RESPONSES OF THE HEART TO REFLEX ACTIVATION OF THE RIGHT AND LEFT VAGUS NERVES BY THE PRESSOR COMPOUNDS, NEOSYNEPHRIN AND PITRESSIN

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Abstract
The pressor compounds neosynephrin, pitressin and angiotonin are capable of producing an inhibition of the heart which is best explained on the basis of reflexes initiated in response to the rise of blood pressure resulting from their vasoconstrictor action. Adm. of these pressor compounds may be said to result in a physiological activation of the vagus nerves. In expts. on dogs whose right vagus has been cut and whose left vagus remains intact, i.e., left vagus dogs, the response to the pressor compounds commonly includes A-V heart block. Thus in this group of animals A-V block followed the inj. of neosynephrin in 27 of 37 expts. on 14 dogs, of pitressin in 7 of 10 expts. on 7 dogs and of angiotonin in a single expt. In dogs whose left vagus had been cut and whose right vagus remained intact, i.e., right vagus dogs, no instances of A-V heart block occurred in response to inj. of neosynephrin in 28 expts. on 12 dogs, of pitressin in 12 expts. on 6 dogs and of angiotonin in a single expt. Physiological activation of either the right or the left vagus results in sinus bradycardia. No remarkable difference in the degree of heart slowing produced by the left as compared with the right vagus is evident. As a result of physiological activation of the right vagus the automaticity of the S-A node frequently is depressed to a level at which the A-V node takes over the function of pacemaker. Thus A-V nodal rhythm was observed in 4 of 28 expts. using neosynephrin and in 1 of 12 experiments using pitressin. A-V nodal rhythm occurred in none of the expts. involving the left vagus group of dogs. The completely dener-vated heart of the dog is not depressed by neosynephrin in the doses in which it was employed. In the majority of expts. pitressin fails to cause significant inhibition of the completely denervated heart. When inhibition occurs it usually is of small degree.