Norepinephrine :-     

                             Norepinephrine, administered to a normotensive adult either subcutaneously or by slow intravenous injection,constricts most blood vessels.Venules as well as arterioles are constricted.As a consequence, there is a net increase in the total peripheral resistance. 

           The effects of norepinephrine on cardiac function are complex because of the dynamic interaction of the direct effects of norepinephrine on the heart and the initiation of powerful cardiac reflexes.

➤ Important considerations are as follows:

 (1) The direct effect of norepinephrine on the heart is stimulatory. 

(2) The reflex initiated is inhibitory, that is, opposite to the direct effect.

(3) The reflex varies with the level of sympathetic and parasympathetic activity just before the initiation of the reflex.

(4) The distribution of sympathetic and parasympathetic nerves is not uniform in the heart.

➤The net effect of norepinephrine administration on heart rate and ventricular contractile force therefore varies with the dose of norepinephrine,the physical activity of the subject,any prior cardiovascular and baroreceptor pathology,and the presence of other drugs that may alter reflexes.

➤In a normal resting subject who is receiving no drugs, there is a moderate parasympathetic tone to the heart, and sympathetic activity is relatively low. The ventricular muscle receives little, if any, parasympathetic innervation.