephrine and exogenous noradrenaline result from stimulation of the alpha1A- and alpha1B-adrenoceptor subtypes, whereas that of endogenous noradrenaline is mediated via the alpha1B-adrenoceptor subtype. This could be explained by assuming that the alpha1B-adrenoceptor subtype population may be located on a space confronting more closely to the sympathetic nerve endings than the alpha1A-adrenoceptor subtype population.