The vagal cardiac accelerator system in the reflex control of heart rate in conscious dogs

The reactions of the vagal cardioaccelerator (VCA) system to changes in mea n arterial pressure (MAP) were studied in five beta -adrenoceptor blocked c onscious dogs. An increase in MAP was obtained by administration of vasopre ssin or methoxamine, a decrease by doxazosin or nitroprusside. In the first series of experiments the MAP changes were induced after muscarinic recept or blockade, in a second series both before and after muscarinic blockade. Prior to these experiments, the maximum VCA activity, defined as the differ ence between maximum heart rate after muscarinic blockade and the rate afte r additional nicotinic blockade, was determined. We hypothesized that this quantity, as a measure of VCA activity, depends on the prevailing vagal ton e. In the first series of experiments, a rise in MAP evoked an increase in heart rate, a fall in MAP indicated decrease. In the second series, when pr ior to muscarinic blockade the vagal tone was reflexly raised, the subseque nt VCA reflex response to the rise in MAP was attenuated. Prior to the musc arinic blockade the vagal tone was reflexly lowered, the VCA reflex respons e was enhanced. Direct chronotropic effects of MAP-varying drugs were ruled out by the absence of a heart-rate response in experiments on vagotomized animals. We concluded that the vagal cardioaccelerator system is involved i n the reflex control of heart rate. Both the VCA reflex response to changes in systemic blood pressure and the maximum VCA activity however, are deter mined by the prevailing vagal tone.