Angiotensin II, methoxamine and phenylephrine were evaluated in two tests of reflex responsiveness that involve arterial pressure increases (delta BP) and reflex bradycardia (delta HR). First, bolus injections produced comparable ratios (delta HR/delta BP, in beats . min-1 . mmHg-1) for the three agents: 1.69 +/- 0.31, 1.80 +/- 0.30, and 1.59 +/- 0.32, respectively (SE, n = 6). All three agents also had some undesirable direct chronotropic effects on the heart. Phenylephrine and methoxamine produced rapid and predictable pressure changes and were stable in solution. Blockade by atropine and propranolol was used to show that the bradycardia after bolus injection has an early component (1 s after pressure maximum) that is wholly parasympathetic. In the second test, 5-min infusions of the three agents produced comparable final delta HR/delta BP ratios: 2.0 +/- 0.3, 2.2 +/- 0.5, and 2.3 +/- 0.2 for angiotensin, methoxamine, and phenylephrine, respectively (n = 6). delta HR in this case had a predominant (estimated 67% of total) beta-adrenergic component. Therefore, rapid vagal control of heart rate in the conscious rat is temporary; it is replaced and strengthened in time by a more slowly developing beta-adrenergic component. Bolus injection and infusion of vasopressors can be conveniently used in combination to more fully quantitate heart rate responsiveness in conscious animals that have implanted arterial and venous catheters.
T. G. Coleman (1980) studied this question.