Fig. 1. Effect of intravenous injection of specific agonists on heart rate (f_H) before and after blockade with the appropriate antagonist in one seal. Each data point represents the mean f_H for the preceding 10 s interval. Arrows denote the time of injection of agonists. (A) 1-phenylephrine hydrochloride (0.06 µg kg^-1) alone and after S.C. injection of the muscarinic antagonist methoctramine (0.23 mg kg^-1). (B) Phenylephrine (0.06 µg kg^-1) alone and after oral administration of the ₁-adrenergic antagonist prazosin (0.24 mg kg^-1). (C) Isoproterenol hydrochloride (0.01 µg kg^-1) alone and after the oral administration of ß₁-adrenergic antagonist metoprolol (4 mg kg^-1). Control saline injections caused no significant effect on f_H.