POSITIVE CHRONOTROPIC REPONSES TO CARDIAC α1-ADRENORECEPTOR ACTIVATION IN THE PITHED RAT

• 1 Adrenaline (0.1–10 μg/kg), noradrenaline (0.1–10 μg/kg) and phenylephrine (1–100 μg/kg) acted on both cardiac α₁- and β-adrenoreceptors to induce positive chronotropic responses in the pithed rat.
• 2 When β-adrenoreceptors were blocked by propranolol (1 mg/kg), the residual chronotropic responses were due to activation of α₁-adrenoreceptors since they were significantly reduced by prazosin (10–100 μg/kg).
• 3 Methoxamine (10–300 μg/kg) acted solely on cardiac α₁-adrenoreceptors to induce positive chronotropic responses which were abolished by prazosin (10–100 μg/kg) alone, as has been demonstrated previously for amidephrine.
• 4 The rank order of potency for eliciting the positive chronotropic response to α₁-adrenoreceptor activation was adrenaline > noradrenaline > phenylephrine > methoxamine.
• 5 The positive chronotropic responses to adrenaline (3–10 μg/kg), noradrenaline (3–10 μg/kg) and phenylephrine (30–100 μg/kg) produced by activating α₁-adrenoceptors had a slower time course than did the chronotropic responses produced by activation of β-adrenoreceptors.