Cardiac response to norepinephrine and sympathetic nerve stimulation following experimental subarachnoid hemorrhage

This study aimed to investigate the cardiac response to sympathetic stimulation and norepinephrine exposure following subarachnoid hemorrhage (SAH). Cardiac functional response was assessed 3 days following an injection of 300 microl of homologous blood in the cisterna magna using an in situ perfused, innervated rat heart model. Sympathetic nervous activity was indirectly assessed from measurements of arterial plasma and tissue norepinephrine concentration and cardiac beta-receptor density. In in situ perfused hearts, sympathetic nerve stimulation (2, 4 and 8 Hz, 1 min duration) induced a frequency-dependent increase in left ventricular pressure (VP), with the response being more pronounced in the SAH group of animals at the higher frequency (P<0.05). However, the concomitant release of norepinephrine was identical in the two groups of animals. Increasing doses of norepinephrine (10(-9) to 10(-5) M) added to the perfusate induced a dose-dependent increase in VP and its first derivative (dP/dt). Both responses were greater in the SAH animals compared to the sham rats (P<0.01). ECG recordings from SAH animals presented a higher incidence of different types of arrhythmias, both at rest and when submitted to electrical stimulation or norepinephrine exposure. No difference was found between groups in left ventricle norepinephrine content, plasma norepinephrine nor left ventricle beta-receptor density. In conclusion, hearts from animals following acute experimental SAH exhibit enhanced sensitivity to norepinephrine infusion and sympathetic nerve stimulation, and are more prone to develop arrhythmias. However, hypersensitivity of the heart may not be explained by changes in norepinephrine release or by beta-receptor density.