The Effect of Calcium, Practolol, Atropine and Phenoxybenzamine on the Rat Atrial Action Potential at Threshold and Supra‐threshold Stimulation

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Abstract
An increase in the concentration of calcium in the Ringer's solution increased the maximum rate of rise (dv/dt) and shortened the duration of the action potential of rat left atria in vitro. Supra‐threshold stimulation increased dv/dt at low Ca++ concentration (2.0 meq./l). This increase was completely abolished by pretreatment with an adrenergic β‐receptor blocker, practolol 5 × 10‐6 M. At this concentration the drug is devoid of a quinidine‐like effect. The duration of the action potential was decreased by an increase in the strength of the electrical stimulation. This decrease was completely blocked by pretreatment with atropine 1.43 × 10‐6 M and phenoxybenzamine 2.5 × 10‐6 M, an adrenergic α‐receptor blocker with an atropine‐like activity. At threshold stimulation neither practolol 5 × 10‐6 M nor atropine 1.43 × 10‐6 M nor phenoxybenzamine 2.5 × 10‐6 M caused any changes in the shape or the duration of the intracellularly recorded rat left atrial action potential. It is concluded that the possible liberation of autonomic mediators as well as the calcium concentration of the Ringer solution, should be taken into consideration in the interpretation of the results of experiments which involve the effects of antagonists on intracellularly recorded atrial action potentials.