Chronotropic effects of angiotensin I, angiotensin II, bradykinin and vasopressin in guinea pig atria.

Chronotropic responses to angiotensin I and angiotensin II, vasopressin and bradykinin were measured in guinea pig isolated right atria. Angiotensin II (100-30,000 pg/ml) was slightly more potent than angiotensin I and caused a maximum tachycardia of 30-40 b/min; only 20% of the maximum response to (--)-noradrenaline. Propranolol (1 micro M) or reserpine pretreatment (1 mg/kg i.p., 24 h) did not alter the response to angiotensin II or bradykinin. Converting enzyme inhibition by captopril (10 micrograms/ml) did not affect resting rate nor the response to angiotensin II but shifted the location of the angiotensin I curve by 40 fold to the right. Bradykinin (5-500 ng/ml) caused small increases in rate while vasopressin 1-100 ng/ml was completely without effect. These results suggest that angiotensin II has a small positive chronotropic effect that is not dependent on tissue noradrenaline release or beta-adrenoceptors and that tissue converting enzyme is active in right atria. Relatively high concentrations of angiotensin and bradykinin were required to directly stimulate the sino-atrial node compared with plasma levels measured during physiological stimuli. Therefore these effects on atria are probably of little physiological significance for peptide concentrations in plasma but may be important in relation to local tissue generation of angiotensin II.