Role of the vagus in modulation by Ca2+ of the depressant action of adenosine and adenosine 5'-triphosphate on the canine sinus node in vivo.

Abstract

1 The effect of elevated plasma Ca2+ level on the depressant action of adenosine and adenosine 5'-triphosphate (ATP) on the sinus node was studied in a canine model in vivo. 2 Under baseline conditions, both adenosine and ATP (3 mumol/kg) administered as a rapid bolus injection into the right atrium, caused a transient (less than 120 s) prolongation of sinus cycle length (SCL) of 128 +/- 25 ms and 439 +/- 31 ms respectively (P less than 0.02). 3 Infusion of CaCl2 (0.025 mmol/kg/min for 40 min) prolonged SCL from 257 +/- 17 ms to 418 +/- 17 ms (at t = 40 min; P less than 0.05), but did not significantly change arterial blood pressure. 4 Ca2+ infusion had no significant effect on SCL prolongation caused by adenosine (128 +/- 25 ms vs 151 +/- 31 ms; before and at t = 40 min respectively), but caused pronounced enhancement of SCL prolongation caused by ATP (439 +/- 31 ms vs 1215 +/- 401 ms; before and at t = 40, respectively, P less than 0.02). 5 Atropine (0.2 mg/kg, i.v.) did not affect the action of adenosine before or during Ca2+ infusion. In contrast, atropine abolished SCL prolongation by Ca2+ and abolished Ca2+ enhancement of the action of ATP. 6 The present data give further support to the hypotheses that (a) calcium potentiates vagally mediated actions and (b) the vagus is involved in the differential electrophysiological actions of adenosine and ATP in the canine heart in vivo.